1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
|
<?xml version="1.0" encoding="iso-8859-1"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" lang="en" xml:lang="en">
<head>
<title>C++ Object Persistence with ODB</title>
<meta name="copyright" content="© 2009-2010 Code Synthesis Tools CC"/>
<meta name="keywords" content="odb,c++,object,persistence,ORM,relational,database,RDBMS,ODBMS,OODBMS"/>
<meta name="description" content="C++ Object Persistence with ODB"/>
<meta name="revision" content="1.0"/>
<meta name="version" content="1.0.0"/>
<link rel="stylesheet" type="text/css" href="default.css" />
<style type="text/css">
pre {
padding : 0 0 0 0em;
margin : 0em 0em 0em 0;
font-size : 102%
}
body {
min-width: 48em;
}
h1 {
font-weight: bold;
font-size: 200%;
line-height: 1.2em;
}
h2 {
font-weight : bold;
font-size : 150%;
padding-top : 0.8em;
}
h3 {
font-size : 140%;
padding-top : 0.8em;
}
/* Adjust indentation for three levels. */
#container {
max-width: 48em;
}
#content {
padding: 0 0.1em 0 4em;
/*background-color: red;*/
}
#content h1 {
margin-left: -2.06em;
}
#content h2 {
margin-left: -1.33em;
}
/* Title page */
#titlepage {
padding: 2em 0 1em 0;
border-bottom: 1px solid black;
}
#titlepage .title {
font-weight: bold;
font-size: 200%;
text-align: center;
padding: 1em 0 2em 0;
}
#titlepage p {
padding-bottom: 1em;
}
#titlepage #revision {
padding-bottom: 0em;
}
/* Lists */
ul.list li, ol.list li {
padding-top : 0.3em;
padding-bottom : 0.3em;
}
div.img {
text-align: center;
padding: 2em 0 2em 0;
}
/* */
dl dt {
padding : 0.8em 0 0 0;
}
/* TOC */
table.toc {
border-style : none;
border-collapse : separate;
border-spacing : 0;
margin : 0.2em 0 0.2em 0;
padding : 0 0 0 0;
}
table.toc tr {
padding : 0 0 0 0;
margin : 0 0 0 0;
}
table.toc * td, table.toc * th {
border-style : none;
margin : 0 0 0 0;
vertical-align : top;
}
table.toc * th {
font-weight : normal;
padding : 0em 0.1em 0em 0;
text-align : left;
white-space : nowrap;
}
table.toc * table.toc th {
padding-left : 1em;
}
table.toc * td {
padding : 0em 0 0em 0.7em;
text-align : left;
}
</style>
</head>
<body>
<div id="container">
<div id="content">
<div class="noprint">
<div id="titlepage">
<div class="title">C++ Object Persistence with ODB</div>
<p>Copyright © 2009-2010 Code Synthesis Tools CC</p>
<p>Permission is granted to copy, distribute and/or modify this
document under the terms of the
<a href="http://www.codesynthesis.com/licenses/fdl-1.3.txt">GNU Free
Documentation License, version 1.3</a>; with no Invariant Sections,
no Front-Cover Texts and no Back-Cover Texts.</p>
<!-- REMEMBER TO CHANGE VERSIONS IN THE META TAGS ABOVE! -->
<p id="revision">Revision 1.0, September 2010</p>
<p>This revision of the manual describes ODB 1.0.0 and is available
in the following formats:
<a href="http://www.codesynthesis.com/products/odb/doc/manual.xhtml">XHTML</a>,
<a href="http://www.codesynthesis.com/products/odb/doc/odb-manual.pdf">PDF</a>, and
<a href="http://www.codesynthesis.com/products/odb/doc/odb-manual.ps">PostScript</a>.</p>
</div>
<h1>Table of Contents</h1>
<table class="toc">
</table>
</div>
<!-- Hello World Example -->
<h1><a name="2">2 Hello World Example</a></h1>
<p>In this chapter we will examine how to create a simple C++
application that relies on ODB for object persistence using
the traditional "Hello World" example. In particular, we will
discuss how to declare persistent classes, generate database
support code, as well as compile and run our application. We
will also learn how to make objects persistent as well as
query, update and delete persistent objects.</p>
<p>The code presented in this chapter is based on the
<code>hello</code> example which can be found in the
<code>odb-examples</code> package of the ODB distribution.</p>
<h2><a name="2.1">2.1 Declaring a Persistent Class</a></h2>
<p>In our "Hello World" example we will depart slighly from
the norm and say hello to people instead of the world. People
in our application will be represented as objects of C++ class
<code>person</code> which is saved in <code>person.hxx</code>:</p>
<pre class="c++">
// person.hxx
//
#include <string>
class person
{
public:
person (const std::string& first,
const std::string& last,
unsigned short age);
const std::string&
first () const;
const std::string&
last () const;
unsigned short
age () const;
void
age (unsigned short);
private:
std::string first_;
std::string last_;
unsigned short age_;
};
</pre>
<p>In order not to miss anyone whom we need to greet, we would like
to save the person objects in a database. To achive this we declare
the <code>person</code> class as persistent:</p>
<pre class="c++">
// person.hxx
//
#include <string>
#include <odb/code.hxx> // (1)
#pragma db object // (2)
class person
{
...
private:
person () {} // (3)
friend class odb::access; // (4)
#pragma db id auto // (5)
unsigned long id_; // (5)
std::string first_;
std::string last_;
unsigned short age_;
};
</pre>
<p>To be able to save person objects in the database we had to make
five changes, marked with (1) to (5), to the orignal class
definition. The first change is the inclusion of the ODB
headers <code>core.hxx</code>. This headers provides a number
of core ODB declarations, such as <code>odb::access</code>, that
are used to define peristent classes.</p>
<p>The second change is the addition of <code>db object</code>
pragma just before the class definition. This pragma tells the
ODB compiler that the class that follows is persistent. Note
that making a class persistent does not mean that all objects
of this class will automatiacally be stored in the database.
You would still create ordinary or <em>transient</em> instances
of this class just as you would before. The difference is that
now you can make such transient instances persistent, as we will
see shortly.</p>
<p>The third change is the addition of the default constructor.
The ODB-generated database support code will use this constructor
when instantiating an object from the persistent state. As we have
done for the <code>person</code> class, you can make the default
constructor private or protected if you don't want to make it
available to the ordinary users of your class.</p>
<p>With the fourth change we make the <code>odb::access</code> class
friend of our <code>person</code> class. This is necessary to make
the default constructor and the data members accessible to the
ODB support code. If your class has public default constructor and
public data members, then the <code>friend</code> declaration is
unnecessary.</p>
<p>The final change adds a data member called <code>id_</code> which
is preceded by another pragma. In ODB every persistent object must
have a unique, within its class, identifier. Or, in other words,
no two persistent instances of the same type have equal
identifiers. For our class we use an integer id. The
<code>db id auto</code> pragma that preceeds the <code>id_</code>
member tells the ODB compiler that the following member is the
object's id. The <code>auto</code> specifier indicates that it is
a database-assigned id. A unique id will be automatically generated
by the database and assigned to the object when it is made
persistent.</p>
<p>In this example we choose to add an identifier because none of
the existing members could serve the same purpose. However, if
a class already has a member with suitable properties, then it
is natural to use that member for an identifier. For example,
if our <code>person</code> class contained some form of personal
identification (SSN in the United States or ID/passport number
in other countries), then we could use that as an id. Or, if
we stored an email associated with each person, then we could
have used that since each person is presumed to have a unqiue
email address:</p>
<pre class="c++">
class person
{
...
#pragma db id
std::string email_;
std::string first_;
std::string last_;
unsigned short age_;
};
</pre>
<p>Now that we have the header file with the persistent class, let's
see how to generate that database support code that we talked
about.</p>
<h2><a name="2.2">2.2 Generating Database Support Code</a></h2>
<p>The persistent class definition that we created in the previous
section was particularly light on code that could actualy
do the job and store the person't data to a database. There
was no serialization or deserialization code, not even data member
registration, that you would normally have to write by hand in
other ORM libraries for C++. This is because in ODB code
that translates between the database and C++ representations
of an object is automatically generated by the ODB compiler.</p>
<p>To compile the <code>person.hxx</code> header we created in the
previous section and generate the support code for the MySQL
database we invoke the ODB compiler from a terminal (UNIX) or
a command prompt (Windows):</p>
<pre class="terminal">
odb -d mysql --generate-query person.hxx
</pre>
<p>We will use MySQL in the reminder of this chapter though other
supported database systems can be used instead.</p>
<p>If you haven't installed the common ODB runtime library
(<code>libodb</code>) or installed it into a directory where
the C++ compiler doesn't search for headers by default,
then you may get the following error:</p>
<pre class="terminal">
person.hxx:10:24: fatal error: odb/core.hxx: No such file or directory
</pre>
<p>To resolve this you will need to specify <code>libodb</code> headers
location with the <code>-I</code> preprocessor option, for example:</p>
<pre class="terminal">
odb -I.../libodb -d mysql --generate-query person.hxx
</pre>
<p>Here <code>.../libodb</code> represents the path to the
<code>libodb</code> directory.</p>
<p>The above invocation of the ODB compiler produces three C++ files:
<code>person-odb.hxx</code>, <code>person-odb.ixx</code>,
<code>person-odb.cxx</code>. You normally don't use types
or functions contained in these files directly. Rather, all
you have to do is include <code>person-odb.hxx</code> in
C++ files where you are performing database operations
with classes from <code>person.hxx</code> as well as compile
<code>person-odb.cxx</code> and link the resulting object
file to your application.</p>
<p>You may be wondering what is the <code>--generate-query</code>
option for. It instructs the ODB compiler to generate
optional query support code that we will use later in our
"Hello World" example. Another option that we will find
useful is <code>--generate-schema</code>. This option
makes the ODB compiler generate a fourth file,
<code>person.sql</code>, which contains the database
schema for the classes defined in <code>person.hxx</code>:</p>
<pre class="terminal">
odb -d mysql --generate-query --generate-schema person.hxx
</pre>
<p>If you would like to see the list of all the available options,
refer to the <a href="http://www.codesynthesis.com/products/odb/doc/odb.xhtml">ODB
Compiler Command Line Manual</a>.</p>
<p>Now that we have the persistent class and the database support
code, the only part that is left is the application code that
does something useful with all this. But before we move on to
the fun part, let first learn how to build and run an application
that uses ODB. This way when we have some application code
to try, there are no more delays before we can run it.</p>
<h2><a name="2.3">2.3 Compiling and Running</a></h2>
<p>Assuming that the <code>main()</code> function with some application
code is saved in <code>driver.cxx</code> and the database support
code and schema are generated as described in the previous section,
to build our application we will first need to compile all the C++
source files and then link them with two ODB runtime libraries.</p>
<p>On UNIX, the compilation part can be done with the following commands
(for Microsoft Visual Studio setup, see the <code>odb-examples</code>
package):</p>
<pre class="terminal">
c++ -c driver.cxx
c++ -c person-odb.cxx
</pre>
<p>Similar to the ODB compilation, if you get an error stating that
a headers in <code>odb/</code> or <code>odb/mysql</code> directory
in not found. In this case you will need to use the <code>-I</code>
preprocessor option to specify the location of the common ODB runtime
library (<code>libodb</code>) and MySQL ODB runtime library
(<code>libodb-mysql</code>).</p>
<p>Once the compilation is done, we can link the application with
the following command:</p>
<pre class="terminal">
c++ -o driver driver.o person-odb.o -lodb-mysql -lodb
</pre>
<p>Notice that we link our application with two ODB libraries:
<code>libodb</code> which is a common runtime library and
<code>libodb-mysql</code> which is a MySQL runtime library
(if you use another database, then the name of this library
will change accordingly). If you get an error saying that
one of these libraries could not be found, then you will need
to use the <code>-L</code> linker option to specify their locations.</p>
<p>Before we can run our application we need to create a database
schema using the generated <code>person.sql</code> file. For MySQL
we can use the <code>mysql</code> client program, for example:</p>
<pre class="terminal">
mysql --user=odb_test --database=odb_test < person.sql
</pre>
<p>The above command will login to a local MySQL server as user
<code>odb_test</code> without a password and use database
named <code>odb_test</code>. Note that after executing this
command all data stored in the <code>odb_test</code> database
will be deleted.</p>
<p>Once the database schema is ready, we run our application
using the same login and database name:</p>
<pre class="terminal">
./driver --user odb_test --database odb_test
</pre>
<h2><a name="2.4">2.4 Making Objects Persistent</a></h2>
<p>Now that we have the infrastructure work out of the way, it
is time to see our first code fragment that interracts with the
database. In this section we will learn how to make <code>person</code>
objects persistent:</p>
<pre class="c++">
// driver.cxx
//
#include <memory> // std::auto_ptr
#include <iostream>
#include <odb/database.hxx>
#include <odb/transaction.hxx>
#include <odb/mysql/database.hxx>
#include "person.hxx"
#include "person-odb.hxx"
using namespace std;
using namespace odb;
int
main (int argc, char* argv[])
{
try
{
auto_ptr<database> db (new mysql::database (argc, argv));
unsigned long john_id, jane_id, joe_id;
// Create a few persistent person objects.
//
{
person john ("John", "Doe", 33);
person jane ("Jane", "Doe", 32);
person joe ("Joe", "Dirt", 30);
transaction t (db->begin_transaction ());
db->persist (john);
db->persist (jane);
db->persist (joe);
t.commit ();
// Save object ids for later use.
//
john_id = john.id ();
jane_id = jane.id ();
joe_id = joe.id ();
}
}
catch (const odb::exception& e)
{
cerr << e.what () << endl;
return 1;
}
}
</pre>
<p>Let's examine this code piece by piece. At the beginnig we include
a bunch of headers. Those include <code>odb/database.hxx</code> and
<code>odb/transaction.hxx</code> which define database
system-independant <code>odb::database</code> and
<code>odb::transaction</code> interfaces. Then we include
<code>odb/mysql/database.hxx</code> which defines the
MySQL implementation of the <code>database</code> interface. Finaly,
we include <code>person.hxx</code> and <code>person-odb.hxx</code>
which define our persistent <code>person</code> class.</p>
<p>Once we are in <code>main()</code>, the first thing we do is create
the MySQL database object. Notice that this is the last line in
<code>driver.cxx</code> that mentions MySQL explicitly; the rest
of the code works though the common interfaces and is database
system-independant. We use the <code>argc</code>/<code>argv</code>
<code>mysql::database</code> constructor which automatically
extract the database parameters, such as login name, passowrd,
database name, etc., from the command line. In your own applications
you may prefer to use other versions of the <code>mysql::database</code>
constructors which allow you to pass this information directly
(@@ ref MySQL database).</p>
<p>Next we create three <code>person</code> objects. Right now they are
transient objects, which means that if we terminate the application
at this point, they will be gone without any evidence of them ever
existed. The next line starts a database transaction. We discuss
transactions in detail later in this manual. For now all we need
to know is that all ODB database operations must be performed within
a transaction and that a transaction is an atomic unit of work; all
database operations performed within a transaction either succeed
(commited) together or are automatically undone (rolled back).</p>
<p>Once we are in a transaction, we call the <code>persist()</code>
database function on each of our <code>person</code> objects.
At this point the state of each object is saved in the database.
However, note that this state is not permanent until and unless
the transaction is commited. If, for example, our application
crashes at this point, there will still be no evidence of our
objects ever existed.</p>
<p>In our case one more thing happens when we call <code>persist()</code>
on a <code>person</code> object. Remember that we decided to use
database-assigned identifiers for our objects. The call to
<code>persist()</code> is where this assignment happens. Once
this function returns, the <code>id_</code> member contains this
object's unique identifier.</p>
<p>After we have persisted our objects, it is time to commit the
transaction and make the changes permanent. Only after the
<code>commit()</code> function returns succefully are we
guaranteed that the objects are made persistent. Following
the crashing example, if our application terminates after
the commit for whatever reason, the objects' state in the
database will remain intact. In fact, as we will discover
shortly, our application can be restarted and load the
orignal objects from the database. Note also that a
transaction must be commited explicitly with the
<code>commit()</code> call. If the <code>transaction</code>
object leaves scope without the transaction beeing
explicitly commited or rolled back, it will be automatically
rolled back. This behavior allows you not to worry about
exceptions being thrown within a transaction; if they
cross the transaction boundaries, the transaction will
be automatically rolled back and all the changes made
to the database undone.</p>
<p>After the transaction has been commited, we save the persistent
objects' ids in local variables. We will use them later in this
chapter to perform other database operations on our persistent
objects. You might have noticed that our <code>person</code>
class doesn't have the <code>id()</code> function that we use
here. To make our code work we need to add a simple accessor
with this name that returns the value of the <code>id_</code>
data member.</p>
<p>The final bit of code in our example is the <code>catch</code>
block that handles the ODB exceptions. We do this by catching
the base ODB exception and printing the diagnostics. (@@ Ref
exceptions)</p>
<p>Let's now compile (see @@ Ref "Compiling and Running") and then
run our first ODB application:</p>
<pre class="terminal">
mysql --user=odb_test --database=odb_test < person.sql
./driver --user odb_test --database odb_test
</pre>
<p>Our first application doesn't print anything except for error
messages so we can't really tell whether it actually stored the
objects' state in the database. While we will extend our application
to be more enternaining, for now we can use the <code>mysql</code>
client to examine the database content. It will also give us a feel
for how the object are stored:</p>
<pre class="terminal">
mysql --user=odb_test --database=odb_test
Welcome to the MySQL monitor.
mysql> select * from person;
+----+-------+------+-----+
| id | first | last | age |
+----+-------+------+-----+
| 1 | John | Doe | 33 |
| 2 | Jane | Doe | 32 |
| 3 | Joe | Dirt | 30 |
+----+-------+------+-----+
3 rows in set (0.00 sec)
mysql> quit
</pre>
<p>In the next section we will examine how to query persistent objects
from our application.</p>
<h2><a name="2.4">2.4 Querying Persistent Objects</a></h2>
<p>So far our application doesn't resemble a typical "Hello World"
example. It doesn't print anything except for error messages.
Let's change that and teach our application to say hello to
people from our database. To make it a bit more interesting,
let's say hello only to people over 30:</p>
<pre class="c++">
// driver.cxx
//
...
int
main (int argc, char* argv[])
{
try
{
...
// Create a few persistent person objects.
//
{
...
}
typedef odb::query<person> query;
typedef odb::result<person> result;
// Say hello to those over 30.
//
{
transaction t (db->begin_transaction ());
result r (db->query<person> (query::age > 30));
for (result::iterator i (r.begin ()); i != r.end (); ++i)
{
cout << "Hello, " << i->first () << "!" << endl;
}
t.commit ();
}
}
catch (const odb::exception& e)
{
cerr << e.what () << endl;
return 1;
}
}
</pre>
<p>The first half of our application is the same as before and is
replaced with "..." in the above listing for brievety. Again, let's
examine the rest of it piece by piece.</p>
<p>The two <code>typedef</code>s create convenient aliases for two
template instantiations that will be used a lot in our application.
The first is the query type for the <code>person</code> objects
and the second is the result type of that query.</p>
<p>Then we begin a new transaction and call the <code>query()</code>
database function. We pass a query expression
(<code>query::age > 30</code>) which limits the returned objects
only to those with age greater than 30. We also save the result
of the query in a local variable.</p>
<p>The next few lines perform a pretty standard for-loop iteration
over the result sequence printing hello for every returned person.
Then we commit the transaction and we are node. Let's see what
this application will print:</p>
<pre class="terminal">
mysql --user=odb_test --database=odb_test < person.sql
./driver --user odb_test --database odb_test
Hello, John!
Hello, Jane!
</pre>
<p>That looks about right but how do we know that the query actually
used the database instead of just using some in-memory artifacts of
the earlier <code>persist()</code> calls. One way to test this
would be to comment out the first transaction in our application
and re-run it without re-creating the database schema so that the
objects that were persisted during the previous run will be returned.
Alternatively, we can just re-run the same application without
re-creating the schema and notice that we now how duplicate
objects:</p>
<pre class="terminal">
./driver --user odb_test --database odb_test
Hello, John!
Hello, Jane!
Hello, John!
Hello, Jane!
</pre>
<p>What happens here is that the previous run of our application
persisted a set of <code>person</code> objects and when we re-run
the application, we persist another set with the same names but
with different id. When we later run the query, matches from
both sets are returned. We can change the line where we print
the "Hello" string as follows to illustrate this point:</p>
<pre class="c++">
cout << "Hello, " << i->first () << " (" << i->id () << ")!" << endl;
</pre>
<p>If we now re-run this modified program, we will get the following
output:</p>
<pre class="terminal">
./driver --user odb_test --database odb_test
Hello, John (1)!
Hello, Jane (2)!
Hello, John (4)!
Hello, Jane (5)!
Hello, John (7)!
Hello, Jane (8)!
</pre>
<p>The identifiers 3, 6, and 9 that miss from the above list belong to
the "Joe Dirt" objects which are not selected by this query.</p>
<h2><a name="2.5">2.5 Updating Persistent Objects</a></h2>
<p>While making objects persistent and then querying them are
useful oprations, most applications will also need to change
the object's state and then make these changes persistent. Let's
illustrate this by updating Joe's age who just had a birthday:</p>
<pre class="c++">
// driver.cxx
//
...
int
main (int argc, char* argv[])
{
try
{
...
unsigned long john_id, jane_id, joe_id;
// Create a few persistent person objects.
//
{
...
// Save object ids for later use.
//
john_id = john.id ();
jane_id = jane.id ();
joe_id = joe.id ();
}
// Joe Dirt just had a birthday, so update his age.
//
{
transaction t (db->begin_transaction ());
auto_ptr<person> joe (db->load<person> (joe_id));
joe->age (joe->age () + 1);
db->store (*joe);
t.commit ();
}
// Say hello to those over 30.
//
{
...
}
}
catch (const odb::exception& e)
{
cerr << e.what () << endl;
return 1;
}
}
</pre>
<p>The beginning and the end of this transaction are the same as
the previous two. Once within a transaction, we call the
<code>load()</code> database function to instantiate a
<code>person</code> object with Joe's persistent state. We
pass Joe's object identifer that we stored earlier when we
made this object persistent.</p>
<p>With the instantiated object in hand we increment the age
and call the <code>store()</code> database function to update
the object's state in the database. Once the transaction is
commited, the changes are made permanent in the database.</p>
<p>If we now run this application, we will see Joe in the output
since he is now over 30:</p>
<pre class="terminal">
mysql --user=odb_test --database=odb_test < person.sql
./driver --user odb_test --database odb_test
Hello, John!
Hello, Jane!
Hello, Joe!
</pre>
<p>What if we didn't have an identifier for Joe? Maybe this object
was made persisted in another run of our application or by another
application altogether. Provided that we have only one Joe Dirt
in the database, we can use query to come up with an alternative
implementation of the above transaction:</p>
<pre class="c++">
// Joe Dirt just had a birthday, so update his age. An
// alternative implementation without using the object id.
//
{
transaction t (db->begin_transaction ());
result r (db->query<person> (query::first == "Joe" &&
query::last == "Dirt"));
result::iterator i (r.begin ());
if (i != r.end ())
{
auto_ptr<person> joe (*i);
joe->age (joe->age () + 1);
db->store (*joe);
}
t.commit ();
}
</pre>
<h2><a name="2.5">2.5 Deleting Persistent Objects</a></h2>
<p>The last operation that we will discuss in this chapter is deleting
the persistent object from the database. The following code
fragment shows how we can delete an object given its identifier:</p>
<pre class="c++">
// John Doe is no longer in our database.
//
{
transaction t (db->begin_transaction ());
db->erase<person> (john_id);
t.commit ();
}
</pre>
<p>To delete John from the database we start a transaction, call
the <code>erase()</code> database function with John's object
id, and commit the transaction. After the transaction is commited
the erased object is no longer persistent.</p>
<p>If we don't have an object id handy, we can use query to find and
delete the object:</p>
<pre class="c++">
// John Doe is no longer in our database. An alternative
// implementation without using the object id.
//
{
transaction t (db->begin_transaction ());
result r (db->query<person> (query::first == "John" &&
query::last == "Doe"));
result::iterator i (r.begin ());
if (i != r.end ())
{
auto_ptr<person> john (*i);
db->erase (*john);
}
t.commit ();
}
</pre>
<h2><a name="2.5">2.5 Summary</a></h2>
<p>This chapter presented a very simple application which, nevertheless,
excercised all core database functions: <code>persist()</code>,
<code>query()</code>, <code>load()</code>, <code>store()</code>,
and <code>erase()</code>. We also saw that writing an application
that uses ODB involves the following steps:</p>
<ol>
<li>Declare persistent classes in header files.</li>
<li>Compile these headers to generate database support code.</li>
<li>Link the application with the support code and two ODB runtime libraries.</li>
</ol>
<p>Do not be concerned if, at this point, much appears unclear. The intent
of this chapter is to give you only a general idea of how to persist C++
objects with ODB. We will cover all the details throughout the remainder
of this manual.</p>
<h1><a name="3">3 Working Title</a></h1>
<h2><a name="3.1">3.1 Base Concepts</a></h2>
<p>The term <em>database</em> can refer to three distinct things:
a general notion of a place where an application stores its data,
a software implementation for managing this data (for example
MySQL), and, finally, some database software implementations
may manage several data stores which are usually distinguished
by name. This name is also commonly referred to as database.</p>
<p>In this manual, when we use just the word <em>database</em>, we
refer to the first meaning above, for example,
"The <code>store()</code> function saves the object's state to
the database." The term Database Management System (DBMS) is
often used to refer to the second meaning of the words database.
In this manual we will use the term <em>database system</em>
for short, for example, "Database system-independant
application code." Finally, to distinguish the third meaning
from the other two we will use the term <em>database name</em>,
for example, "The second option specfies the database name
that the application should use to store its data."</p>
<p>In C++ there is only one notion of a type and an instance
of a type. For example, a fundamental type, such as <code>int</code>,
is, for the most part, treated the same as a user defined class
type. However, when it comes to persistence, we have to place
certain restrictions and requirements on certain C++ types that
can be stored in the database. As a result, we devide persistent
C++ types into two groups: <em>object types</em> and <em>value
types</em>. An stances of an object type is called an <em>object</em>
and an instance of a value type — a <em>value</em>.</p>
<p>An object is an independant entity. It can be stored, updated,
and deleted in the database independant of other objects or values.
An object has an identifier, called <em>object id</em>, that is
unique among all instances of an object type within a database.
An object consits of data members which are either values or
references to other objects. In contrast, a value can only be
stored in the database as part of an object and doesn't have
its own unique identifier.</p>
<p>An object type is a C++ class. Because of this one to one
relationship, we will use terms <em>object type</em>
and <em>object class</em> interchangably. In contrast,
a value type can be a fundamental C++ type, such as
<code>int</code> or a class type, such as <code>std::string</code>.
If a value consists of other values then is is called a
<em>composite value</em> and its type — a
<em>composite value type</em>. Otherwise the the value is
called <em>simple value</em> and its type — a
<em>simple value type</em>. Note that the distinction between
simple and composite values is conceptual rather than
representational. For example, <code>std::string</code>
is a simple value type because conceptually string is a
single value even though the representation of the string
class may contain several data member each of which would be
considered a value. In fact, the same value type can be
viewed (and mapped) as both simple and composite by different
applications.</p>
<p>Seeing how all these concepts map to the relational model
will hopefully make these distinctions more clear. In a relational
database an object type is mapped to a table and a value type is
mapped to one or more columns. A simple value type is mapped
to a single column while a composite value type is mapped to
several columns. Conversly, an object is stored as a row in this
table and a value is stored as one or more cells in this row.
A simple value is stored in a single cell while a composite
value occupies several cells.</p>
<p>Going back to the distinction beetween simple and composite
values, consider a date type which has three integer data
members: year, month, and day. In one application it can be
conidered a composite value and each member will get its
own column in the relational database. In another application
it can considered as a simple value and stored a single
column as a number of day from some predefined date.</p>
<p>Until now, we have been using the term <em>persistent class</em>
to refer to object classes. We will continue to do so even though
a value type can also be a class. The reason for this assimetry
is the subordinate nature of value types when it comes to
database operations. Remember that values are never stored
directly but rather as part of an object that contains them.
As a result, when we say that we want to make a C++ class
persistent or persist an instance of a class in the database,
we invariably refer to an object class rather than a value
class.</p>
<p>To make a C++ class a persistent object class we need to declare
it as such using the <code>db object</code> pragma:</p>
<pre class="c++">
#pragma db object
class person
{
...
};
</pre>
<p>The other pargma that we need to use is the <code>db id</code>
which designates one of the data members as an object id:</p>
<pre class="c++">
#pragma db object
class person
{
private:
#pragma db id
unsigned long id_;
};
</pre>
<p>These two pragmas are the minimum required to declare a
persistent class. Other pragmas can be used to fine-tune
the persistence-related properties of a class and its
members.</p>
<p>You may be wondering whether we aslo have to do declare value types
as persistent. We don't need to do anything special for simple value
types such as <code>int</code> or <code>std::string</code> since the
ODB compiler knows how to map them to the database system types and
how to convert between the two. On the other hand, if a simple value
is unknown to the ODB compiler then you will need to provide the
mapping to the database system type and, possibly, the code to
convert between the two. For more information on this see @@ Ref
Custom value types/pragma value type. Composite value types are
not yet supported by ODB.</p>
<p>Normally, you would use object types to model real-world entities,
things that have their own identity. For example, in the
previous chapter we created a <code>person</code> class to model
a person which is a real-world enitity. Name and age, which we
used as data members in our <code>person</code> class are clearly
values. It is hard to think of age 31 or name "Joe" as having their
own identity.</p>
<p>A good test to determine whether something is an object or
a value is to consider if other objects might reference
it. A person is clearly an object because it can be refered
to by other object's such as a spouce, an employer, or a
bank. On the other hand, a person's age or name is not
something that other objects would normally refer to.</p>
<p>Also, when an object represents a real entity, it is easy to
choose a suitable object identifier. For example, for a
person there is an established notion of an identifier
(SSN, student id, passport number, etc). Another alternative
is to use person't email address as an identifier.</p>
<p>Note, however, that these are only guidelines. There could
be goot reasons to make something that would normally be
a value an object. Consider, for example, a database that
stores a vast number of people. Many of the person objects
in this database have the same names and surnames and the
overhead of repeating them in every object may negatively
affect the performance. In this case we could make first name
and last name each an object and only store references to
these objects in the <code>person</code> class.</p>
<p>An instance of a persistent class can be in one of two states:
<em>transient</em> and <em>persistent</em>. A transient
instance only has a representation in the applciation's
memory and will ceas to exist when the application terminates
unless it is explicitly made persistent. A persistent instance
has a representation in both the application's memory and the
database. A persistent instance will remain even after the
application terminates unless and until it is explicitly
deleted from the database. In other words, a transient instance
of a persistent class behaves just like an instance of any
ordinary C++ class.</p>
<h2><a name="3.2">3.2 Transactions and Concurrency</a></h2>
<p>A transaction is an atomic, consistent, isolated and durable
(ACID) unit of work. All database operations can only be
performed within a transaction and each thread of execution
in an application can have only one active transaction at a
time.</p>
<p>By atomicity we mean that when it comes to making changes to
the database state within a transaction,
either all the changes succeed or none at all. Consider,
for example, a transaction that transfers funds between two
objects representing bank accounts. If the debit function
on the first object succeeds but the credit function on
the second fails, the transaction is rolled back and the
database state of the first object remains unchanged.</p>
<p>By consistency we mean that a transaction must take all the
objects stored in the database from one consistent state
to another. For example, if a bank account object must
reference a person object as its owner and we forget to
set this reference before making the object persistent,
the transaction will be rolled back and the database
will remain unchanged.</p>
<p>By isolation we mean that the changes made to the database
state during a transaction are only visible inside this
transaction until and unless it is commited. Using the
above example with bank transfer, the results of the
debit operation performed on the first object is not
visible to other transactions until the credit operation
is successfully completed and the transaction is commited.</p>
<p>By durability we mean that once the transaction is committed,
the changes that it made to the database state are permanent
and will survive failures such as an application crash. From
now the only way to alter this state is to execute and commit
another transaction.</p>
<p>Note that all of the above guarantees only apply to the
object's state in the database as opposed to the object's
state in the application's memory. It is possible to roll
a transaction back but still have changes from this
transaction in the application's memory. An easy way to
avoid this potentiall inconsistency is to instantiate
persistent objects withing the transaction's scope. Consider,
for example, this two implementations of the same transaction:</p>
<pre class="c++">
void
update_age (database& db, person& p)
{
transaction t (db.begin_transaction ());
p.age (p.age () + 1);
db.store (p);
t.commit ()
}
</pre>
<p>In the above implementation, if the <code>store()</code> call fails
and the transaction is rolled back, the state of the person
object in the database and the state of the same object in the
application's memory will differ. Now consider an
alternative implementation which only instantiates the
person object for the duration of the transaction:</p>
<pre class="c++">
void
update_age (database& db, unsigned long id)
{
transaction t (db.begin_transaction ());
auto_ptr<person> p (db.load<person> (id));
p.age (p.age () + 1);
db.store (p);
t.commit ()
}
</pre>
<p>Of course, it may be not always be possible to write the
application in this style. Oftentimes we need to access and
modify application's state of persistent objects out of
transactions. In this case it may make sense to try to
roll back the changes made to the application state if
the transaction was rolled back and the database state
remains unchanged. One way to do this is to re-load
the object's state from the database:</p>
<pre class="c++">
void
update_age (database& db, person& p)
{
try
{
transaction t (db.begin_transaction ());
p.age (p.age () + 1);
db.store (p);
t.commit ()
}
catch (...)
{
transaction t (db.begin_transaction ());
db.load (p.id (), p);
t.commit ();
throw;
}
}
</pre>
<p>A transaction is started by calling the <code>begin_transaction()</code>
database function. The returned transaction handle is stored in
an instance of the <code>odb::transaction</code> class which has
the following interface:</p>
<pre class="c++">
namespace odb
{
class transaction
{
public:
typedef odb::database database_type;
void
commit ();
void
rollback ();
database_type&
database ();
static transaction&
current ();
static bool
has_current ();
};
}
</pre>
<p>The <code>commit()</code> function commits a transaction and
<code>rollback()</code> rolls it back. Unless the transaction
has been <em>finalized</em>, (explicitly commited or rolled back),
the destructor of the <code>odb::transaction</code> class will
automatically roll it back when the transaction instance goes
out of scope.</p>
<p>The <code>database()</code> function returns the database this
transaction is working on. The <code>current()</code> static
function returns the currently active transaction for this
thread. If there is no active transaction, this function
throws the <code>odb::not_in_transaction</code> exception.
You can check whether there is a transaction in effect using
the <code>has_current()</code> static function.</p>
<p>If two or more transaction access or modify more than one object
and are executed concurrently by different applications or by
different threads within the same application, then it is possible
that these transactions will try to access objects in an incompatible
order and deadlock. The canonical example of a deadlock are
two transactions in which the first has modified <code>object1</code>
and is waiting for the second transaction to commit its changes to
<code>object2</code> so that it can update <code>object2</code>. At
the same time the second transaction has modified <code>object2</code>
and is waiting for the first transaction to commit its changes to
<code>object1</code> because it also needs to modify <code>object1</code>.
As a result none of the two transactions can complete.</p>
<p>The database system detects such situations and automatically
aborts the waiting operation in one of the deadlocked transactions.
In ODB this translates to the <code>odb::deadlock</code> exception
being thrown from one of the database functions. You would normally
handle a deadlock by restarting the transaction, for example:</p>
<pre class="c++">
for (;;)
{
try
{
transaction t (db.begin_transaction ());
...
t.commit ()
break;
}
catch (const odb::deadlock&)
{
continue;
}
}
</pre>
</div>
</div>
</body>
</html>
|
