-
Notifications
You must be signed in to change notification settings - Fork 924
/
basic_mapping.xml
2367 lines (2171 loc) · 120 KB
/
basic_mapping.xml
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
<chapter id="mapping">
<title>Basic O/R Mapping</title>
<sect1 id="mapping-declaration">
<title>Mapping declaration</title>
<para>
Object/relational mappings are defined in an XML document. The mapping document
is designed to be readable and hand-editable. The mapping language is object-centric,
meaning that mappings are constructed around persistent class declarations, not
table declarations.
</para>
<para>
Note that, even though many NHibernate users choose to define XML mappings by hand,
a number of tools exist to generate the mapping document, including
NHibernate.Mapping.Attributes library and various template-based code
generators (CodeSmith, MyGeneration).
</para>
<para>
Let's kick off with an example mapping:
</para>
<programlisting><![CDATA[<?xml version="1.0"?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2" assembly="Eg"
namespace="Eg">
<class name="Cat" table="CATS" discriminator-value="C">
<id name="Id" column="uid" type="Int64">
<generator class="hilo"/>
</id>
<discriminator column="subclass" type="Char"/>
<property name="BirthDate" type="Date"/>
<property name="Color" not-null="true"/>
<property name="Sex" not-null="true" update="false"/>
<property name="Weight"/>
<many-to-one name="Mate" column="mate_id"/>
<set name="Kittens">
<key column="mother_id"/>
<one-to-many class="Cat"/>
</set>
<subclass name="DomesticCat" discriminator-value="D">
<property name="Name" type="String"/>
</subclass>
</class>
<class name="Dog">
<!-- mapping for Dog could go here -->
</class>
</hibernate-mapping>]]></programlisting>
<para>
We will now discuss the content of the mapping document. We will only describe the
document elements and attributes that are used by NHibernate at runtime. The mapping
document also contains some extra optional attributes and elements that affect the
database schemas exported by the schema export tool. (For example the <literal>
not-null</literal> attribute.)
</para>
<sect2 id="mapping-declaration-xmlns">
<title>XML Namespace</title>
<para>
All XML mappings should declare the XML namespace shown. The actual schema definition
may be found in the <literal>src\nhibernate-mapping-2.2.xsd</literal> file in the
NHibernate distribution.
</para>
<para><emphasis>
Tip: to enable IntelliSense for mapping and configuration files, copy the appropriate
<literal>.xsd</literal> files to
<literal><VS.NET installation directory>\Common7\Packages\schemas\xml</literal>.
</emphasis></para>
</sect2>
<sect2 id="mapping-declaration-mapping">
<title>hibernate-mapping</title>
<para>
This element has several optional attributes. The <literal>schema</literal> attribute
specifies that tables referred to by this mapping belong to the named schema. If specified,
tablenames will be qualified by the given schema name. If missing, tablenames will be
unqualified. The <literal>default-cascade</literal> attribute specifies what cascade style
should be assumed for properties and collections which do not specify a
<literal>cascade</literal> attribute. The <literal>auto-import</literal> attribute lets us
use unqualified class names in the query language, by default. The <literal>assembly</literal>
and <literal>namespace</literal> attributes specify the assembly where persistent classes
are located and the namespace they are declared in.
</para>
<programlistingco>
<areaspec>
<area id="hm1" coords="2 55"/>
<area id="hm2" coords="3 55"/>
<area id="hm3" coords="4 55"/>
<area id="hm4" coords="5 55"/>
<area id="hm5" coords="6 55"/>
</areaspec>
<programlisting><![CDATA[<hibernate-mapping
schema="schemaName"
default-cascade="none|save-update"
auto-import="true|false"
assembly="Eg"
namespace="Eg"
/>]]></programlisting>
<calloutlist>
<callout arearefs="hm1">
<para>
<literal>schema</literal> (optional): The name of a database schema.
</para>
</callout>
<callout arearefs="hm2">
<para>
<literal>default-cascade</literal> (optional - defaults to <literal>none</literal>):
A default cascade style.
</para>
</callout>
<callout arearefs="hm3">
<para>
<literal>auto-import</literal> (optional - defaults to <literal>true</literal>):
Specifies whether we can use unqualified class names (of classes in this mapping)
in the query language.
</para>
</callout>
<callout arearefs="hm4 hm5">
<para>
<literal>assembly</literal> and <literal>namespace</literal>(optional): Specify
assembly and namespace to assume for unqualified class names in the mapping
document.
</para>
</callout>
</calloutlist>
</programlistingco>
<para>
If you are not using <literal>assembly</literal> and <literal>namespace</literal>
attributes, you have to specify fully-qualified class names, including the name
of the assembly that classes are declared in.
</para>
<para>
If you have two persistent classes with the same (unqualified) name, you should set
<literal>auto-import="false"</literal>. NHibernate will throw an exception if you attempt
to assign two classes to the same "imported" name.
</para>
</sect2>
<sect2 id="mapping-declaration-class">
<title>class</title>
<para>
You may declare a persistent class using the <literal>class</literal> element:
</para>
<programlistingco>
<areaspec>
<area id="class1" coords="2 55"/>
<area id="class2" coords="3 55" />
<area id="class3" coords="4 55"/>
<area id="class4" coords="5 55" />
<area id="class5" coords="6 55"/>
<area id="class6" coords="7 55" />
<area id="class7" coords="8 55"/>
<area id="class8" coords="9 55" />
<area id="class9" coords="10 55" />
<area id="class10" coords="11 55"/>
<area id="class11" coords="12 55"/>
<area id="class12" coords="13 55"/>
<area id="class13" coords="14 55"/>
<area id="class14" coords="15 55"/>
<area id="class15" coords="16 55"/>
</areaspec>
<programlisting><![CDATA[<class
name="ClassName"
table="tableName"
discriminator-value="discriminator_value"
mutable="true|false"
schema="owner"
proxy="ProxyInterface"
dynamic-update="true|false"
dynamic-insert="true|false"
select-before-update="true|false"
polymorphism="implicit|explicit"
where="arbitrary sql where condition"
persister="PersisterClass"
batch-size="N"
optimistic-lock="none|version|dirty|all"
lazy="true|false"
/>]]></programlisting>
<calloutlist>
<callout arearefs="class1">
<para>
<literal>name</literal>: The fully qualified .NET class name of the persistent class
(or interface), including its assembly name.
</para>
</callout>
<callout arearefs="class2">
<para>
<literal>table</literal>: The name of its database table.
</para>
</callout>
<callout arearefs="class3">
<para>
<literal>discriminator-value</literal> (optional - defaults to the class name): A value
that distiguishes individual subclasses, used for polymorphic behaviour. Acceptable
values include <literal>null</literal> and <literal>not null</literal>.
</para>
</callout>
<callout arearefs="class4">
<para>
<literal>mutable</literal> (optional, defaults to <literal>true</literal>): Specifies
that instances of the class are (not) mutable.
</para>
</callout>
<callout arearefs="class5">
<para>
<literal>schema</literal> (optional): Override the schema name specified by
the root <literal><hibernate-mapping></literal> element.
</para>
</callout>
<callout arearefs="class6">
<para>
<literal>proxy</literal> (optional): Specifies an interface to use for lazy
initializing proxies. You may specify the name of the class itself.
</para>
</callout>
<callout arearefs="class7">
<para>
<literal>dynamic-update</literal> (optional, defaults to <literal>false</literal>):
Specifies that <literal>UPDATE</literal> SQL should be generated at runtime and
contain only those columns whose values have changed.
</para>
</callout>
<callout arearefs="class8">
<para>
<literal>dynamic-insert</literal> (optional, defaults to <literal>false</literal>):
Specifies that <literal>INSERT</literal> SQL should be generated at runtime and
contain only the columns whose values are not null.
</para>
</callout>
<callout arearefs="class9">
<para>
<literal>select-before-update</literal> (optional, defaults to <literal>false</literal>):
Specifies that NHibernate should <emphasis>never</emphasis> perform an SQL <literal>UPDATE</literal>
unless it is certain that an object is actually modified. In certain cases (actually, only
when a transient object has been associated with a new session using <literal>update()</literal>),
this means that NHibernate will perform an extra SQL <literal>SELECT</literal> to determine
if an <literal>UPDATE</literal> is actually required.
</para>
</callout>
<callout arearefs="class10">
<para>
<literal>polymorphism</literal> (optional, defaults to <literal>implicit</literal>):
Determines whether implicit or explicit query polymorphism is used.
</para>
</callout>
<callout arearefs="class11">
<para>
<literal>where</literal> (optional) specify an arbitrary SQL <literal>WHERE</literal>
condition to be used when retrieving objects of this class
</para>
</callout>
<callout arearefs="class12">
<para>
<literal>persister</literal> (optional): Specifies a custom <literal>IClassPersister</literal>.
</para>
</callout>
<callout arearefs="class13">
<para>
<literal>batch-size</literal> (optional, defaults to <literal>1</literal>) specify a "batch size"
for fetching instances of this class by identifier.
</para>
</callout>
<callout arearefs="class14">
<para>
<literal>optimistic-lock</literal> (optional, defaults to <literal>version</literal>):
Determines the optimistic locking strategy.
</para>
</callout>
<callout arearefs="class15">
<para>
<literal>lazy</literal> (optional): Lazy fetching may be completely disabled by setting
<literal>lazy="false"</literal>.
</para>
</callout>
</calloutlist>
</programlistingco>
<para>
It is perfectly acceptable for the named persistent class to be an interface. You would then
declare implementing classes of that interface using the <literal><subclass></literal>
element. You may persist any inner class. You should specify the
class name using the standard form ie. <literal>Eg.Foo+Bar, Eg</literal>.
Due to an HQL parser limitation inner classes can not be used in queries in NHibernate 1.0.
</para>
<para>
Immutable classes, <literal>mutable="false"</literal>, may not be updated or deleted by the
application. This allows NHibernate to make some minor performance optimizations.
</para>
<para>
The optional <literal>proxy</literal> attribute enables lazy initialization of persistent
instances of the class. NHibernate will initially return proxies which implement
the named interface. The actual persistent object will be loaded when a method of the
proxy is invoked. See "Proxies for Lazy Initialization" below.
</para>
<para><emphasis>Implicit</emphasis> polymorphism means that instances of the class will be returned
by a query that names any superclass or implemented interface or the class and that instances
of any subclass of the class will be returned by a query that names the class itself.
<emphasis>Explicit</emphasis> polymorphism means that class instances will be returned only
be queries that explicitly name that class and that queries that name the class will return
only instances of subclasses mapped inside this <literal><class></literal> declaration
as a <literal><subclass></literal> or <literal><joined-subclass></literal>. For
most purposes the default, <literal>polymorphism="implicit"</literal>, is appropriate.
Explicit polymorphism is useful when two different classes are mapped to the same table
(this allows a "lightweight" class that contains a subset of the table columns).
</para>
<para>
The <literal>persister</literal> attribute lets you customize the persistence strategy used for
the class. You may, for example, specify your own subclass of
<literal>NHibernate.Persister.EntityPersister</literal> or you might even provide a
completely new implementation of the interface
<literal>NHibernate.Persister.IClassPersister</literal> that implements persistence via,
for example, stored procedure calls, serialization to flat files or LDAP. See
<literal>NHibernate.DomainModel.CustomPersister</literal> for a simple example (of "persistence"
to a <literal>Hashtable</literal>).
</para>
<para>
Note that the <literal>dynamic-update</literal> and <literal>dynamic-insert</literal>
settings are not inherited by subclasses and so may also be specified on the
<literal><subclass></literal> or <literal><joined-subclass></literal> elements.
These settings may increase performance in some cases, but might actually decrease
performance in others. Use judiciously.
</para>
<para>
Use of <literal>select-before-update</literal> will usually decrease performance. It is very
useful to prevent a database update trigger being called unnecessarily.
</para>
<para>
If you enable <literal>dynamic-update</literal>, you will have a choice of optimistic
locking strategies:
</para>
<itemizedlist>
<listitem>
<para>
<literal>version</literal> check the version/timestamp columns
</para>
</listitem>
<listitem>
<para>
<literal>all</literal> check all columns
</para>
</listitem>
<listitem>
<para>
<literal>dirty</literal> check the changed columns
</para>
</listitem>
<listitem>
<para>
<literal>none</literal> do not use optimistic locking
</para>
</listitem>
</itemizedlist>
<para>
We <emphasis>very</emphasis> strongly recommend that you use version/timestamp
columns for optimistic locking with NHibernate. This is the optimal strategy with
respect to performance and is the only strategy that correctly handles modifications
made outside of the session (ie. when <literal>ISession.Update()</literal> is used).
Keep in mind that a version or timestamp property should never be null, no matter
what <literal>unsaved-value</literal> strategy, or an instance will be detected as
transient.
</para>
<para>
Beginning with NHibernate 1.2.0, version numbers start with 1, not 0 as in previous
versions. This was done to allow using 0 as <literal>unsaved-value</literal> for the
version property.
</para>
</sect2>
<sect2 id="mapping-declaration-id">
<title>id</title>
<para>
Mapped classes <emphasis>must</emphasis> declare the primary key column of the database
table. Most classes will also have a property holding the unique identifier
of an instance. The <literal><id></literal> element defines the mapping from that
property to the primary key column.
</para>
<programlistingco>
<areaspec>
<area id="id1" coords="2 50" />
<area id="id2" coords="3 50" />
<area id="id3" coords="4 50" />
<area id="id4" coords="5 50" />
<area id="id5" coords="6 50" />
</areaspec>
<programlisting><![CDATA[<id
name="PropertyName"
type="typename"
column="column_name"
unsaved-value="any|none|null|id_value"
access="field|property|nosetter|ClassName">
<generator class="generatorClass"/>
</id>]]></programlisting>
<calloutlist>
<callout arearefs="id1">
<para>
<literal>name</literal> (optional): The name of the identifier property.
</para>
</callout>
<callout arearefs="id2">
<para>
<literal>type</literal> (optional): A name that indicates the NHibernate type.
</para>
</callout>
<callout arearefs="id3">
<para>
<literal>column</literal> (optional - defaults to the property name): The
name of the primary key column.
</para>
</callout>
<callout arearefs="id4">
<para>
<literal>unsaved-value</literal> (optional - defaults to a "sensible" value):
An identifier property value that indicates that an instance is newly instantiated
(unsaved), distinguishing it from transient instances that were saved or loaded
in a previous session.
</para>
</callout>
<callout arearefs="id5">
<para>
<literal>access</literal> (optional - defaults to <literal>property</literal>): The
strategy NHibernate should use for accessing the property value.
</para>
</callout>
</calloutlist>
</programlistingco>
<para>
If the <literal>name</literal> attribute is missing, it is assumed that the class has no
identifier property.
</para>
<para>
The <literal>unsaved-value</literal> attribute is almost never needed in NHibernate 1.0.
</para>
<para>
There is an alternative <literal><composite-id></literal> declaration to allow access to
legacy data with composite keys. We strongly discourage its use for anything else.
</para>
<sect3 id="mapping-declaration-id-generator">
<title>generator</title>
<para>
The required <literal><generator></literal> child element names a .NET class used
to generate unique identifiers for instances of the persistent class. If any parameters
are required to configure or initialize the generator instance, they are passed using the
<literal><param></literal> element.
</para>
<programlisting><![CDATA[<id name="Id" type="Int64" column="uid" unsaved-value="0">
<generator class="NHibernate.Id.TableHiLoGenerator">
<param name="table">uid_table</param>
<param name="column">next_hi_value_column</param>
</generator>
</id>]]></programlisting>
<para>
All generators implement the interface <literal>NHibernate.Id.IIdentifierGenerator</literal>.
This is a very simple interface; some applications may choose to provide their own specialized
implementations. However, NHibernate provides a range of built-in implementations. There are shortcut
names for the built-in generators:
<variablelist>
<varlistentry>
<term><literal>increment</literal></term>
<listitem>
<para>
generates identifiers of any integral type that are unique only when
no other process is inserting data into the same table.
<emphasis>Do not use in a cluster.</emphasis>
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>identity</literal></term>
<listitem>
<para>
supports identity columns in DB2, MySQL, MS SQL Server and Sybase. The identifier
returned by the database is converted to the property type using <literal>
Convert.ChangeType</literal>. Any integral property type is thus supported.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>sequence</literal></term>
<listitem>
<para>
uses a sequence in DB2, PostgreSQL, Oracle or a generator
in Firebird. The identifier returned by the database is converted to the property
type using <literal>Convert.ChangeType</literal>. Any integral property type is
thus supported.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>hilo</literal></term>
<listitem>
<para id="mapping-declaration-id-hilodescription" revision="1">
uses a hi/lo algorithm to efficiently generate identifiers of any integral type,
given a table and column (by default <literal>hibernate_unique_key</literal> and
<literal>next_hi</literal> respectively) as a source of hi values. The hi/lo algorithm
generates identifiers that are unique only for a particular database. <emphasis>Do not
use this generator with a user-supplied connection.</emphasis>
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>seqhilo</literal></term>
<listitem>
<para>
uses a hi/lo algorithm to efficiently generate identifiers of any integral type,
given a named database sequence.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>uuid.hex</literal></term>
<listitem>
<para>
uses <literal>System.Guid</literal> and its <literal>ToString(string format)</literal> method
to generate identifiers of type string. The length of the string returned depends on the
configured <literal>format</literal>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>uuid.string</literal></term>
<listitem>
<para>
uses a new <literal>System.Guid</literal> to create a <literal>byte[]</literal> that is
converted to a string.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>guid</literal></term>
<listitem>
<para>
uses a new <literal>System.Guid</literal> as the identifier.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>guid.comb</literal></term>
<listitem>
<para>
uses the algorithm to generate a new <literal>System.Guid</literal>
described by Jimmy Nilsson in the article
http://www.informit.com/articles/article.asp?p=25862.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>native</literal></term>
<listitem>
<para>
picks <literal>identity</literal>, <literal>sequence</literal> or
<literal>hilo</literal> depending upon the capabilities of the
underlying database.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>assigned</literal></term>
<listitem>
<para>
lets the application to assign an identifier to the object before
<literal>Save()</literal> is called.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>foreign</literal></term>
<listitem>
<para>
uses the identifier of another associated object. Usually used in conjunction
with a <literal><one-to-one></literal> primary key association.
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
</sect3>
<sect3 id="mapping-declaration-id-hilo">
<title>Hi/Lo Algorithm</title>
<para>
The <literal>hilo</literal> and <literal>seqhilo</literal> generators provide two alternate
implementations of the hi/lo algorithm, a favorite approach to identifier generation. The
first implementation requires a "special" database table to hold the next available "hi" value.
The second uses an Oracle-style sequence (where supported).
</para>
<programlisting><![CDATA[<id name="Id" type="Int64" column="cat_id">
<generator class="hilo">
<param name="table">hi_value</param>
<param name="column">next_value</param>
<param name="max_lo">100</param>
</generator>
</id>]]></programlisting>
<programlisting><![CDATA[<id name="Id" type="Int64" column="cat_id">
<generator class="seqhilo">
<param name="sequence">hi_value</param>
<param name="max_lo">100</param>
</generator>
</id>]]></programlisting>
<para>
Unfortunately, you can't use <literal>hilo</literal> when supplying your own
<literal>IDbConnection</literal> to NHibernate. NHibernate must be able to
fetch the "hi" value in a new transaction.
</para>
</sect3>
<sect3 id="mapping-declaration-id-uuid-hex">
<title>UUID Hex Algorithm</title>
<programlisting><![CDATA[<id name="Id" type="String" column="cat_id">
<generator class="uuid.hex">
<param name="format">format_value</param>
<param name="seperator">seperator_value</param>
</generator>
</id>]]></programlisting>
<para>
The UUID is generated by calling <literal>Guid.NewGuid().ToString(format)</literal>.
The valid values for <literal>format</literal> are described in the MSDN documentation.
The default <literal>seperator</literal> is <literal>-</literal> and should rarely be
modified. The <literal>format</literal> determines if the configured
<literal>seperator</literal> can replace the default seperator used by
the <literal>format</literal>.
</para>
</sect3>
<sect3 id="mapping-declaration-id-uuid-string">
<title>UUID String Algorithm</title>
<para>
The UUID is generated by calling <literal>Guid.NewGuid().ToByteArray()</literal> and
then converting the <literal>byte[]</literal> into a <literal>char[]</literal>. The
<literal>char[]</literal> is returned as a <literal>String</literal> consisting of
16 characters.
</para>
</sect3>
<sect3 id="mapping-declaration-id-guid">
<title>GUID Algorithms</title>
<para>
The <literal>guid</literal> identifier is generated by calling <literal>Guid.NewGuid()</literal>.
To address some of the performance concerns with using Guids as primary keys, foreign keys, and
as part of indexes with MS SQL the <literal>guid.comb</literal> can be used. The benefit of using
the <literal>guid.comb</literal> with other databases that support GUIDs has not been measured.
</para>
</sect3>
<sect3 id="mapping-declaration-id-sequences">
<title>Identity columns and Sequences</title>
<para>
For databases which support identity columns (DB2, MySQL, Sybase, MS SQL), you
may use <literal>identity</literal> key generation. For databases that support
sequences (DB2, Oracle, PostgreSQL, Interbase, McKoi, SAP DB) you may use
<literal>sequence</literal> style key generation. Both these strategies require
two SQL queries to insert a new object.
</para>
<programlisting><![CDATA[<id name="Id" type="Int64" column="uid">
<generator class="sequence">
<param name="sequence">uid_sequence</param>
</generator>
</id>]]></programlisting>
<programlisting><![CDATA[<id name="Id" type="Int64" column="uid" unsaved-value="0">
<generator class="identity"/>
</id>]]></programlisting>
<para>
For cross-platform development, the <literal>native</literal> strategy will
choose from the <literal>identity</literal>, <literal>sequence</literal> and
<literal>hilo</literal> strategies, dependent upon the capabilities of the
underlying database.
</para>
</sect3>
<sect3 id="mapping-declaration-id-assigned">
<title>Assigned Identifiers</title>
<para>
If you want the application to assign identifiers (as opposed to having
NHibernate generate them), you may use the <literal>assigned</literal> generator.
This special generator will use the identifier value already assigned to the
object's identifier property. Be very careful when using this feature to assign
keys with business meaning (almost always a terrible design decision).
</para>
<para>
Due to its inherent nature, entities that use this generator cannot be saved
via the ISession's SaveOrUpdate() method. Instead you have to explicitly specify to
NHibernate if the object should be saved or updated by calling either the
<literal>Save()</literal> or <literal>Update()</literal> method of the ISession.
</para>
</sect3>
</sect2>
<sect2 id="mapping-declaration-compositeid">
<title>composite-id</title>
<programlisting><![CDATA[<composite-id
name="PropertyName"
class="ClassName"
unsaved-value="any|none"
access="field|property|nosetter|ClassName">
<key-property name="PropertyName" type="typename" column="column_name"/>
<key-many-to-one name="PropertyName class="ClassName" column="column_name"/>
......
</composite-id>]]></programlisting>
<para>
For a table with a composite key, you may map multiple properties of the class
as identifier properties. The <literal><composite-id></literal> element
accepts <literal><key-property></literal> property mappings and
<literal><key-many-to-one></literal> mappings as child elements.
</para>
<programlisting><![CDATA[<composite-id>
<key-property name="MedicareNumber"/>
<key-property name="Dependent"/>
</composite-id>]]></programlisting>
<para>
Your persistent class <emphasis>must</emphasis> override <literal>Equals()</literal>
and <literal>GetHashCode()</literal> to implement composite identifier equality. It must
also be <literal>Serializable</literal>.
</para>
<para>
Unfortunately, this approach to composite identifiers means that a persistent object
is its own identifier. There is no convenient "handle" other than the object itself.
You must instantiate an instance of the persistent class itself and populate its
identifier properties before you can <literal>load()</literal> the persistent state
associated with a composite key. We will describe a much more
convenient approach where the composite identifier is implemented as a seperate class
in <xref linkend="components-compositeid"/>. The attributes described below apply only
to this alternative approach:
</para>
<itemizedlist spacing="compact">
<listitem>
<para>
<literal>name</literal> (optional, required for this approach): A property of
component type that holds the composite identifier (see next section).
</para>
</listitem>
<listitem>
<para>
<literal>access</literal> (optional - defaults to <literal>property</literal>):
The strategy NHibernate should use for accessing the property value.
</para>
</listitem>
<listitem>
<para>
<literal>class</literal> (optional - defaults to the property type determined by
reflection): The component class used as a composite identifier (see next section).
</para>
</listitem>
</itemizedlist>
</sect2>
<sect2 id="mapping-declaration-discriminator" revision="1">
<title>discriminator</title>
<para>
The <literal><discriminator></literal> element is required for polymorphic persistence
using the table-per-class-hierarchy mapping strategy and declares a discriminator column of the
table. The discriminator column contains marker values that tell the persistence layer what
subclass to instantiate for a particular row. A restricted set of types may be used:
<literal>String</literal>, <literal>Char</literal>, <literal>Int32</literal>,
<literal>Byte</literal>, <literal>Short</literal>, <literal>Boolean</literal>,
<literal>YesNo</literal>, <literal>TrueFalse</literal>.
</para>
<programlistingco>
<areaspec>
<area id="discriminator1" coords="2 40"/>
<area id="discriminator2" coords="3 40" />
<area id="discriminator3" coords="4 40" />
<area id="discriminator4" coords="5 40" />
</areaspec>
<programlisting><![CDATA[<discriminator
column="discriminator_column"
type="discriminator_type"
force="true|false"
insert="true|false"
/>]]></programlisting>
<calloutlist>
<callout arearefs="discriminator1">
<para>
<literal>column</literal> (optional - defaults to <literal>class</literal>) the
name of the discriminator column.
</para>
</callout>
<callout arearefs="discriminator2">
<para>
<literal>type</literal> (optional - defaults to <literal>String</literal>) a
name that indicates the NHibernate type
</para>
</callout>
<callout arearefs="discriminator3">
<para>
<literal>force</literal> (optional - defaults to <literal>false</literal>)
"force" NHibernate to specify allowed discriminator values even when retrieving
all instances of the root class.
</para>
</callout>
<callout arearefs="discriminator4">
<para>
<literal>insert</literal> (optional - defaults to <literal>true</literal>)
set this to <literal>false</literal> if your discriminator column is also part
of a mapped composite identifier.
</para>
</callout>
</calloutlist>
</programlistingco>
<para>
Actual values of the discriminator column are specified by the
<literal>discriminator-value</literal> attribute of the <literal><class></literal> and
<literal><subclass></literal> elements.
</para>
<para>
The <literal>force</literal> attribute is (only) useful if the table contains rows with
"extra" discriminator values that are not mapped to a persistent class. This will not
usually be the case.
</para>
</sect2>
<sect2 id="mapping-declaration-version">
<title>version (optional)</title>
<para>
The <literal><version></literal> element is optional and indicates that
the table contains versioned data. This is particularly useful if you plan to
use <emphasis>long transactions</emphasis> (see below).
</para>
<programlistingco>
<areaspec>
<area id="version1" coords="2 60"/>
<area id="version2" coords="3 60"/>
<area id="version3" coords="4 60"/>
<area id="version4" coords="5 60"/>
<area id="version5" coords="6 60"/>
</areaspec>
<programlisting><![CDATA[<version
column="version_column"
name="PropertyName"
type="typename"
access="field|property|nosetter|ClassName"
unsaved-value="null|negative|undefined|value"
/>]]></programlisting>
<calloutlist>
<callout arearefs="version1">
<para>
<literal>column</literal> (optional - defaults to the property name): The name
of the column holding the version number.
</para>
</callout>
<callout arearefs="version2">
<para>
<literal>name</literal>: The name of a property of the persistent class.
</para>
</callout>
<callout arearefs="version3">
<para>
<literal>type</literal> (optional - defaults to <literal>Int32</literal>):
The type of the version number.
</para>
</callout>
<callout arearefs="version4">
<para>
<literal>access</literal> (optional - defaults to <literal>property</literal>): The
strategy NHibernate should use for accessing the property value.
</para>
</callout>
<callout arearefs="version5">
<para>
<literal>unsaved-value</literal> (optional - defaults to a "sensible" value):
A version property value that indicates that an instance is newly instantiated
(unsaved), distinguishing it from transient instances that were saved or loaded
in a previous session. (<literal>undefined</literal> specifies that the identifier
property value should be used.)
</para>
</callout>
</calloutlist>
</programlistingco>
<para>
Version numbers may be of type <literal>Int64</literal>, <literal>Int32</literal>,
<literal>Int16</literal>, <literal>Ticks</literal>, <literal>Timestamp</literal>,
or <literal>TimeSpan</literal> (or their nullable counterparts in .NET 2.0).
</para>
</sect2>
<sect2 id="mapping-declaration-timestamp" revision="1">
<title>timestamp (optional)</title>
<para>
The optional <literal><timestamp></literal> element indicates that the table contains
timestamped data. This is intended as an alternative to versioning. Timestamps are by nature
a less safe implementation of optimistic locking. However, sometimes the application might
use the timestamps in other ways.
</para>
<programlistingco>
<areaspec>
<area id="timestamp1" coords="2 45"/>
<area id="timestamp2" coords="3 45" />
<area id="timestamp3" coords="4 45" />
<area id="timestamp4" coords="5 45" />
</areaspec>
<programlisting><![CDATA[<timestamp
column="timestamp_column"
name="PropertyName"
access="field|property|nosetter|ClassName"
unsaved-value="null|undefined|value"
/>]]></programlisting>
<calloutlist>
<callout arearefs="timestamp1">
<para>
<literal>column</literal> (optional - defaults to the property name): The name
of a column holding the timestamp.
</para>
</callout>
<callout arearefs="timestamp2">
<para>
<literal>name</literal>: The name of a property of .NET type
<literal>DateTime</literal> of the persistent class.
</para>
</callout>
<callout arearefs="timestamp3">
<para>
<literal>access</literal> (optional - defaults to <literal>property</literal>): The
strategy NHibernate should use for accessing the property value.
</para>
</callout>
<callout arearefs="timestamp4">
<para>
<literal>unsaved-value</literal> (optional - defaults to <literal>null</literal>):
A timestamp property value that indicates that an instance is newly instantiated
(unsaved), distinguishing it from transient instances that were saved or loaded
in a previous session. (<literal>undefined</literal> specifies that the identifier
property value should be used.)
</para>
</callout>
</calloutlist>
</programlistingco>
<para>
Note that <literal><timestamp></literal> is equivalent to
<literal><version type="timestamp"></literal>.
</para>
</sect2>
<sect2 id="mapping-declaration-property">
<title>property</title>
<para>
The <literal><property></literal> element declares a persistent property