https://luminoussheep.net/mediawiki/index.php?action=history&feed=atom&title=Persistence_Strategies 2026-04-16T19:04:20Z Revision history for this page on the wiki MediaWiki 1.43.8 https://luminoussheep.net/mediawiki/index.php?title=Persistence_Strategies&diff=61&oldid=prev 2021-09-14T21:29:33Z

<p>Created page with &quot;= Inheritance and persistence = == Single Table == &#039;&#039;&#039;Default mechanism in EJB3 &#039;&#039;&#039; * Objects discriminated by extra column in table ** specify mechanism: @Inheritance(Strat...&quot;</p> <p><b>New page</b></p><div>= Inheritance and persistence =<br /> <br /> == Single Table ==<br /> &#039;&#039;&#039;Default mechanism in EJB3 &#039;&#039;&#039;<br /> * Objects discriminated by extra column in table <br /> ** specify mechanism: @Inheritance(Strategy=InheritanceType.SINGLE_TABLE)<br /> ** must specify the column name: @DiscriminatorColumn(name=&quot;...<br /> ** Each entity must specify values: @DiscriminatorValue(value=&quot;... - default = name of subclass<br /> <br /> === Implications ===<br /> * column for each field in every subclass<br /> ** can result in a lot of redundant columns<br /> * unable to exploit constraints - null values may be required for some subclasses and not others<br /> ** can overcome with triggers<br /> <br /> == Joined Table ==<br /> ** specify mechanism: @Inheritance(Strategy=InheritanceType.JOINED)<br /> * separate table for each entity in hierarchy<br /> * core table with primary key and columns for common elements<br /> * sub tables use same primary key as core table and include columns for additional attributes<br /> * discriminator column used to easily identify subtype see [[#Single Table]]<br /> * Also specify table join column @PrimaryKeyJoinColumn(name=&quot;...<br /> <br /> === Implications ===<br /> * cleaner design - easy to enforce constraints<br /> * worse performance than Single Table as requires joining tables<br /> <br /> == Table per Class ==<br /> &#039;&#039;&#039;Optional for providers to implement&#039;&#039;&#039;<br /> ** specify mechanism: @Inheritance(Strategy=InheritanceType.TABLE_PER_CLASS)<br /> * All classes have unique tables<br /> ** No relationship exists between tables<br /> * primary keys across all tables must be unique<br /> * All inherited columns duplicated in each sub table<br /> <br /> === Implications ===<br /> * Poor OO design<br /> * Poor performance<br /> * data can be pulled from one table in single query<br /> * poor support for polymorphic queries - queries must union all tables<br /> <br /> = Inheritance from non Entity classes =<br /> Annotate with: @MappedSuperClass<br /> * no associated table<br /> <br /> = Polymorphic associations =<br /> Polymorphic associations work in JPA<br /> * When base class has an association to another object<br /> * Sub classes inherit this association</div>

Martin