Persistence Strategies

Inheritance and persistence

Single Table

Default mechanism in EJB3

• Objects discriminated by extra column in table
  • specify mechanism: @Inheritance(Strategy=InheritanceType.SINGLE_TABLE)
  • must specify the column name: @DiscriminatorColumn(name="...
  • Each entity must specify values: @DiscriminatorValue(value="... - default = name of subclass

Implications

• column for each field in every subclass
  • can result in a lot of redundant columns
• unable to exploit constraints - null values may be required for some subclasses and not others
  • can overcome with triggers

Joined Table

• • specify mechanism: @Inheritance(Strategy=InheritanceType.JOINED)
• separate table for each entity in hierarchy
• core table with primary key and columns for common elements
• sub tables use same primary key as core table and include columns for additional attributes
• discriminator column used to easily identify subtype see #Single Table
• Also specify table join column @PrimaryKeyJoinColumn(name="...

Implications

• cleaner design - easy to enforce constraints
• worse performance than Single Table as requires joining tables

Table per Class

Optional for providers to implement

• • specify mechanism: @Inheritance(Strategy=InheritanceType.TABLE_PER_CLASS)
• All classes have unique tables
  • No relationship exists between tables
• primary keys across all tables must be unique
• All inherited columns duplicated in each sub table

Implications

• Poor OO design
• Poor performance
• data can be pulled from one table in single query
• poor support for polymorphic queries - queries must union all tables

Inheritance from non Entity classes

Annotate with: @MappedSuperClass

• no associated table

Polymorphic associations

Polymorphic associations work in JPA

• When base class has an association to another object
• Sub classes inherit this association