Java Persistence/Inheritance - Wikibooks, open books for an open world - 0 views
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Inheritance
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hardest part of persisting inheritance is choosing how to represent the inheritance in the database
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There are three inheritance strategies defined from the InheritanceType enum,
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SINGLE_TABLE
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TABLE_PER_CLASS
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JOINED
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Single table inheritance is the default
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@MappedSuperclass
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@Inheritance
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mapped superclass is
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not a persistent class
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but allow common mappings to be define for its subclasses
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Single Table Inheritance
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implemented as a sparse table. ie. all attributes from all entities end up as columns in the 'super' table
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single table is used to store all of the instances of the entire inheritance hierarchy
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table will have a column for
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every attribute
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every class
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in the hierarchy
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discriminator column
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is used to determine which class the particular row belongs to
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abstract
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Project
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extends Project
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extends Project
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@DiscriminatorValue("S")
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@DiscriminatorValue("L")
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@DiscriminatorColumn(name="PROJ_TYPE")
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@Inheritance
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@Table(name="PROJECT")
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single table inheritance
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Joined, Multiple Table Inheritance
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mirrors the object model in the data model
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table is defined for each class in the inheritance hierarchy to store only the local attributes of that class
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Each table in the hierarchy must also store the object's id (primary key), which is
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only defined in the root class
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share the same id attribute
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joined inheritance
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@Inheritance(strategy=
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InheritanceType.JOINED
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@DiscriminatorColumn(name="PROJ_TYPE")
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@Table(name="PROJECT")
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abstract
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Project
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@DiscriminatorValue("L")
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@Table(name=
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"LARGEPROJECT"
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LargeProject
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Project
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@DiscriminatorValue("S")
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@Table(name=
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"SMALLPROJECT"
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SmallProject
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Project
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Table Per Class Inheritance
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Advanced
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table is defined for
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each concrete class
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in the inheritance hierarchy to store
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all the attributes
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of that class and
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all of its superclasses
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table per class inheritance
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@Inheritance(strategy=
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InheritanceType.TABLE_PER_CLASS
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abstract
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Project
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@Table(name="LARGEPROJECT")
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LargeProject
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Project
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@Table(name="SMALLPROJECT")
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SmallProject
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Project
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Mapped Superclasses
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similar to table per class inheritance, but does not allow querying, persisting, or relationships to the superclass
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mapped superclass
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@MappedSuperclass
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abstract
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Project
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@Column(name="NAME")
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@Table(name="LARGEPROJECT")
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LargeProject
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Project
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@AttributeOverride
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"PROJECT_NAME"
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"name"
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@Table("SMALLPROJECT")
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SmallProject
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Project
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cannot have a relationship to a mapped superclass
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Joined, Multiple Table Inheritance
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oined, Multiple Table Inheritance
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abstract
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abstract c
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extends Project
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Mapped Superclasses
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Mapped Superclasses
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apped Superclasses
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allows inheritance to be used in the object model, when it does not exist in the data model
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@MappedSuperclass
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MappedSuperclass
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abstract
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abstract
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extends Project
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extends Project