Chapter 2. Mapping Entities - 0 views
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Composite identifier
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You can define a composite primary key through several syntaxes:
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@EmbeddedId
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map multiple properties as @Id properties
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annotated the property as
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map multiple properties as @Id properties and declare an external class to be the identifier type
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declared on the entity via the @IdClass annotation
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The identifier type must contain the same properties as the identifier properties of the entity: each property name must be the same, its type must be the same as well if the entity property is of a
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basic type
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last case is far from obvious
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recommend you not to use it (for simplicity sake)
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@EmbeddedId property
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@EmbeddedId
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@Embeddable
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@EmbeddedId
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@Embeddable
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@Embeddable
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@EmbeddedId
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Multiple @Id properties
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arguably more natural, approach
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place @Id on multiple properties of my entity
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only supported by Hibernate
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does not require an extra embeddable component.
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@IdClass
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@IdClass on an entity points to the class (component) representing the identifier of the class
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WarningThis approach is inherited from the EJB 2 days and we recommend against its use. But, after all it's your application and Hibernate supports it.
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Mapping entity associations/relationships
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One-to-one
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three cases for one-to-one associations:
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associated entities share the same primary keys values
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foreign key is held by one of the entities (note that this FK column in the database should be constrained unique to simulate one-to-one multiplicity)
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association table is used to store the link between the 2 entities (a unique constraint has to be defined on each fk to ensure the one to one multiplicity)
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@PrimaryKeyJoinColumn
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shared primary keys:
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explicit foreign key column:
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@JoinColumn(name="passport_fk")
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foreign key column named passport_fk in the Customer table
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may be bidirectional
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owner is responsible for the association column(s) update
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In a bidirectional relationship, one of the sides (and only one) has to be the owner
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To declare a side as
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not responsible for the relationship
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the attribute
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mappedBy
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is used
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mappedBy
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Indexed collections (List, Map)
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Lists can be mapped in two different ways:
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as ordered lists
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as indexed lists
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@OrderBy("number")
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List<Order>
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List<Order>
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List<Order>
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To use one of the target entity property as a key of the map, use
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@MapKey(name="myProperty")
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@MapKey(name"number")
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Map<String,Order>
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String number
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@MapKeyJoinColumn/@MapKeyJoinColumns
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if the map key type is another entity
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@ManyToAny
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2.4.5.2. @Any
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@Any annotation defines a polymorphic association to classes from multiple tables
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this is most certainly not meant as the usual way of mapping (polymorphic) associations.
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@ManyToAny allows polymorphic associations to classes from multiple tables
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first column holds the type of the associated entity
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remaining columns hold the identifier
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not meant as the usual way of mapping (polymorphic) associations