Relations between entities

Remarks#

Relations Between Entities Basics

A foreign key can be one or more columns that reference a unique key, usually the primary key, in another table.

A foreign key and the primary parent key it references must have the same number and type of fields.

Foreign keys represents relationships from a column or columns in one table to a column or columns in another table.

Multiplicity in Entity Relationships

Multiplicities are of the following types:

One-to-one: Each entity instance is related to a single instance of another entity.
One-to-many: An entity instance can be related to multiple instances of the other entities.
Many-to-one: multiple instances of an entity can be related to a single instance of the other entity.
Many-to-many: The entity instances can be related to multiple instances of each other.

One-to-One Mapping

One-to-one mapping defines a single-valued association to another entity that has one-to-one multiplicity. This relationship mapping use the @OneToOne annotation on the corresponding persistent property or field.

Example: Vehicle and ParkingPlace entities.

One-to-Many Mapping

An entity instance can be related to multiple instances of the other entities.

One-to-many relationships use the @OneToMany annotation on the corresponding persistent property or field.

The mappedBy element is needed to refer to the attribute annotated by ManyToOne in the corresponding entity:

 @OneToMany(mappedBy="attribute")

A one-to-many association needs to map the collection of entities.

Many-to-One Mapping

A many-to-one mapping is defined by annotating the attribute in the source entity (the attribute that refers to the target entity) with the @ManyToOne annotation.

A @JoinColumn(name="FK_name") annotation discribes a foreing key of a relationship.

Many-to-Many Mapping

The entity instances can be related to multiple instances of each other.

Many-to-many relationships use the @ManyToMany annotation on the corresponding persistent property or field.

We must use a third table to associate the two entity types (join table).

@JoinTable Annotation Example

When mapping many-to-many relationships in JPA, configuration for the table used for the joining foreign-keys can be provided using the @JoinTable annotation:

@Entity
public class EntityA {
   @Id
   @Column(name="id")
   private long id;
   [...]
   @ManyToMany
   @JoinTable(name="table_join_A_B",
              joinColumns=@JoinColumn(name="id_A"), referencedColumnName="id"
              inverseJoinColumns=@JoinColumn(name="id_B", referencedColumnName="id"))
   private List<EntityB> entitiesB;
   [...]
}

@Entity
public class EntityB {
   @Id
   @Column(name="id")
   private long id;
   [...]
}

In this example, which consists of EntityA having a many-to-many relation to EntityB, realized by the entitiesB field, we use the @JoinTable annotation to specify that the table name for the join table is table_join_A_B, composed by the columns id_A and id_B, foreign keys respectively referencing column id in EntityA’s table and in EntityB’s table; (id_A,id_B) will be a composite primary-key for table_join_A_B table.