To map a composite key, you can use the EmbeddedId
or the IdClass
annotations. I know this question is not strictly about JPA but the rules defined by the specification also applies. So here they are:
2.1.4 Primary Keys and Entity Identity
…
A composite primary key must correspond to either a single persistent field or property or to a set of such fields or properties as described below. A primary key class must be defined to represent a composite primary key. Composite primary keys typically arise when mapping from legacy databases when the database key is comprised of several columns. The EmbeddedId
and IdClass
annotations are used to denote composite primary keys. See sections 9.1.14 and 9.1.15.
…
The following rules apply for composite primary keys:
- The primary key class must be public and must have a public no-arg constructor.
- If property-based access is used, the properties of the primary key class must be public or protected.
- The primary key class must be
serializable
. - The primary key class must define
equals
andhashCode
methods. The semantics of value equality for these methods must be consistent with the database equality for the database types to which the key is mapped. - A composite primary key must either be represented and mapped as an embeddable class (see Section 9.1.14, “EmbeddedId Annotation”) or must be represented and mapped to multiple fields or properties of the entity class (see Section 9.1.15, “IdClass Annotation”).
- If the composite primary key class is mapped to multiple fields or properties of the entity class, the names of primary key fields or properties in the primary key class and those of the entity class must correspond and their types must be the same.
With an IdClass
The class for the composite primary key could look like (could be a static inner class):
public class TimePK implements Serializable {
protected Integer levelStation;
protected Integer confPathID;
public TimePK() {}
public TimePK(Integer levelStation, Integer confPathID) {
this.levelStation = levelStation;
this.confPathID = confPathID;
}
// equals, hashCode
}
And the entity:
@Entity
@IdClass(TimePK.class)
class Time implements Serializable {
@Id
private Integer levelStation;
@Id
private Integer confPathID;
private String src;
private String dst;
private Integer distance;
private Integer price;
// getters, setters
}
The IdClass
annotation maps multiple fields to the table PK.
With EmbeddedId
The class for the composite primary key could look like (could be a static inner class):
@Embeddable
public class TimePK implements Serializable {
protected Integer levelStation;
protected Integer confPathID;
public TimePK() {}
public TimePK(Integer levelStation, Integer confPathID) {
this.levelStation = levelStation;
this.confPathID = confPathID;
}
// equals, hashCode
}
And the entity:
@Entity
class Time implements Serializable {
@EmbeddedId
private TimePK timePK;
private String src;
private String dst;
private Integer distance;
private Integer price;
//...
}
The @EmbeddedId
annotation maps a PK class to table PK.
Differences:
- From the physical model point of view, there are no differences
@EmbeddedId
somehow communicates more clearly that the key is a composite key and IMO makes sense when the combined pk is either a meaningful entity itself or it reused in your code.@IdClass
is useful to specify that some combination of fields is unique but these do not have a special meaning.
They also affect the way you write queries (making them more or less verbose):
- with
IdClass
select t.levelStation from Time t
- with
EmbeddedId
select t.timePK.levelStation from Time t
References
- JPA 1.0 specification
- Section 2.1.4 “Primary Keys and Entity Identity”
- Section 9.1.14 “EmbeddedId Annotation”
- Section 9.1.15 “IdClass Annotation”