问题
Given the following example (departments - projects):
A department has the following properties (composite primary key):
@Entity
@IdClass(DeptId.class)
public class Department
{
@Id
@Column(name="number")
private Integer number;
@Id
@Column(name="country")
private String country;
@Column(name="name")
private String name;
@OneToMany(mappedBy="dept")
private Collection<Project> projects;
...
}
Here the PK class:
public class DeptId implements Serializable
{
private Integer number;
private String country;
...
}
The relationship between projects and departments is many-to-one, that is a deptartment can have many projects. The Project class is itself using a composite key referencing Department's composite key. Important note: it's only about the implementation with @IdClass not @EmbeddedId.
Then the (problematic) JPA 1.0 @IdClass implementation would have to look something like that (redundant deptNum and deptCtry properties): -> it's just a unique name within a department
@Entity
@IdClass(ProjectId.class)
public class Project
{
@Id
@Column(name="dept_number")
private Integer deptNumber;
@Id
@Column(name="dept_country")
private String deptCountry;
@Id
@Column(name="name")
private String name;
@ManyToOne
@JoinColumns({
@JoinColumn(name="dept_number", referencedColumnName="number"),
@JoinColumn(name="dept_country", referencedColumnName="country")
})
private Department dept;
...
}
The ProjectId is:
public class ProjectId implements Serializable
{
private String name;
private DeptId dept;
...
}
The problem with this is that neither Hibernate nor EclipseLink know how to map the two redundant properties deptNum and deptCtry in Project to the dept property in DeptId (or the properies within it). -> MappingException etc.
My question is:
Is this a limitation of JPA 1.0, that tables with composite keys referencing other composite keys with @IdClass implementations generally WON'T work, because the JPA implementation simply can't know how to map these fields?
As a workaround, you'd have to use @EmbeddedId for these classes or use JPA 2.0 syntax to annotate the @XToX associations with @Id. I just want to make sure my view on this is right.
Thanks
回答1:
Yes, this is a limitation of JPA 1.0, corrected in JPA 2.0. In the new JPA 2.0, you can put the ID annotation on your dept relationship and completely avoid having the redundent deptCountry and deptNumber attributes, with the key class using nesting. In JPA 1.0, only basic mappings can be marked as apart of the ID, requiring redundent mappings and some code to ensure that the values/relationships get put into the cache correctly when persisting. Because of the redundancy, as mentioned in other answers, one of the mappings for a field needs to be marked read-only via the insertable/updatable=false. Doing so though means that value is not merged into the cache - so changes (such as on insert, since you can't change an objects ID once it exists) will not be reflected unless the object is refreshed from the database. If you mark the JoinColumns as read-only, you will need to get the values from the referenced dept and put them into the correspoinding basic id attributes manually when you want to persist a Project. But, you can also mark the basic attributes as read-only. Eclipselink anyway will not have any problems and will correctly set the field values using the associated dept entity (as long as it is set before persist is called on the Project). Notice though that the basic attributes may or may not be populated when you read back the project in a different context- this will depend on if the entity is refreshed from the database or not. If they are read-only, they do not get merged into the shared cache since they, being read only, should not have changed. So they can be just ignored, or if they must be populated, the entity refreshed or the values set from the dept in an event.
This same model can be reused by using the JPA2.0 @MapsId, which will also maintain the basic mappings using the values from the relationship for you. Only benifit I see is that you don't need to access the relationship (potentially causing unneccessary joins or database access on lazy relationships) to get the foreign key/id field values.
As for the ZipArea EclipseLink exceptions, they are due to ZipAreaId having a ZipId zip attribute instead it being flattened out. JPA 1.0 requires the key class to have an attribute of the same type and name for each @ID attribute in the Entity.
回答2:
The problem with this is that neither Hibernate nor EclipseLink know how to map the two redundant properties deptNum and deptCtry in Project to the dept property in DeptId
This is why you need to define the ManyToOne foreign key(s) as read-only with this kind of mapping. This is done by setting the JoinColumn attributes insertable and updatable to false.
So try the following:
@Entity
@IdClass(ProjectId.class)
public class Project
{
@Id
@Column(name="dept_number")
private Integer deptNumber;
@Id
@Column(name="dept_country")
private String deptCountry;
@Id
@Column(name="name")
private String name;
@ManyToOne
@JoinColumns({
@JoinColumn(name="dept_number", referencedColumnName="number", insertable=false, updatable=false),
@JoinColumn(name="dept_country", referencedColumnName="country", insertable=false, updatable=false)
})
private Department dept;
...
}
回答3:
The problem with the posted code is, that JPA 1.0 really doesn't allow nesting of composite primary key classes. This ProjectId is invalid:
public class ProjectId implements Serializable
{
private String name;
private DeptId dept;
...
}
DeptId has to be flattened, like:
public class ProjectId implements Serializable
{
private Integer deptNumber;
private String deptCountry;
private String name;
...
}
I just got an EclipseLink version to go, but Hibernate has problems with that. I wonder how to tell Hibernate that JPA 1.0 is assumed.
来源：https://stackoverflow.com/questions/4058592/limitation-of-jpa-1-0-using-idclass-with-nested-composite-primary-keys