How do I repsent OneToMany relationship in JPA with multiple tables inbetween?

Question

At a high level, I have a class "MainClass" that has a list of "OtherEntity". There are essentially two join tables. The two join tables have meta-data in them that other components use but I have not defined entities for as we don't need them outside of a join operation. But basically MainClass => TableOne => TableTwo => OtherEntity

My entity definitions

@Entity
public class MainClass {

    // other stuff ...

    // how to?
    @OneToMany
    private List<OtherEntity> otherEntities;
}


@Entity
public class OtherClass { // other stuff ... }

My schema (legacy and cannot be modified)

table MainClass
    PK id
    ...

table TableOne
    PK id
    FK main_class_id
    FK table_two_id

table TableTwo
    PK id
    FK table_one_id
    FK other_entity_id

table OtherEntity
     PK id
     ...

I'd like avoid creating entities for TableOne and TableTwo if I could.

Answer 1

The issue is a design problem that will eventually be addressed. We have a database view that provides a unified look up table. In the database layer we can assign the pieces individually.

@Entity
public class MainClass {

    @OneToMany
    @JoinTable(name = "TheViewName", 
        joinColumns = @JoinColumn(name = "id", insertable = false, updatable= false), 
        inverseJoinColumns = @JoinColumn(name = "other_id", insertable = false, 
            updatable = false))
    private List<OtherEntity> otherEntities;
}

This means I cannot assign a new other entity to the MainClass and persist both. I have to do two separate operations to get things finalized. But, that will go away when we refactor the schema.

Alternatively, we could create entities for the other two tables to make the operations more transparent.

Answer 2

Consider using @SecondaryTable it should work for legacy schemas, using this annotation you can have fields in one entity, and just define which ones will form part of other table, but you only have 2 entities.

Check this tutorial

Idea is split fields from one entity in different tables without need the creation of other entities, just define which fields are in which table.

@SecondaryTables{@SecondaryTable(name="XX")} 

and to move fields between tables use

@Column(table="XX")

In your config try something like this.

@Entity
@SecondaryTables{@SecondaryTable(name="TableOne"), @SecondaryTable(name="TableTwo")}
table MainClass
    PK id
    @Column(table="TableOne")
    FK main_class_id
    @Column(table="TableOne")
    FK table_two_id

    @Column(table="TableTwo")
    PK id
    @Column(table="TableTwo")
    FK table_one_id
    @Column(table="TableTwo")
    FK other_entity_id

For ID in middle tables consider remove @Id and set it automatically, it should work !!