Hibernate foreign key with a part of composite primary key

Question

I have to work with Hibernate and I am not very sure how to solve this problem, I have 2 tables with a 1..n relationship like this:

-------
TABLE_A
-------
col_b (pk)
col_c (pk)
[other fields]

-------
TABLE_B
-------
col_a (pk)
col_b (pk) (fk TABLE_A.col_b)
col_c (fk TABLE_A.col_c)
[other fields]

How can I manage this with Hibernate?

I do not have any idea how to declare a foreign key that would contain a part of primary key.

My database schema is generated from the Hibernate model.

Byronbyrum

If you want to fight hibernate... go ahead and use composite keys. If you want a much easier life, ensure each table has a single primary key column. Trust me, it's not worth the fight... it's a battle which will cost you many hours in the future – Obey

What do you mean by "part of primary key"? Composite primary keys in Hibernate are quite simple, but you shouldn't try to complicate that by trying to use only a part of it. Basicly you need a composite key class. There is an answer here with all basic details: #3585534 – Klement

There's a fairly thorough answer here: https://mcmap.net/q/116555/-how-to-map-a-composite-key-with-jpa-and-hibernate – Rhiamon

For one-to-many mappings you don't require a composite key. It is so so easier to manage this situation without using composite key. – Sheaff

@LanceJava In this case I need to use composite keys because of readability. – Byronbyrum

@Rhiamon This answer does not help at all in my case. It is only about how to define a composite key. – Byronbyrum

https://mcmap.net/q/746116/-hibernate-model Could you please suggest on this query. – Femmine

Answer 1

I have found two solutions to this problem.

The first one is rather a workaround and is not so neat as the second one.

Define the primary key of the B entity as composite key containing col_a, col_b, and col_c and what was supposed to be the primary key in the first place, define as unique constraint. The disadvantage is that the column col_c is not really conceptually a part of primary key.

@Entity
class A {
  @Id
  private int b;
  @Id
  private int c;
}

@Entity
@Table(uniqueConstraints = {@UniqueConstraint(columnNames = { "a", "b" }) })
class B {
  @Id
  private int a;

  @Id
  @ManyToOne(optional = false)
  @JoinColumns(value = {
          @JoinColumn(name = "b", referencedColumnName = "b"),
          @JoinColumn(name = "c", referencedColumnName = "c") })
  private A entityA;
}

The second uses @EmbeddedId and @MapsId annotations and does exactly what I wanted to be done at the very beginning.

@Entity
class A {
  @Id
  private int b;
  @Id
  private int c;
}

@Embeddable
class BKey {
  private int a;
  private int b;
}

@Entity
class B {
  @EmbeddedId
  private BKey primaryKey;

  @MapsId("b")
  @ManyToOne(optional = false)
  @JoinColumns(value = {
          @JoinColumn(name = "b", referencedColumnName = "b"),
          @JoinColumn(name = "c", referencedColumnName = "c") })
  private A entityA;
}

Byronbyrum

To get above code work you also need to implement Serializable for all above classes – Polyzoan

I tried this way and I'm getting Unable to find column reference in the @MapsId mapping: c. Any idea?. TIA – Farmergeneral

Is it a runtime or compile error? Could you post your code somewhere so that I can have a look at it? – Byronbyrum

Is it possible to have MapsId for two attrributes? For example, if there was an attribute 'd' which is also part of both keys (same as b)? – Hernia

@Hernia Maybe try with another @MapsId("d") annotation, so that you will have two annotations in the end: @MapsId("b") and @MapsId("d"). – Byronbyrum

Unfortunately it is not possible to have multiple ´@MapsId´ annotations on the same attribute. But I managed to solve it for my use case by using ´@JoinFormula´ instead of ´@JoinColumn´ for the columns that are used in Primary Key as well as Foreign Key. – Hernia

Answer 2

Jagger's second solution that is my first reaction, with @EmbededId and @MapsId. The following is another way based on his second solution but without using @MapsId. `

@Entity
class A {
  @Id
  private int b;
  @Id
  private int c;
}

@Embeddable
class BKey {
  private int a;
  private int b;
}

@Entity
class B {
  @EmbeddedId
  private BKey primaryKey;

  @ManyToOne(optional = false)
  @JoinColumns(value = {
          @JoinColumn(name = "b", referencedColumnName = "b", insertable= false, updatable= false),
          @JoinColumn(name = "c", referencedColumnName = "c") }, )
  private A entityA;
}

Aloha

This won't work and ends with an exception org.hibernate.AnnotationException: Mixing insertable and non insertable columns in a property is not allowed There is a Hibernate bug for it . – Byronbyrum