android - Nullable foreign key on room

Question

I would know if that is possible to create a possible null reference on foreign key with room.

For now my database structure is like this :

@Entity(tableName = "A")
class A {
    @PrimaryKey(autoGenerate = true)
    public long id;
}

@Entity(tableName = "B")
class B{
    @PrimaryKey(autoGenerate = true)
    public long id;
}

@Entity(tableName = "C", foreignKeys = {@ForeignKey(entity = A.class, parentColumns = "id", childColumns = "foreign_id_a"), @ForeignKey(entity = B.class, parentColumns = "id", childColumns = "foreign_id_b")})
class C{
    public long id;
    public long foreign_id_a;
    public long foreign_id_b;   
}

I would like to be able to insert the following objects :

C(id=1, foreign_id_a=1, foreign_id_b=1)
C(id=1, foreign_id_a=null, foreign_id_b=1)
C(id=1, foreign_id_a=1, foreign_id_b=null)

But the previous insert with null value give this error : FOREIGN KEY constraint failed (Sqlite code 787 SQLITE_CONSTRAINT_FOREIGNKEY)

Is there a way to make it possible ?

Answer 1

Yes, just change foreign key types from long to Long.

In Java, long type cannot be null and therefore Room generates this column as NOT NULL. Also, the C class does not have @PrimaryKey specified.

@Entity(tableName = "C", foreignKeys = {@ForeignKey(entity = A.class, parentColumns = "id", childColumns = "foreign_id_a"), @ForeignKey(entity = B.class, parentColumns = "id", childColumns = "foreign_id_b")})
class C{
    @PrimaryKey
    public long id;
    public Long foreign_id_a;
    public Long foreign_id_b;   
}

In Kotlin, Long type is non-null. If you want to insert nullable foreign keys, you need to change the fields to nullable type Long?.

@Entity(tableName = "C", foreignKeys = [ForeignKey(entity = A::class, parentColumns = ["id"], childColumns = ["foreign_id_a"]), ForeignKey(entity = B::class, parentColumns = ["id"], childColumns = ["foreign_id_b"])])
class C{
    @PrimaryKey
    var id: Long = 0
    var foreign_id_a: Long? = null
    var foreign_id_b: Long? = null
}