房间上可以为空的外键

Question

我会知道是否可以在外键上创建一个可能的空引用和空间。

目前我的数据库结构是这样的：

@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;   
}

我希望能够插入以下对象：

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)

但是之前的空值插入给出了这个错误： FOREIGN KEY 约束失败（Sqlite 代码 787 SQLITE_CONSTRAINT_FOREIGNKEY）

有没有办法让它成为可能？

Answer 1

是的，只需将外键类型从 long 更改为 Long。

在 Java 中，long 类型不能是 null，因此 Room 将此列生成为 NOT NULL。此外，C 类没有指定 @PrimaryKey。

@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;   
}

在 Kotlin 中，Long 类型是非空的。如果要插入可空外键，则需要将字段更改为可空类型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
}