房间外键可为空

我想知道是否可以在带有空间的外键上创建可能的空引用。

现在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)

但之前插入空值会出现此错误： FOREIGN KEY 约束失败（Sqlite 代码 787 SQLITE_CONSTRAINT_FOREIGNKEY）

有没有办法让它成为可能？

是的，只需更改外键类型即可long to Long.

在爪哇，long类型不能是null因此 Room 生成此列为NOT NULL。另外，C类没有@PrimaryKey指定的。

@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;   
}

在科特林中，Long类型非空。如果要插入可为空的外键，则需要将字段更改为可为空的类型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
}