房间内可空外键 [英] Nullable foreign key on room
问题描述
我会知道是否有可能在带有空间的外键上创建一个可能的空引用.
I would know if that is possible to create a possible null reference on foreign key with room.
现在我的数据库结构是这样的:
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;
}
我希望能够插入以下对象:
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)
但是前一个具有空值的插入会出现此错误:FOREIGN KEY约束失败(SQLite代码787 SQLITE_CONSTRAINT_FOREIGNKEY)
But the previous insert with null value give this error : FOREIGN KEY constraint failed (Sqlite code 787 SQLITE_CONSTRAINT_FOREIGNKEY)
有没有办法使之成为可能?
Is there a way to make it possible ?
推荐答案
是的,只需将外键类型从 long
更改为 Long
.
Yes, just change foreign key types from long
to Long
.
在Java中, long
类型不能为 null
,因此Room将此列生成为 NOT NULL
.另外, C
类未指定 @PrimaryKey
.
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;
}
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