如何使用Android Room创建一个包含两个或多个外键的表？

Question

根据entity-relationship model，可以使用Room Persistency Library指定tbl_post和tbl_category之间的关系，如下所示：

@Entity(foreignKeys = @ForeignKey(
    entity = TblPost.class,
    parentColumns = "id",
    childColumns = "tbl_post_id")
)
class TblPostCategory {
    @PrimaryKey
    public String id;

    @ColumnInfo(name = "user_id")
    public String postId;
}

但TblPostCategory取决于两个外键：来自post_id和category_id的{​​{1}}和TblPost。

如何使用Room annotations描述关系？

Answer 1

<强> TblCategory.java

@Entity
class TblCategory {
    @PrimaryKey
    @ColumnInfo(name="cat_id")
    public String id;

    @ColumnInfo(name = "cat_name")
    public String name;
}

TblPost.java （它缺少外键引用，但对于这种情况并不重要）

@Entity
class TblPost {
    @PrimaryKey
    @ColumnInfo(name="post_id")
    public String id;

    public String title, content, create_time, author_id;
}

<强> TblPostCategory.java

@Entity(foreignKeys = {
    @ForeignKey(
        entity = TblPost.class,
        parentColumns = "post_id",
        childColumns = "tbl_post_id"
    ),
    @ForeignKey(
        entity = TblCategory.class,
        parentColumns = "cat_id",
        childColumns = "tbl_category_id"
    )
})
class TblPostCategory {
    @PrimaryKey
    @ColumnInfo(name="tbl_post_id")
    public String id;

    @ColumnInfo(name = "tbl_category_id")
    public String categoryId;
}

Answer 2

在科特林：

@Entity(
    tableName = "some_table",
    indices = [Index("id"), Index("brand_id"), Index("model_id")],
    foreignKeys = [
        ForeignKey(entity = BrandEntity::class, parentColumns = ["id"],
            childColumns = ["brand_id"]),
        ForeignKey(entity = ModelEntity::class, parentColumns = ["id"],
            childColumns = ["model_id"]),
        ForeignKey(entity = Table1Entity::class, parentColumns = ["id"],
            childColumns = ["table1_id"]),
        ForeignKey(entity = Table2Entity::class, parentColumns = ["id"],
            childColumns = ["table2_id"])
    ]
)