[英]How to create a table with a two or more foreign keys using Android Room?
根據實體關系模型,可以使用Room Persistency Library指定tbl_post
和tbl_category
之間的關系,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;
}
然而TblPostCategory
取決於兩個外鍵:來自TblPost
和TbCategory
post_id
和category_id
。
應該如何使用 Room 注釋來描述這種關系?
TblCategory.java
@Entity
class TblCategory {
@PrimaryKey
@ColumnInfo(name="cat_id")
public String id;
@ColumnInfo(name = "cat_name")
public String name;
}
TblPost.java (它缺少外鍵引用,但對於這種情況並不重要)
@Entity
class TblPost {
@PrimaryKey
@ColumnInfo(name="post_id")
public String id;
public String title, content, create_time, author_id;
}
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;
}
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"])
]
)
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