According the entity-relationship model, the relationship between tbl_post
and tbl_category
could be specified using Room Persistency Library as follows:
@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;
}
However TblPostCategory
depends on two foreign keys: post_id
and category_id
from TblPost
and TbCategory
.
How the relationship should be described using Room annotations?
A table may have multiple foreign keys, and each foreign key can have a different parent table. Each foreign key is enforced independently by the database system. Therefore, cascading relationships between tables can be established using foreign keys.
What is a Room database? Room is a database layer on top of an SQLite database. Room takes care of mundane tasks that you used to handle with an SQLiteOpenHelper . Room uses the DAO to issue queries to its database. By default, to avoid poor UI performance, Room doesn't allow you to issue queries on the main thread.
TblCategory.java
@Entity
class TblCategory {
@PrimaryKey
@ColumnInfo(name="cat_id")
public String id;
@ColumnInfo(name = "cat_name")
public String name;
}
TblPost.java (It is missing the foreign key reference but it is not important for the case)
@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;
}
In Kotlin:
@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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