与Room&LiveData的多对多关系

Question

我有一个休息api,它返回一个地方列表,其中包含一个类别列表:

{
      "id": "35fds-45sdgk-fsd87",
      "name" : "My awesome place",
       "categories" : [
          {
            "id": "cat1",
            "name" : "Category 1"
          },
          {
            "id": "cat2",
            "name" : "Category 2"
          },
          {
            "id": "cat3",
            "name" : "Category 3"
          }
       ]
}

因此,使用改造我从远程服务器获取这些模型类:

data class Category(var id: String, var name: String)

data class Place(
  var id: String,
  var name: String,
  var categories: List<Category>
)

问题是 - 我希望viewModel始终从返回Flowables的本地Room数据库中检索,并且只触发将更新数据库以及视图的刷新操作.

DAO方法示例:

@Query("select * from Places where placeId = :id")
fun getPlace(id: String): Flowable<Place>

所以我尝试像这样建模这两个类:

@Entity
data class Category(var id: String, var name: String)


@Entity
data class Place(
  @PrimaryKey
  var id: String,
  var name: String,
  var categories: List<Category>
)

但当然,Room无法自行处理关系.我看过这篇帖子只是从本地数据库中检索出以前的城市列表,但这种情况与那个不匹配.

我能想到的唯一选择是将数据库中的类别保存为JSON字符串,但这会失去数据库的关系质量......

这似乎是一个非常常见的用例,但我没有找到很多关于它的信息.

Answer 1

在Room中有可能有多对多的关系.

首先@Ignore在您的Place班级中添加注释.它会告诉Room忽略此属性,因为它无法保存没有转换器的对象列表.

data class Category(
        @PrimaryKey var id: String, 
        var name: String
)

data class Place(
        @PrimaryKey var id: String,
        var name: String,
        @Ignore var categories: List<Category>
) 

然后创建一个表示这两个类之间连接的类.

@Entity(primaryKeys = ["place_id", "category_id"],
        indices = [
            Index(value = ["place_id"]),
            Index(value = ["category_id"])
        ],
        foreignKeys = [
            ForeignKey(entity = Place::class,
                    parentColumns = ["id"],
                    childColumns = ["place_id"]),
            ForeignKey(entity = Category::class,
                    parentColumns = ["id"],
                    childColumns = ["category_id"])
        ])
data class CategoryPlaceJoin(
        @ColumnInfo(name = "place_id") val placeId: String,
        @ColumnInfo(name = "category_id") val categoryId: String
)

如你所见,我使用了外键.

现在,您可以指定特殊DAO以获取场所的类别列表.

@Dao
interface PlaceCategoryJoinDao {

    @SuppressWarnings(RoomWarnings.CURSOR_MISMATCH)
    @Query("""
        SELECT * FROM category INNER JOIN placeCategoryJoin ON
        category.id = placeCategoryJoin.category_id WHERE
        placeCategoryJoin.place_id = :placeId
        """)
    fun getCategoriesWithPlaceId(placeId: String): List<Category>

    @Insert
    fun insert(join: PlaceCategoryJoin)
}

最后一件重要的事情是每次插入新的时插入连接对象Place.

val id = placeDao().insert(place)
for (place in place.categories) {
   val join = CategoryPlaceJoin(id, category.id)
   placeCategoryJoinDao().insert(join)
}

现在当你从placeDao()他们那里获得地点时他们有空的类别列表.要添加类别,您可以使用以下代码:

fun getPlaces(): Flowable<List<Place>> {
    return placeDao().getAll()
            .map { it.map { place -> addCategoriesToPlace(place) } }
}

private fun addCategoriesToPlace(place: Place): Place {
    place.categories = placeCategoryJoinDao().getCategoriesWithPlaceId(place.id)
    return place
}

要了解更多详情,请参阅这篇文章.