Many to Many relations with Room & LiveData

Question

I have a rest api that returns a list of places, which have a list of categories:

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

So using retrofit I get these from the remote server with these model classes:

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

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

Problem is -- I want the viewModel to always retrieve from a local Room Database returning Flowables and just trigger refresh actions that will update the database and thus the view.

DAO method example:

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

So I tried modeling those two classes like this:

@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>
)

But of course Room is not able to process relations on its own. I have seen this post which just retrieves from the local database the previous list of cities, but this case doesnt match that one.

Only option I could think of is to save the categories in the database as a JSON string but this is losing the relational quality of the database...

This seems like a pretty common use case but I haven't found much info about it.

Answer 1

It's possible in Room to have many to many relationship.

First add @Ignore annotation to your Place class. It will tell Room to ignore this property, because it can't save the list of objects without converter.

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>
) 

Then create a class that will represent the connection between this two classes.

@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
)

As you can see I used foreign keys.

Now you can specify special DAO for getting list of categories for a place.

@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)
}

And the last important thing is to insert join object each time you insert new Place.

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

Now when you get places from placeDao() they have empty category list. In order to add categories you can use this part of code:

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
}

To see more details see this article.

Answer 2

I had a similar use case. As Room doesn't manage relations, I ended up with this solution following the blog you mentioned :/

@Entity
data class Category(
    @PrimaryKey(autoGenerate = true)
    val id: Long = 0,
    var catId: String, 
    var name: String,
    @ForeignKey(entity = Place::class, parentColumns = ["id"], childColumns = ["placeId"], onDelete = ForeignKey.CASCADE)
    var placeId: String = ""
)

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

PlaceDao

@Dao
interface PlaceDao {
    @Insert(onConflict = OnConflictStrategy.REPLACE)
    fun insert(place: Place)

    @Query("SELECT * FROM place WHERE id = :id")
    fun getPlace(id: String?): LiveData<Place>
}

fun AppDatabase.getPlace(placeId: String): LiveData<Place> {
    var placeLiveData = placeDao().getPlace(placeId)
    placeLiveData = Transformations.switchMap(placeLiveData, { place ->
        val mutableLiveData = MutableLiveData<Place>()
        Completable.fromAction { // cannot query in main thread
            place.categories = categoryDao().get(placeId)
        }
                .subscribeOn(Schedulers.io())
                .observeOn(AndroidSchedulers.mainThread())
                .subscribe { mutableLiveData.postValue(place) }
        mutableLiveData
    })
    return placeLiveData
}

// run in transaction
fun AppDatabase.insertOrReplace(place: Place) {
    placeDao().insert(place)
    place.categories?.let {
        it.forEach {
            it.placeId = place.id
        }
        categoryDao().delete(place.id)
        categoryDao().insert(it)
    }
}

CategoryDao

@Dao
interface CategoryDao {
    @Insert(onConflict = OnConflictStrategy.REPLACE)
    fun insert(categories: List<Category>)

    @Query("DELETE FROM category WHERE placeId = :placeId")
    fun delete(placeId: String?)

    @Query("SELECT * FROM category WHERE placeId = :placeId")
    fun get(placeId: String?): List<Category>
}

Not a big fan but I didn't find a better way for the moment.