Cant get data from Room database

Question

I have a problem with getting data from check_list_table. insert method executes normally and there is no error or warning, but allItems property doesn't work.

Item, ItemDao and ViewModel class are shown below:

@Entity(tableName = "check_list_table")
data class Item(
    @PrimaryKey(autoGenerate = true)
    var id: Int = 0,
    var title: String,
    var priority: Int=0
)

interface ItemDao {

    @Insert
    suspend fun insert(item: Item)

    @Delete
    suspend fun delete(item: Item)

    @Query("SELECT * FROM check_list_table ORDER BY id DESC")
    fun getAllItems():LiveData<List<Item>>

}

class DailyCheckListItemViewModel(private val itemDao: ItemDao, application: Application) :
    AndroidViewModel(application) {

    private val viewModelJob = Job()
    private val uiScope = CoroutineScope(Dispatchers.Main + viewModelJob)

    val allItems = itemDao.getAllItems().value


    override fun onCleared() {
        super.onCleared()
        viewModelJob.cancel()
    }

    fun onAdd(item: Item) {
        uiScope.launch {
            insertItem(item)
        }
    }

    suspend fun insertItem(item: Item) {
        withContext(Dispatchers.IO) {
            itemDao.insert(item)
        }
    }

}

• insert method called.
• insert method called with a proper feed.
• insert method is called from a non-ui thread

Answer 1

Why I cant getData from database [using allItems Property]?

You've defined an observable read operation:

@Query("SELECT * FROM check_list_table ORDER BY id DESC")
fun getAllItems():LiveData<List<Item>>

But are reading it in a direct manner:

val allItems = itemDao.getAllItems().value

This will not work because the LiveData object must be observed to actually trigger the database read.

You can either ...

Make the read operation a "one-shot" (non-observable) operation (if you only need to read the data once)

Change the DAO to just return a list of items, not a LiveData:

@Query("SELECT * FROM check_list_table ORDER BY id DESC")
suspend fun getAllItems():List<Item>

And change the ViewModel to fetch that data all at once:

viewModelScope.launch(Dispatchers.IO){ allItems = itemDao.getAllItems() }

Or make the ViewModel variable a LiveData as well and observe it in the UI (if the data can change and you want to refresh the UI as it does)

The DAO stays the same:

@Query("SELECT * FROM check_list_table ORDER BY id DESC")
fun getAllItems():LiveData<List<Item>>

The ViewModel just references the LiveData:

val allItems = itemDao.getAllItems()

And your Activity/Fragment observes the changes. Now that the LiveData is being observed, the read operation will be triggered and the data returned:

viewModel.allItems.observe(/*lifecycle*/) { items ->
    // Do something with the list of items that was read
}