Room query returns null value

问题

This is my query:

and here, when I retrive it, the value is null :

I have debugged my database and the data is there:

This is my entity object :

I have investigated the database query, but couldn't figure what's wrong

回答1:

Your Query returns a LiveData. You will have to observe the changes. Initially, the value is null before the query. After the query, the list dataset changes and it will return you the desired correct values.

Assuming ViewModel class is something like this:

class MyViewModel:ViewModel(){
    private val gymDao: GymDao
    private val listLiveData: LiveData<List<Country>>

    init {
        val roomDatabase = myRoomDatabase.getDatabase(application)
        gymDao= roomDatabase ?.gymDao()!!
        listLiveData = gymDao?.getCountriesList()
    }

    fun getAllCountries(): LiveData<List<Country>> {
        return listLiveData
    }
}

In your Activity class, you can observe the list as:

class MyActivity : AppCompatActivity() {
    private lateinit var vm: MyViewModel

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.my_activity)

        vm = ViewModelProviders.of(this).get(MyViewModel::class.java)

        vm.getAllCountries().observe(this, Observer { items ->
            if (!items.isEmpty()) {
                Log.d(TAG, "ITEMS: $items")
            }

            Log.d(TAG, "ITEMS: $items")
        })
    }
}

来源：https://stackoverflow.com/questions/61914751/room-query-returns-null-value