How to get a specific Entity in Room

Question

So, I'm using Room database to store courses and I'm stuck on the method that returns the course with the name(course) that I want because it's always returning null. I have diminished my database to have 2 courses with the course variable as:

As you can see in the picture above, when I try to get the CourseEnt in the Repository with course = fun, which I can see below that it exists, it returns a LiveData with a null value instead of the CourseEnt that I wanted.

Any idea on what I'm doing wrong or on what should I look into with debugger?

Here's the code:

Entity:

@Entity(tableName = "courses_table")
data class CoursesEnt (@PrimaryKey val course: String, 
                                   val location: String, 
                                   val description: String,                           
                                   val difficulty: Double, 
                                   val distance: Double, 
                                   val photos: ListInt, 
                                   val category: String, 
                                   val activities: ListString)//ListString is a type converter that converts a String into a List<String> and vice-versa

DAO:

@Dao
interface CoursesDao {

   @Query("SELECT * from courses_table ORDER BY course ASC")
    fun getAllCourses(): LiveData<List<CoursesEnt>>

   @Query("SELECT * FROM courses_table WHERE course LIKE :str")
    fun getCourse(str: String):LiveData<CoursesEnt>

   ...
}

Repository:

class CoursesRepository(private val coursesDao: CoursesDao){

    val allCourses: LiveData<List<CoursesEnt>> = coursesDao.getAllCourses()
    var singleCourse: LiveData<CoursesEnt> = coursesDao.getCourse("")

    @WorkerThread
    fun getCourse(str: String) {

        singleCourse = coursesDao.getCourse(str)
    }

    ...
}

Are you observing the new `LiveData` instance that you're putting into the `singleCourse` property with the `getCourse` method? If you start observation before that method runs, it could be that you're still observing the old `LiveData` instance that queried for an empty string. As a debugging step, I'd try fetching the course in a direct, blocking way (i.e. with an additional method that returns `CoursesEnt?`) inside `getCourse` and looking at the value that you get that way. — zsmb13, Jul 07 '19 at 13:31
Yes, I am observing the LiveData on my Fragment and I do it after I use getCourse. Regarding the debugging step, as I said in my question what gets returned as null is ```coursesDao.getCourse(str).value``` that I call in my repository, which should return a ```CoursesEnt``` — Migaloco, Jul 07 '19 at 14:55

score 0 · Answer 1 · answered Aug 06 '19 at 05:05

Read documentation, liveData will always return null for straight call, you have to observe LiveData, the result value from Room will be in block. This is some example of usage

mViewModel.getAllUsers().observe( this@YourActivity, Observer {
        // it - is all users from DB
    })

but if you will call

val users = mViewModel.getAllUsers()

the result will be null

How to get a specific Entity in Room

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