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How to get values of LiveData from repository in viewmodel?

Quiz app, categories have questions and questions have answers.
I have two queries in the DAO.
In the first one, I get all the categories from the data:
#Query("SELECT * from category_table ORDER BY category_id")
fun getAllCategories(): LiveData<List<Category>>
In the second one, I get a list of questions and answers by category id:
#Query("SELECT * FROM question_table WHERE parent_category_id = :categoryId ")
fun getQuestionsWithAnswersByCategoryId(categoryId: Long): LiveData<List<QuestionWithAnswers>>
Repository:
val getAllCategories: LiveData<List<Category>> = quizDao.getAllCategories()
fun getQuestionsWithAnswersByCategoryId(id: Long): LiveData<List<QuestionWithAnswers>> {
return quizDao.getQuestionWithAnswers(id)
}
This is my viewmodel:
val getAllCategories: LiveData<List<Category>>
var questionById: LiveData<List<QuestionWithAnswers>>
init {
getAllCategories = repository.getAllCategories
questionByCategoryId = repository.getQuestionWithAnswersByCategoryId(???????)
}
The problem is that I don't know the categories' id beforehand and need to obtain them from the database.
when I try to obtain a category id in the viewmodel like this:
var categoryId = getAllCategories.value[0].categoryId
it returns null.
Is there any way to obtain categories id which wrapped in livedata inside a viewmodel?

How do I pass my Data object to DAO and access all data object properties in Query?

I have simple data object that I want to insert into room database but as I am using auto increment on my primary key I am doing it as below
#Dao
interface T1Dao {
#Query("INSERT INTO tbl_t1(data1, data2) VALUES ( :T1.data1, :T1.data2) ")
fun insert(note: T1): Long
}
I have many properties in T1 so I don't want pass them separately if possible.
In above example I am just showing two properties.

But you can just use #Insert and not to set your primary key field, can't you?
#Insert
fun insert(note: T1): Long
Let's say you have T1 class:
#Entity
data class T1(
#PrimaryKey(autoGenerate = true)
val id: Int = 0, // This lets you not to set id before inserting
val data1: String,
val data2: String
)
Then you can insert:
dao.insert(T1(data1 = "data1", data2 = "data2")) // just don't set id

One-shot request single result in Room Android

I am practicing my android skills (beginner) by coding a grocery list app. I have two tables in my db, a shopping_item table (The items I want to buy) and a reference_item table (The items I know the category and the unit price). Each time I add a shopping item, there is an refId field referencing to the reference item id corresponding. It is a default value to a default reference item if the shopping item is not referenced yet.
I use a MVVM model. I then have a DAO, a repository, a viewModel and my fragments that display data.
When I add a new shopping item, I want to know if there is a corresponding reference item. I want to do the following Query:
#Query(value = "SELECT refId FROM reference_items WHERE reference_item_name = :refName")
suspend fun getRefItem(refName : String) : Int
It returns the id of the reference item corresponding as an Int or is null if it is not referenced yet. In my repository, I have a function like that:
suspend fun getRefItem(refName : String) = db.getShoppingDao().getRefItem(refName)
For now, I think I am doing alright. No mistake in sight I guess.
The problem begin when I try to implement my viewModel. What should I do? What about my fragment?
I have a addNewItem(name: String, amount: Int) function in my fragment to add the new item. I can find the reference item corresponding with the name provided.
I tried multiple things, using LiveData, suspend functions, mutableLiveData/LiveData, but I am getting lost right now. Every tutorials or examples use LiveData or Query all data from the db. I just want one Integer, one Time, no need of LiveData I think.

here is the complete solution. Hope this is useful for you.
DAO
#Query(value = "SELECT refId FROM reference_items WHERE reference_item_name = :refName")
suspend fun getRefItem(refName : String) : Int
Repository
// Specify return datatype as Int
suspend fun getRefItem(refName : String): Int = db.getShoppingDao().getRefItem(refName)
ViewModel
fun getRefItem(name: String): LiveData<Int> {
val result : MutableLiveData<Int>() <-- setup livedata to return as value
viewModelScope.lanuch {
result.postValue(repository.getRefItem(name))
}
return result <-- return livedata
}
Fragment
fun addNewItem(name: String, amount: Int) {
// setup viewModel observer
viewModel.getRefItem(name).observer { viewLifecycleOwner, { value ->
// GET YOUR INT VALUE HERE
Log.i("VALUE", value)
}
}
}

Access and Delete a row in one query in SQLite

I am using Room Database to make a database to store information in a table. I want to access one entry from the table and delete the same entry without the need to call two functions.
#Query("SELECT * FROM history_packet_table ORDER BY timestamp ASC LIMIT 1")
fun get(): HistoryPacket?
#Query("DELETE FROM history_packet_table ORDER BY timestamp ASC LIMIT 1")
fun delete()
I want these two operations to happen only by calling get. Is there a way?

I believe that you can add the following to the Dao :-
#Transaction
fun getAndDelete() {
get()
delete()
}
Obviously you can call the function what you wish. However, the get seems to be useless as it is.
So you may want something like :-
#Query("SELECT * FROM history_packet_table WHERE timestamp = (SELECT min(timestamp) FROM history_packet_table)")
fun get() :HistoryPacketTable
#Query("DELETE FROM history_packet_table WHERE timestamp = (SELECT min(timestamp) FROM history_packet_table)")
fun delete() :Int
#Transaction
fun getAndDelete() :HistoryPacketTable {
// Anything inside this method runs in a single transaction.
var rv: HistoryPacketTable = get()
val rowsDeleted: Int = delete()
if (rowsDeleted < 1) {
rv = HistoryPacketTable();
//....... set values of rv to indicate not deleted if needed
}
return rv
}
Note as LIMIT on delete is turned off by default, the queries can be as above, this assumes that timestamp is unique otherwise multiple rows may be deleted, in which case the Dao could be something like
:-
#Delete
fun delete(historyPacketTable: HistoryPacketTable) :Int
#Transaction
fun getAndDelete() :HistoryPacketTable {
// Anything inside this method runs in a single transaction.
var rv: HistoryPacketTable = get()
val rowsDeleted: Int = delete(rv)
if (rowsDeleted < 1) {
rv = HistoryPacketTable();
//....... set values to indicate not deleted
}
return rv
}

Value emit even if data isn't updated

I try yo use Room with Kotlin and RxJava 2.
I have an entity with a string primary key and I would like to be notified when values are updated but I receive new value from flowable even if the value isn't updated when I had whatever value.
Entity :
#Entity
class MarketSummaryModel(#PrimaryKey var marketName: String)...
Dao :
#Dao
interface MarketSummaryDao {
#Insert(onConflict = OnConflictStrategy.REPLACE)
fun insert(marketSummaryModel: MarketSummaryModel)
#Query("select * from MarketSummaryModel where marketName = :marketName LIMIT 1")
fun getByName(marketName: String): Flowable<MarketSummaryModel>
}