Android Room LiveData select query parameters

Question

I decided to teach myself Java and Android by doing a simple database app. I already implemented some functionality the "lazy" way - all selects are done on the main thread.

Now I want to use LiveData for selects. I've read the simplistic training guide on android developers and implemented a more complex solution from codelabs guide, with LiveData and RecyclerView. Inserts, updates, deletes, and selects for whole tables work flawlessly, however I have no idea how to pass parameters to selects.

Example: I have an activity with scrollable list of all records and I want to apply some filters to the list (search). From what I understand the actual select method from DAO is called only once (when ViewModel is created), so how do I update query with new parameters?

Other example: I have other activity that displays all columns of a record (for viewing and editing). How do I pass id to query to select a single row?

My database code is more less the same as in this codelab

Edit: I finally did it like that: every time I want to update query parameters I call select from DAO (through ViewModel and Repo) and add a new observer to that new list. This solution doesn't seem optimal but I guess it works...

I thought of that, but let's say I want a text field in which user can type any search string — Sancho, Feb 24 '18 at 20:04
so ? whats the issue here ?? though these doesnt require a select statement. — Santanu Sur, Feb 24 '18 at 20:05
you have your list from **select all** statement...then from that whole list filter the list which the user searches...and show the searched list — Santanu Sur, Feb 24 '18 at 20:10
hmmm... so basically filter using java not sql... maybe I'll try. Thanks for the idea — Sancho, Feb 24 '18 at 20:49
yes..there are a lot of demos.. you can do whichever way you feel like.. :) it depends upon the function — Santanu Sur, Feb 24 '18 at 20:50

score 36 · Accepted Answer · edited Jun 15 '20 at 20:07

36

I think your solution is a Transformations.switchMap. The simplest example how it may works in the case from codelab

public class WordViewModel extends AndroidViewModel {
    private WordRepository mRepository;
    private LiveData<List<Word>> mAllWords;
    private LiveData<List<Word>> searchByLiveData;
    private MutableLiveData<String> filterLiveData = new MutableLiveData<>();

    public WordViewModel (Application application) {
        super(application);
        mRepository = new WordRepository(application);
        mAllWords = mRepository.getAllWords();
        searchByLiveData = Transformations.switchMap(filterLiveData, 
                                                     v -> mRepository.searchBy(v));
    }

    LiveData<List<Word>> getSearchBy() { return searchByLiveData; }
    void setFilter(String filter) { filterLiveData.setValue(filter); }
    LiveData<List<Word>> getAllWords() { return mAllWords; }
    public void insert(Word word) { mRepository.insert(word); }
}

So, when you set filter value, it will change value of searchByLiveData

I hope this helps you.

edited Jun 15 '20 at 20:07

Riot Goes Woof

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answered Apr 05 '18 at 10:01

basilkot

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Finally got some time to try your solution and it works wonderfully! Thank you so much! That's exactly what I was looking for. – Sancho Apr 26 '18 at 13:29
In repository.searchBy(v) it must be **mRepository**? – mjn42 Nov 07 '18 at 13:39
1

@mjn42 yes, i fix it. regarding the error, show me the code and perhaps i will be able to help you – basilkot Nov 07 '18 at 17:13
in the line void setFilter(String filter) { filterLiveData.value = filter; } the ".value" does not exist in filterLiveData – mjn42 Nov 08 '18 at 10:38
1

@mjn42 setValue, sure – basilkot Nov 08 '18 at 11:06
@basilkot it was helpful for me where I had to fetch data from Room based on various sort options selected by the user. Earlier, I was doing it in a very unusual way. Thanks! – Shahood ul Hassan Feb 10 '19 at 06:57
Thank you very much! This has resolved a fair bit of headache for me :) – Adi B Mar 26 '19 at 21:16
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Your code helped a lot. But there is a problem in the line : `private LiveData filterColor = new MutableLiveData();`instead use `private MutableLiveData filterColor = new MutableLiveData(); `because the **LiveData's** `setValue()` has a protected access while **MutableLiveData's** `setValue()` has a public access. – Sayok Majumder May 31 '19 at 08:00
2

have save mylife – QuestionAndroid Jun 07 '19 at 09:46

Android Room LiveData select query parameters

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