Room "Not sure how to convert a Cursor to this method's return type": which method?

Question

Error:Not sure how to convert a Cursor to this method's return type
Error:Execution failed for task ':app:compileDebugJavaWithJavac'.
Compilation failed; see the compiler error output for details.

Using Room I'm getting this error and I'd like to find out which method causes it.

I have multiple DAOs, with approximately 60 methods in total, and this error just popped up after adding a method (copy&pasted from another one that worked perfectly, just changed the field to set).

I could post the whole class of DAOs, but I'm asking for a way to know which method failed. I tried with Run with --stacktrace, Run with --info and --debug option, but none of these show any valuable information.

The method I added is a @Query UPDATE with Int return type, as suggested in the documentation

UPDATE or DELETE queries can return void or int. If it is an int, the value is the number of rows affected by this query.

EDIT: I'd like to add that I tried deleting the method, bringing the DAO back to the working state, but it still gives me this error.

EDIT2: Adding gradle console output because unreadable in comments:

error: Not sure how to convert a Cursor to this method's return type
error: Not sure how to convert a Cursor to this method's return type
2 errors

:app:compileDebugJavaWithJavac FAILED

FAILURE: Build failed with an exception.

* What went wrong:
Execution failed for task ':app:compileDebugJavaWithJavac'.
Compilation failed; see the compiler error output for details.

* Try:
Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output.

* Get more help at https://help.gradle.org

BUILD FAILED in 22s

Answer 1

Recently I've had the same problem but I was using Coroutines within the Dao function, e.g.:

@Query("SELECT * FROM Dummy")
suspend fun get(): LiveData<List<Dummy>>

And was unable to compile, but after removing the suspend everything worked just fine. It's not needed when returning LiveData. suspend and LiveData seem not to work together (as of now).

Answer 2

I Spend the entire day on this issue. the solution was very simple. I was using something like this before

@Query("SELECT * FROM myTable")
fun getAll(): MutableLiveData<ArrayList<myData>>

Now when I changed ArrayList to List & MutableLiveData to LiveData it is working fine.

@Query("SELECT * FROM myTable")
fun getAll(): LiveData<List<myData>>

based on the answers & comments on this issue I think room support only List & LiveData because I tried with on MutableLiveData & only ArrayList too. none of the combinations worked.

Hope this will help someones few hours.

Answer 3

For anyone landing here, using a coroutine Flow as a return type, you will get this error if you accidentally make the function suspend. Since it is returning a flow, there is no need to suspend.

So instead of this:

@Query("SELECT * FROM myTable WHERE id = :id")
suspend fun findById(id: Long): Flow<MyDataType>

use this (without suspend modifier):

@Query("SELECT * FROM myTable WHERE id = :id")
fun findById(id: Long): Flow<MyDataType> 

Answer 4

Yup, based on what you have mentioned in the comments, you are not allowed to change the return type from List to anything else inside the Dao. I'd assume Room doesn't know how to deal with other return types. Take the List and cast/convert it into your desired type outside of the Dao.