Android room compiler error when trying to return a list of own objects in the DAO: incompatible types: <null> cannot be converted to int - sql

Question

I am using room for database operations. I have a class TableQuestion which holds a string and a id and I have a class TableAnswer which holds a string, an id plus the id of the question where it is refering to. QuizTask brings together the Question with all its answers. The query getQuestionsWithAnswer should return a QuizTask which wraps the question with all its answers. The error metioned in the title happens in auto-generated code of room.
The relevant part of the interface:
#android.arch.persistence.room.Dao
public interface dbDao {
#Transaction
#Query("SELECT table_question.question, table_answer.answer FROM table_question, table_answer WHERE table_question.id = table_answer.id_question")
LiveData<List<QuizTask>> getQuestionsWithAnswers();
}
Class TableQuestion:
#Entity(foreignKeys = #ForeignKey(entity = TableQuestionnaire.class,
parentColumns = "id",
childColumns = "id_questionnaire",
onDelete = CASCADE),
tableName = "table_question")
public class TableQuestion {
#PrimaryKey
public final int id;
#NonNull
public String question;
#NonNull
public int id_questionnaire;
public String subject;
public String category;
public String sub_category;
#Ignore
public String questionnaire;
public TableQuestion(int id, #NonNull String question, int id_questionnaire, String subject, String category, String sub_category) {
this.id = id;
this.question = question;
this.id_questionnaire = id_questionnaire;
this.questionnaire = null;
this.subject = subject;
this.category = category;
this.sub_category = sub_category;
}
public void setQuestionnaire(String questionnaire){
this.questionnaire = questionnaire;
}
}
Class TableAnswer:
#Entity(foreignKeys = #ForeignKey(entity = TableQuestion.class,
parentColumns = "id",
childColumns = "id_question",
onDelete = CASCADE),
tableName = "table_answer")
public class TableAnswer {
#PrimaryKey
public final int id;
#NonNull
public String answer;
#NonNull
public final int id_question;
public boolean rightAnswer;
public TableAnswer(int id, String answer, int id_question, boolean rightAnswer) {
this.id = id;
this.answer = answer;
this.id_question = id_question;
this.rightAnswer = rightAnswer;
}
}
Class QuizTask:
public class QuizTask {
#Embedded
private TableQuestion question;
#Relation(parentColumn = "id", entityColumn = "id_question")
private List<TableAnswer> answers;
public void setQuestion(TableQuestion question){ this.question = question; }
public TableQuestion getQuestion(){
return question;
}
public void setAnswers(List<TableAnswer> answers) { this.answers = answers; }
public List<TableAnswer> getAnswers() {
return answers;
}
}
AndroidStudio doesn't show any error upon compilation. When room auto-generates the code for getQuestionWithAnswers it shows a compiler error "incompatible types: cannot be converted to int". In the auto-generated dbDao_Impl.java is a row where a TableQuestion object is tried to create but with null for the id parameter. That's where the error occurs. What do I have to change?

Answer 1

I found the issue:
#Query("SELECT table_question.question, table_answer.answer FROM table_question, table_answer WHERE table_question.id = table_answer.id_question")
There is no id selected but used later on. That's why room tries to create objects with Id = null which is not possible. So simply change to:
#Query("SELECT * FROM table_question, table_answer WHERE table_question.id = table_answer.id_question")