Error in Room Query to Return Subset of Columns

Question

I was playing with Room, where I couldn't find some solution to my queries.

Below is the data.

Table

CREATE TABLE `Employee` (
    `id` INTEGER NOT NULL,
    `first_name` TEXT,
    `last_name` TEXT,
    PRIMARY KEY(`id`)
);

Table Data

Entity

@Entity(tableName = "Employee")
public class Employee {

    @PrimaryKey
    private int id;

    @ColumnInfo(name = "first_name")
    private String firstName;

    @ColumnInfo(name = "last_name")
    private String lastName;

    ..Getters & Setters..
}

Query 1

@Query("Select * from Employee")
List<Employee> getEmployees();

Result Its successfull

Query 2

@Query("Select first_name, last_name from Employee")
List<Employee> getEmployees();

Result

error: The columns returned by the query does not have the fields [id] in ***.Employee even though they are annotated as non-null or primitive. Columns returned by the query: [first_name, last_name]

If I add id to above Query 2, it works.

Same goes, if we have a Foreign Key in the Table and we try to Query Subset of Columns, it throws Error. The Error goes when we add both Primary Key & Foreign Key Column in the Query.

Question 1 Does that mean we have to always include Primary Key & Foreign Key (if present) in a Query ?

Question 2 What actually happens under the hood that it throws such error ? Or Am I doing anything wrong ?

Room Version 1.1.1

Also, referred this link but it doesn't solve my issue with Primary Keys.

Answer 1

To select data from multiple fields consider below example.

From the docs

Room allows you to return any Java-based object from your queries as long as the set of result columns can be mapped into the returned object. For example, you can create the following plain old Java-based object (POJO) to fetch the user's first name and last name:

public class NameTuple {
    @ColumnInfo(name = "first_name")
    public String firstName;

    @ColumnInfo(name = "last_name")
    public String lastName;
}

Now, you can use this POJO in your query method:

@Dao
public interface MyDao {
    @Query("SELECT first_name, last_name FROM user")
    public List<NameTuple> loadFullName();
}