Hands-On Android UI Development

Creating the Data Access Layer

Now that you have something to write into the database, you need some way to actually write it, and a way to retrieve it again. The most common pattern is to have a dedicated class to deal with this for each class--a Data Access Object class, also known as a DAO. In Room, however, all you have to do is declare what they should look like using an interface; Room will write the implementation code for you. You define your queries using the @Query annotation on a method, like this:

@Query(“SELECT * FROM users WHERE _id = :id”)public User selectById(long id);

This has a huge advantage over traditional O/R mapping layers in that you can still write any form of SQL query, and let Room figure out how to convert it ...