Overview

In this lesson, we'll discuss what a many-to-many relationship is, and show how we can model a many-to-many relationship using Room.

We'll create a many-to-many relationship between Users and Tasks. We'll walk through all of the steps required to create the entity and queries for adding, removing and querying a many-to-many relationship in the database.

Summary of Content:

• What a many-to-many relationship is and how they work
• What SQL join table is and why we need it
• Creating an entity for the join table
• Which primary and foreign keys we'll need for the join table entity
• How to insert into the join table, so that we can assign a task to a user
• Use of OnConflictStrategy to prevent adding duplicate data
• How to query for all tasks, which also retrieves all of the assigned users
• How to query for all users, which also retrieves all of the assigned tasks
• How to join across multiple tables to retrieve all the data required
• How to model the types that will be retrieved from the database, using @Embedded annotation
• How to transform the return type into something easier to work with
• How to convert null relations into an empty list so we don't have to think about nullability

Links:

• Android Training - Room
• Referencing complex data using Room
• OnConflictStrategy
• Embedded annotation