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Storing Data in Android with Room

Many to Many Relationship

11/13/2018

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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

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