200 Migrations [2d]

Reading

• Read the Rails Guide about Active Record migrations Show archive.org snapshot
• Understand why we never use models in migrations.
• Checkout the repository Project Hero and read through some migrations in db/migrate. Find two complex migrations and discuss them with your mentor.

  Tip

  Find the largest files in the db/migrate folder.

• Read and understand How to write complex migrations in Rails

Discuss with your mentor

• Instead of migrations, could we simply log into the production server's SQL console and alter tables there whenever we need a change?
• How do we need to work with migrations when we change feature branches?

Exercise

In MovieDB, add two fields:

• Actor#total_movies: This field should cache the total number of movies an actor stars in
• Actor#first_movie_id: This field should cache the first movie to which the actor starred in. The first movie is the movie with the lowest #created_at timestamp.

Add callbacks Show archive.org snapshot to your ActiveRecord models so the field is always up to date. For the purpose of this exercise, please don't use the :counter_cache option and do everything manually instead.

Whenever you add a new column you need to take care of existing records. Add a migration that sets the total_movies and first_movie_id column for existing records. Write this migration in multiple styles:

• Embedding ActiveRecord models into your migration
• SQL statements (update "UPDATE actors SET ...")