activerecord - Rails: what does schema.rb do?

Question

I used to think the db/schema.rb in a Rails project stored the database schema, so that ActiveRecord can know what table/column it has.

But earlier I surprisingly noticed that my project runs normally after I delete db/schema.rb!

So, since the Rails can work without it, what does schema.rb really do?

Answer 1

The schema.rb serves mainly two purposes:

1. It documents the final current state of the database schema. Often, especially when you have more than a couple of migrations, it's hard to deduce the schema just from the migrations alone. With a present schema.rb, you can just have a look there. ActiveRecord itself will indeed not use it. It will introspect the database during runtime as this is much safer than to expect users to keep the schema.rb up-to-date. However to avoid confusion of your developers, you should always maintain a file that is up-to-date with your migrations.

2. It is used by the tests to populate the database schema. As such a rake db:schema:dump is often run as part of the rake test:prepare run. The purpose is that the schema of the test database exactly matches the current development database.