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Rails: what does schema.rb do?
The schema.rb servers mainly two purposes:
- 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 deduct the schema just from the migrations alone. With a present
schema.rb, you can just have a look there. ActiveRecord it self will indeed not use it. It will introspect the database during runtime as this is much safer than to expect users to keep theschema.rbup-to-date. However to avoid confusion of your developers, you should always maintain a file that is up-do-date with your migrations. - It is used by the tests to populate the database schema. As such a
rake db:schema:dumpis often run as part of thetest:preparerun. The purpose is that the schema of the test database exactly matches the current development database.
From Rails doc
6.1 What are Schema Files for?
Migrations, mighty as they may be, are not the authoritative source for your database schema. Your database remains the authoritative source. By default, Rails generates db/schema.rb which attempts to capture the current state of your database schema.
It tends to be faster and less error prone to create a new instance of your application's database by loading the schema file via bin/rails db:schema:load than it is to replay the entire migration history. Old migrations may fail to apply correctly if those migrations use changing external dependencies or rely on application code which evolves separately from your migrations.
Schema files are also useful if you want a quick look at what attributes an Active Record object has. This information is not in the model's code and is frequently spread across several migrations, but the information is nicely summed up in the schema file.