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On Rails 4.0.0.rc1, Ruby 2.0.0, after I run a migration, I see the following error when I try to run a test through rspec:
/Users/peeja/.rbenv/versions/2.0.0-p0/lib/ruby/gems/2.0.0/gems/activerecord-4.0.0.rc1/lib/active_record/migration.rb:376:in `check_pending!': Migrations are pending; run 'rake db:migrate RAILS_ENV=test' to resolve this issue. (ActiveRecord::PendingMigrationError)
That doesn't seem right. No one migrates their test database, do they? They db:test:prepare it, which—to be fair—I've forgotten to do. So I run rake db:test:prepare and run my rspec command again…and see the same error.
If I actually rake db:migrate RAILS_ENV=test, the error does in fact go away.
What's going on? Is this new in Rails 4?
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Basically it handles cloning the database so you don't have to run the migrations against test to update the test database. rake db:test:prepare is now deprecated. above url is dead, this would be the new one github.com/rails/rails/blob/4-1-stable/activerecord/…
A migration means that you move from the current version to a newer version (as is said in the first answer). Using rake db:migrate you can apply any new changes to your schema. But if you want to rollback to a previous migration you can use rake db:rollback to nullify your new changes if they are incorrectly defined.
Migrations are a convenient way to alter your database schema over time in a consistent way. They use a Ruby DSL so that you don't have to write SQL by hand, allowing your schema and changes to be database independent. You can think of each migration as being a new 'version' of the database.
As of Rails 4.1, the rake db:test:* tasks are deprecated. Instead, your (test|spec)_helper.rb should include:
ActiveRecord::Migration.maintain_test_schema! This means that your test database will get the correct schema every time your tests run, whether you run them from a Rake task or not.
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