Schema Manipulation Outside Migrations
All the migration methods described so far in this chapter are also available as methods on Active Record connection objects and so are accessible within the models, views, and controllers of a Rails application.
For example, you might have discovered that a particular long-running report runs a lot faster if the orders table has an index on the city column. However, that index isn’t needed during the day-to-day running of the application, and tests have shown that maintaining it slows the application appreciably.
Let’s write a method that creates the index, runs a block of code, and then drops the index. This could be a private method in the model or could be implemented in a library:
| | def run_with_index(*columns) |
| | connection.add_index(:orders, *columns) |
| | begin |
| | yield |
| | ensure |
| | connection.remove_index(:orders, *columns) |
| | end |
| | end |
The statistics-gathering method in the model can use this as follows:
| | def get_city_statistics |
| | run_with_index(:city) do |
| | # .. calculate stats |
| | end |
| | end |
What We Just Did
While we had been informally using migrations throughout the development of the Depot application and even into deployment, in this chapter we saw how migrations are the basis for a principled and disciplined approach to configuration management of the schema for your database.
You learned how to create, rename, and delete columns and tables; to manage indices and keys; to apply and back out entire sets of changes; and even to add your own custom SQL into the mix, all in a completely reproducible manner.
At this point we’ve covered the externals of Rails. The next chapter is going to show a few more involved ways of customizing Rails to demonstrate just how flexible Rails can be when you need it. We’ll see how to use RSpec for testing, use Slim instead of ERB for templating, and how to use Webpack to manage your CSS.