Reversible integer to string migrations in Rails

I recently needed to convert an integer column in Rails to a string, and wanted to make sure that the migration would be reversible. I specified the up and down methods, but found that I couldn't reverse the migration because the column type couldn't be automatically cast back into an integer.

As it turns out, Rails allows us to specify how to cast the column with the using option:

def up
  change_column :users, :zip, :string
end

def down
  change_column :users, :zip, :integer, 
    using: "zip::integer"
end

This builds the sql:

ALTER TABLE users 
ALTER COLUMN zip 
TYPE integer
USING zip::integer

Good to go!