Specifying Column Order in ActiveRecord Migrations

ActiveRecord migrations don't allow you to control column order when adding or changing columns. You can always just

execute "ALTER TABLE ..." # but it's ugly and hard to remember the details.

If you're willing to give up vendor neutrality, you can have the feature in pretty-Ruby-migration syntax with a small effort. Here's an implementation for MySQL that adds options :first and :after to add_column and change_column.

ActiveRecord::ConnectionAdapters::MysqlAdapter.class_eval do
  def add_column_options!(sql, options)
    super
    if options[:after]
      sql << " AFTER #{quote_column_name(options[:after])}"
    elsif options[:first]
      sql << " FIRST"
    end
  end
end

(It's also available in this gist if you'd like to suggest a cleaner way.)

With that in place,

a.add_column :foos, :bar, :string, :after => :baz

will execute

ALTER TABLE `foos` ADD `bar` varchar(255) AFTER `baz`

and

a.add_column :foos, :bar, :string, :first => true

will execute

ALTER TABLE `foos` ADD `bar` varchar(255) FIRST

Love that Ruby!

Now this is a monkey-patch, but as monkey-patches go it's pretty innocuous. Rather than redefining a method that's already implemented in MysqlAdapter (and as with many monkey-patches copy-pasting the existing implementation so we can modify it), we're defining a new override of a method (albeit undocumented) inherited from the SchemaStatements module (via AbstractAdapter). If you wanted to, you could subclass MysqlAdapter and then configure Rails to use your own adapter, but I suspect that would be more expensive to own than just adding this method to MysqlAdapter. (Beware: there are many useful Rails-provided rake tasks that look at your adapter string to decide whether to be useful or to spit in your face.)