add_column for references (Rails)

I have the following Rails migration which works perfectly (irrelevant pieces removed):

create_table :comments do |t|
  t.text :body
  t.references :post
end

Now I'd like to add an author column to my comments table (which is the userid of a user), but I have no idea how to do it (I'm tempted to just write the MySql-specific syntax using an execute).

I've been looking at add_column here which doesn't mention references. I've actually found TableDefinition#references but I have no idea how to use it with an add_column statement.

Is this possible? Also, is it true that, for MySql, the "references" functionality does not actually establish relationships between the tables?

While it's too late to get any points out of this, I thought I'd post the best way for posterity :)

use change_table instead of create_table to add columns to a table that already exists, with all the TableDefinition goodness:

self.up do
  change_table :comments do |t|
    t.references :author
  end
end

This might seem trivial, but other gems like Devise make heavy use of their own custom table definitions, and this way you can still use them.

add_reference :table_name, :reference, index: true

Finally got it

add_column :locations, :state_id , :integer, :references => "states"

First, do:

script/generate migration AddAuthorIdToComments

Open the generated file and add this line:

add_column :comments, :author_id, :integer

Then in your model files:

class User < ActiveRecord::Base
  has_many :comments, :foreign_key => "author_id"
end

class Comment
  belongs_to :author, :class_name => User
end

It's been a while since I've looked at this, but last I checked migrations don't support creating foreign keys. Fortunately, however, there is a plug-in for it. I've used this and it works well.

You could add the column by add_column(:table, :column_name, :type, :options) in a new Migration.