add_column for references (Rails)

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"

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