Rails Migration: Remove constraint

In Rails 4+ in order to remove not-null constraint, you can use change_column_null:

change_column_null :users, :address, true

Not sure you can call t.address? Anyway... I would use change_column like so

change_column :users, :address, :string, :null => true

Docs... http://apidock.com/rails/ActiveRecord/ConnectionAdapters/SchemaStatements/change_column