RailsのmigrationでDBの種類を判定して、PostgreSQLでstring→integerへchange_columnする方法。

gistfile1.rb

def change
  adapter = ActiveRecord::Base.connection.instance_values['config'][:adapter]
  if adapter == 'postgresql'
    change_column :table_name, :field_name, 'integer USING CAST(field_name AS integer)'
  else
    change_column :table_name, :field_name, :integer
  end
end