Not able to drop foreign key constraints

I am using ruby on rails 2.3.9. The rake db:migrate passed successfully. However when I run rake db:migrate:redo to test the down part I get error message. I am using mysql with Innodb.

class AddConstraints < ActiveRecord::Migration
  def self.up
    ActiveRecord::Base.connection.execute <<-EOS
      ALTER TABLE venues
      ADD CONSTRAINT FOREIGN KEY (city_id)
        REFERENCES cities (id)
        ON DELETE restrict
        ON UPDATE cascade
    EOS
  end

  def self.down
    ActiveRecord::Base.connection.execute <<-EOS
      ALTER TABLE venues DROP FOREIGN KEY (city_id)
    EOS
  end
end

The error 开发者_如何学运维message I am getting is

You have an error in your SQL syntax; check the manual 
that corresponds to your MySQL server version for the right 
syntax to use near '(city_id)' at line 1:       
ALTER TABLE venues DROP FOREIGN KEY (city_id)

First, you should omit the parentheses after FOREIGN KEY. Second, you must tell mysql the name of the fk constraint you want to drop, not the name of the column affected by that constraint. If you run SHOW CREATE TABLE venues, you can find out the name of the constraint - probably something like venues_ibfk_1 or similar. Use this name for the ALTER TABLE statement, e.g. ALTER TABLE venues DROP FOREIGN KEY venues_ibfk_1.

Maybe remove the parentheses from the statement? A quick google showed statements without parentheses being used in the MySQL documentation.