Question

我在rails 2.3.9上使用ruby。 rake db：migrate成功通过。但是，当我运行rake db：migrate：redo来测试向下部分时，我收到错误消息。我正在使用Innodb的mysql。

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

我收到的错误消息是

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)

Answer 1

首先，您应该在FOREIGN KEY之后省略括号。其次，您必须告诉mysql要删除的fk约束的名称，而不是受该约束影响的列的名称。如果您运行SHOW CREATE TABLE venues，则可以找到约束的名称 - 可能类似于venues_ibfk_1或类似名称。将此名称用于ALTER TABLE语句，例如ALTER TABLE venues DROP FOREIGN KEY venues_ibfk_1。

Answer 2

也许从声明中删除括号？快速谷歌显示MySQL文档中没有使用括号的语句。

无法删除外键约束

2 个答案: