如果外键存在于 Rails 的 Ruby 中，我该如何删除它？

Question

ActionRecord 中有一个名为 index_exists? 的函数，但 Rails 4.2.7 中没有 foreign_key_exists?。

因此，当我在某些数据库上调用 remove_foreign_key :parties, :franchise_groups 时，它会中断。

我应该用什么？

---

更新

我的代码

class RemoveForeignKey < ActiveRecord::Migration
  def up
    if foreign_key_exists?(:parties, :franchise_groups)
      remove_foreign_key :parties, :franchise_groups
    end
  end
end

报错

== 20161107163800 RemoveForeignKey: migrating =================================
-- foreign_key_exists?(:parties, :franchise_groups)
rake aborted!
An error has occurred, all later migrations canceled:

undefined method `foreign_key_exists?' for #<RemoveForeignKey:0x00000007ea0b58>
/home/rje/.rvm/gems/ruby-2.3.0/gems/activerecord-4.2.7/lib/active_record/migration.rb:664:in `block in method_missing'

Answer 1

but no foreign_key_exists?

There is foreign_key_exists?:)

Checks to see if a foreign key exists on a table for a given foreign key definition.

# Checks to see if a foreign key exists.
  foreign_key_exists?(:accounts, :branches)

# Checks to see if a foreign key on a specified column exists. foreign_key_exists?(:accounts, column: :owner_id)

# Checks to see if a foreign key with a custom name exists. foreign_key_exists?(:accounts, name: "special_fk_name")

或者，您可以使用 foreign_keys:

if foreign_keys(:table_name).include?(foreign_key_name)
  # do stuff
end

Answer 2

我想你可以用这样的东西

def up
  remove_foreign_key :parties, column: :franchise_groups
end

def down
  add_foreign_key :parties, :franchise_groups
end

Answer 3

它适用于连接：

ActiveRecord::Base.connection.foreign_key_exists?(:parties, :franchise_groups) 

Answer 4

我的Rails版本好像没有"foreign_key_exists?"（Rails4.2.6），所以我用的是Array#any？搜索 "foreign_keys" 的结果并确定给定的外键是否存在：

foreign_keys("parties").any?{|k| k[:to_table] == "franchise_groups"}

您可以这样使用它：

if foreign_keys("parties").any?{|k| k[:to_table] == "franchise_groups"}
remove_foreign_key :parties, column: :franchise_group_id
end

Answer 5

在Rails 4上没有foreign_key_exists所以我想出了以下解决方案：

remove_foreign_key :events, column: :subscribers_selector_id if foreign_keys(:events).map(&:column).include?("subscribers_selector_id")

Answer 6

Rails 7+ if_exists / if_not_exists 选项

Rails 7 将 if_exists 选项添加到 remove_foreign_key 以便在外键已被删除时不引发错误。

Rails 7 将 if_not_exists 选项添加到 add_foreign_key 以便在已添加外键时不引发错误。

因此，迁移可以这样写：

class RemoveFranchiseGroupForeignKeysFromParties < ActiveRecord::Migration
  def up
    remove_foreign_key :parties, :franchise_groups, if_exists: true
  end

  def down
    add_foreign_key :parties, :franchise_groups, if_not_exists: true
  end
end

来源：

• Add support for if_exists/if_not_exists on remove_foreign_key/add_foreign_key.
• remove_foreign_key docs.
• add_foreign_key docs.