无法销毁多对多关系中的记录

我是 Rails 新手，所以我确信我犯了一个简单的错误。

我在两个模型之间建立了多对多关系：User and Group。它们通过连接模型连接GroupMember.

这是我的模型（删除了不相关的内容）：

class User < ActiveRecord::Base
  has_many :group_members
  has_many :groups, :through => :group_members
end

class GroupMember < ActiveRecord::Base
  belongs_to :group
  belongs_to :user
end

class Group < ActiveRecord::Base
  has_many :group_members
  has_many :users, :through => :group_members
end

该表为GroupMembers包含有关关系的附加信息，因此我没有使用has_and_belongs_to_many（根据 Rails“活动记录关联”指南）。

我遇到的问题是我无法摧毁一个GroupMember.

这是 Rails 控制台的输出：

irb(main):006:0> m = GroupMember.new
=> #<GroupMember group_id: nil, user_id: nil, active: nil, created_at: nil, updated_at: nil>
irb(main):007:0> m.group_id =1
=> 1
irb(main):008:0> m.user_id = 16
=> 16
irb(main):009:0> m.save
=> true
irb(main):010:0> m.destroy
NoMethodError: undefined method `eq' for nil:NilClass
    from /usr/local/lib/ruby/gems/1.8/gems/activesupport-3.0.4/lib/active_support/whiny_nil.rb:48:in `method_missing'
    from /usr/local/lib/ruby/gems/1.8/gems/activerecord-3.0.4/lib/active_record/persistence.rb:79:in `destroy'
    from /usr/local/lib/ruby/gems/1.8/gems/activerecord-3.0.4/lib/active_record/locking/optimistic.rb:110:in `destroy'
    from /usr/local/lib/ruby/gems/1.8/gems/activerecord-3.0.4/lib/active_record/callbacks.rb:260:in `destroy'
    from /usr/local/lib/ruby/gems/1.8/gems/activesupport-3.0.4/lib/active_support/callbacks.rb:413:in `_run_destroy_callbacks'
    from /usr/local/lib/ruby/gems/1.8/gems/activerecord-3.0.4/lib/active_record/callbacks.rb:260:in `destroy'
    from /usr/local/lib/ruby/gems/1.8/gems/activerecord-3.0.4/lib/active_record/transactions.rb:235:in `destroy'
    from /usr/local/lib/ruby/gems/1.8/gems/activerecord-3.0.4/lib/active_record/transactions.rb:292:in `with_transaction_returning_status'
    from /usr/local/lib/ruby/gems/1.8/gems/activerecord-3.0.4/lib/active_record/connection_adapters/abstract/database_statements.rb:139:in `transaction'
    from /usr/local/lib/ruby/gems/1.8/gems/activerecord-3.0.4/lib/active_record/transactions.rb:207:in `transaction'
    from /usr/local/lib/ruby/gems/1.8/gems/activerecord-3.0.4/lib/active_record/transactions.rb:290:in `with_transaction_returning_status'
    from /usr/local/lib/ruby/gems/1.8/gems/activerecord-3.0.4/lib/active_record/transactions.rb:235:in `destroy'
    from (irb):10

这让我发疯，所以任何帮助将不胜感激。

当您从 HABTM 关系切换到 has_many :through 关系时，您可能忘记添加回 id 列。 Active record 需要 GroupMember 有一个 id，.destroy 才能像这样工作。

寻找:id => false在你的迁移中并摆脱它。然后重做迁移。

希望有帮助。