rails mongo查询基于BSON :: ObjectId外键

Question

Mongodb和rails

Parent对象has_many :children

Child有parent_id个班级BSON::ObjectId

Child.first.parent_id
=> BSON::ObjectId('59031cd92936094d04000d31') 

我找不到Parent

的_id

Parent.where(_id: '59031cd92936094d04000d31').first
 => nil 

Parent对象看起来像这样

 => #<Parent _id: 5a959865c8aedf03c1000007, _type: nil, created_at: nil, updated_at: nil, id: nil, name: nil, description: nil, type: nil, starts_at: nil, children_hash: nil>