使用MongoMapper，如何查找另一个表中不存在ID的记录？

Question

I have two models, User and Class. In MySQL, I could find users not in a class with something like:

SELECT * FROM Users WHERE id NOT IN (SELECT user_id FROM Classes)

How can I do something similar with MongoMapper? I've been able to in the Mongo console with:

db.users.find({user_id:{$ne:db.classes.find({}, {user_id:1})}});

but I can't figure out the syntax using MongoMapper.

Answer 1

The Mongo snippet you posted is two queries. The MongoMapper equivalent is:

classy_users_ids = MyClass.fields(:user_id).find_each.map(&:user_id).uniq
classless_users  = User.where(:id.nin => classy_users_ids)

If you have a lot of users, the first query might be more efficient if you skip the conversion to MongoMapper::Document 's with the following:

classy_users_ids = MyClass.collection.distinct(:user_id)

Answer 2

如果它们的关系类似于Class ... has_many :users ，则可以直接与user_ids进行匹配：

Class.where(:user_ids.ne => user.id)