Mongoid named scope comparing two time fields in the same document

Question

I need to create a named scope in Mongoid that compares two Time fields within the same document. Such as

scope :foo, :where => {:updated_at.gt => :checked_at}

This obviously won't work as it treats :checked_at as a symbol, not the actual field. Any suggestions on how this can be done?

Update 1

Here is my model where I have this scope declared, with a lot of extra code stripped out.

class User
  include Mongoid::Document
  include Mongoid::Paranoia
  include Mongoid::Timestamps

  field :checked_at, :type => Time

  scope :unresolved, :where => { :updated_at.gt => self.checked_at }
end

This gives me the following error:

'<class:User>': undefined method 'checked_at' for User:Class (NoMethodError)

Answer 1

As far as I know, mongodb doesn't support queries against dynamic values. But you could use a javascript function:

scope :unresolved, :where => 'this.updated_at >= this.checked_at'

To speed this up you could add an attribute like "is_unresolved" which will be set to true on update when this condition is matched ( and index that ).

Answer 2

scope :foo, :where => {:updated_at.gt => self.checked_at}

For example, this will work:

scope :foo, where(:start_date.lte=>Date.today.midnight)

Answer 3

Not sure if you'll like this method, it's not the best, but it should work.

class User
  include Mongoid::Document
  include Mongoid::Paranoia
  include Mongoid::Timestamps

  field :checked_at, :type => Time

  scope :unresolved, lambda{ |user| where(:updated_at.gt => user.checked_at) }
end

You call it with User.unresolved(my_user_object) It seems now after rereading your post that this probably won't do what you want. If this is true, then you will probably need to use MapReduce or possibly Baju's method (have not tested it)