Environment
mongodb 2.2.3
mongoid 3.1.2
ruby 1.9.3

Structure

class Event
  has_many :children, inverse_of: :parent, class_name: "Event", order: {sequence: 1}
  belongs_to :parent, inverse_of: :children, class_name: "Event", index: true

  field :name, type: String
end
class Activity < Event
class Branch < Event
# There are some other sub-classes of this Event class similar to Activity and Branch.

Now i want to load all events with the given name and eager load their children.

The query is

Event.where(name: 'test').includes(:children).each do |event|
   puts event.children.all.flatten
end

I see the following queries in the moped debug output

database=uat collection=events selector={"name"=>"test"} flags=[] limit=0 skip=0 batch_size=nil fields=nil
database=uat collection=events selector={"parent_id"=>{"$in"=>["7723d05b-d866-4153-bda4-48bcc20d7071"]}} flags=[] limit=0 skip=0 batch_size=nil fields=nil

Till now, it makes complete sense, but then mongoid runs one more query to fetch the children again
database=uat collection=events selector={"parent_id"=>{"$in"=>["7723d05b-d866-4153-bda4-48bcc20d7071"]}} flags=[] limit=0 skip=0 batch_size=nil fields=nil

I guarantee that identity map is enabled in my environment (the following steps would confirm).

So i jumped into the MONGOID gem and added a bunch of print statements to debug this. I found the difference was in these two methods IdentityMap.set_many and IdentityMap.get_many.

Inside set_many
In particular, when mongoid puts the information in identity map, the selector looks like this
{:_type=>

, \"parent_id\"=>\"7723d05b-d866-4153-bda4-48bcc20d7071\"}"
If you see closely, the selector includes the type array.

Inside get_many
However, when mongoid tries to fetch the data from the identity map, the identifier looks like this

"

The type field is not included in the search and hence Identity map returns nil. I am not sure how to fix this but any help would be very useful.

Thank you,
Naren