Using Mongoid's scoping mechanisms can really add up... not always in a good way.

mongoid_scope_building.rb

# Example implementation, ignoring the less important details:
class MyTree
 include Mongoid::Document
 include Mongoid::Tree

 referenced_in :account, :inverse_of => :trees
 field :slug

 scope :slugged, lambda { |s, options={}| where(options.merge(:slug => s)).first }

 def self.with_full_path(path)
   parts = path.split('/')

   # Any query after this using "where" or a scope includes :parent_ids=>{"$size"=>1} in the Mongo query:
   folder = slugged(parts.shift, :parent_ids.size => 1)

   parts.inject(folder) do |parent, part|
     # Don't want the :parent_ids.size part here, but it's being included
     slugged(part, :parent_id => parent.id)
   end
 end
end


# Called like this:
acct = Account.first
acct.trees.with_full_path("first/second")


# Mongoid::Finders has the following, which I think is the cause of the issue, since nothing ever "unbuilds" my criteria 'where' addition:
module Mongoid
  module Finders
    # Initializes and returns the current scope stack.
    def scope_stack
      scope_stack_for = Thread.current[:mongoid_scope_stack] ||= {}
      scope_stack_for[object_id] ||= []
    end
  end
end