require 'mongoid'
Mongoid.configure ...

class Graph
include Mongoid::Document
has_many :people
has_many :organizations
end

class Entity
include Mongoid::Document
field :key, type: String
belongs_to :graph
end

class Person < Entity
 # Optional doesn't matter I promise, it's just to make for less typing
belongs_to :organization, optional: true
end

class Organization < Entity
has_many :people
end
 # Just helpful shorthand
class Mongoid::Criteria
def aggregate
collection.aggregate pipeline
end
end

Graph.destroy_all
Entity.destroy_all

g = Graph.create
o1 = Organization.create key: "first organization", graph: g
o2 = Organization.create key: "second organization", graph: g
p1 = Person.create key: "first person", graph: g # organization: o1
p2 = Person.create key: "second person", graph: g #, organization: o2

puts g.people.aggregate.count # => 4 - WRONG
puts g.people.project(id: 1, _type: 1).aggregate.count # => 4 - WRONG
puts g.people.count # => 2 - CORRECT!
puts g.people.project(id: 1, _type: 1).aggregate.count # => 2 - NOW IT'S CORRECT!
puts g.people.aggregate.count # => 4 - STILL WRONG THO