ruby
require "spec_helper"

module Test_2812
  class Sandwich
    include Mongoid::Document
    has_and_belongs_to_many :meats
  end

  class Meat
    include Mongoid::Document
    has_and_belongs_to_many :sandwiches
  end
end

describe "2812" do

  it "issue" do
    # create a meats collection, without it, no error
    Test_2812::Meat.create!

    sandwich = Test_2812::Sandwich.create!
    sandwich.destroy
    sandwich.meats.count.should == 0
  end

end

  1) 2812 issue
     Failure/Error: sandwich.meats.count.should == 0
     Moped::Errors::OperationFailure:
       The operation: #<Moped::Protocol::Command
         @length=124
         @request_id=8
         @response_to=0
         @op_code=2004
         @flags=[]
         @full_collection_name="mongoid_test.$cmd"
         @skip=0
         @limit=-1
         @selector={:count=>"test_2812_meats", :query=>{"$and"=>[{"_id"=>{"$in"=>nil}}]}}
         @fields=nil>
       failed with error 12580: "exception: invalid query"
       
       See https://github.com/mongodb/mongo/blob/master/docs/errors.md
       for details about this error.
     # ./lib/mongoid/contextual/mongo.rb:54:in `count'
     # ./lib/mongoid/contextual.rb:19:in `count'
     # ./lib/mongoid/relations/referenced/many.rb:10:in `count'
     # ./spec/dblock/2812_spec.rb:23:in `block (2 levels) in <top (required)>'