I've been struggling with this for most of today Here's finally a simple repro.

This repro spec in https://github.com/dblock/mongoid/commit/3cb08404f5c3b2b2611d32c6214556e2d637592e, run rspec spec/functional/mongoid/relations/referenced/many_to_many_spec.rb:213.

Given

class Exhibitor
include Mongoid::Document
belongs_to :exhibition
has_and_belongs_to_many :artworks
end

class Exhibition
include Mongoid::Document
has_many :exhibitors
end

class Artwork
include Mongoid::Document
has_and_belongs_to_many :exhibitors
end

let(:artwork) do
Artwork.create
end

let(:exhibition) do
Exhibition.create
end

let(:exhibitor) do
Exhibitor.create(

{exhibition: exhibition}

)
end

before do
artwork.exhibitors << exhibitor
end

it "doesn't create additional artwork objects" do
Artwork.count.should == 1
end

Fails:

1) Mongoid::Relations::Referenced::ManyToMany#push when the relations are not polymorphic when one side has been persisted doesn't create additional person objects
Failure/Error: Artwork.count.should == 1
expected: 1
got: 2 (using ==)

1. ./spec/functional/mongoid/relations/referenced/many_to_many_spec.rb:195:in `block (6 levels) in <top (required)>'

2) Mongoid::Relations::Referenced::ManyToMany#concat when the relations are not polymorphic when one side has been persisted doesn't create additional person objects
Failure/Error: Artwork.count.should == 1
expected: 1
got: 3 (using ==)

1. ./spec/functional/mongoid/relations/referenced/many_to_many_spec.rb:195:in `block (6 levels) in <top (required)>'