Hi,

I have an issue with self-referencing has_and_belongs_to_many relations.
With the following model and test case, the last two tests fail, while I expected them to succeed:

```ruby
class Item
include Mongoid::Document

has_and_belongs_to_many :parents, inverse_of: :children, class_name: 'Item'
has_and_belongs_to_many :children, inverse_of: :parents, class_name: 'Item'
end

RSpec.describe Item, type: :model do
describe '#create' do
let!(:parent)

{ Item.create! }

let!(:child)

{ Item.create(parents: [parent]) }

before do
parent.reload
child.reload
end

it

{ expect(child.parents).to include(parent) }
it { expect(child.parent_ids).to include(parent.id) }
it { expect(parent.children).to include(child) }
it { expect(parent.child_ids).to include(child.id) }

describe '#update' do
before do
child.update(parents: [parent])
end

it { expect(child.parents).to include(parent) }

it

{ expect(child.parent_ids).to include(parent.id) }
it { expect(parent.children).to include(child) }
it { expect(parent.child_ids).to include(child.id) }

describe '#reload' do
before do
parent.reload
child.reload
end

it { expect(child.parents).to include(parent) }
it { expect(child.parent_ids).to include(parent.id) }

it

{ expect(parent.children).to include(child) }

it

{ expect(parent.child_ids).to include(child.id) }

end
end
end
end
```

Thanks in advance!