Hello,

I'm using mongo (1.3.1), mongoid (4.0.0), moped (2.0.2), rails (4.1.1), ruby 2.1.5p273

This story involves two objects used to build a tree of Steps, with embedded Links documents.
Links are stored in an array within their Step, as embedded documents. Each Link contains a pointer to a step_id.

Here are the minimal objects that can reproduce the behaviour :

class DemoStep
  include Mongoid::Document
  field :name, type: String
  embeds_many :demo_links
end

class DemoLink
  include Mongoid::Document
  field :label, type: String
  embedded_in :demo_step
  belongs_to :next, class_name: 'DemoStep'
end

I create two of these Steps for the demo and confirm that s1 and s2 have no Link for now :

s1 = DemoStep.create id: 1, name: "Step 1"
s2 = DemoStep.create id: 2, name: "Step 2"

s1.demo_links
=> []
s2.demo_links
=> []

Finally, I add a Link inside the freshly created

1

, its

 attribute fed with 

s2

.

The Link is popping exactly as expected, and accessing its 

next

 relation seems correct too :

l = s1.demo_links.create label: "Linking s1 to s2", next: s2
=> #<DemoLink _id: 54920d89626f620958000000, label: "Linking s1 to s2", next_id: 20>

s1.demo_links
=> [#<DemoLink _id: 54920d89626f620958000000, label: "Linking s1 to s2", next_id: 20>]

l.next
=> #<DemoStep _id: 20, name: "Step 2">

But surprise! s2 suddenly receives something for its 

links

 relation:

s2.demo_links
=> #<DemoLink _id: 54920d89626f620958000000, label: "Linking s1 to s2", next_id: 20>

Even stranger, the collection is not an array but just a link. The Link I just created and added to 

s1

 pointing to 

s2

 now corrupted 

s2{{`}} itself too.