Spec:

1. spec/models/view_spec.rb
   require "spec_helper"

class Account
include Mongoid::Document

#this is correct, account has many users, not has many views
has_many :users, as: :account
end

class User
include Mongoid::Document

embeds_many :views
end

class View
include Mongoid::Document

embedded_in :user

#this is correct, view belongs to account, not user belongs to account
belongs_to :account , polymorphic: true
attr_accessible :account, :account_id, :account_type
validates :account, :presence => true
end

describe View do

let!(:account)

{ Account.create! }

let!(:user)

{ User.create! }

let!(:view) do
user.views.build(
:account_id => account.id,
:account_type => account.class.name
)
end

it "works" do
user.save!
end

it "works" do
view.save!(:validate => false)
end

it "doesn't work" do
view.save!
end

end

Failure:

$ rspec spec -f d -b

View
works
works
doesn't work (FAILED - 1)

Failures:

1) View doesn't work
Failure/Error: view.save!
NoMethodError:
undefined method `insert_modifier' for #<Mongoid::Atomic::Paths::Root:0x00000003aa9058>

1. mongoid-2.3.4/lib/mongoid/atomic.rb:71:in `atomic_insert_modifier'
2. mongoid-2.3.4/lib/mongoid/persistence/operations.rb:65:in `inserts'
3. mongoid-2.3.4/lib/mongoid/persistence/operations/embedded/insert.rb:34:in `block in persist'
4. mongoid-2.3.4/lib/mongoid/persistence/insertion.rb:26:in `block (3 levels) in prepare'
5. activesupport-3.1.3/lib/active_support/callbacks.rb:390:in `_run_create_callbacks'
6. activesupport-3.1.3/lib/active_support/callbacks.rb:81:in `run_callbacks'
7. mongoid-2.3.4/lib/mongoid/callbacks.rb:42:in `block in run_callbacks'
8. mongoid-2.3.4/lib/mongoid/callbacks.rb:67:in `call'
9. mongoid-2.3.4/lib/mongoid/callbacks.rb:67:in `run_cascading_callbacks'
10. mongoid-2.3.4/lib/mongoid/callbacks.rb:41:in `run_callbacks'
11. mongoid-2.3.4/lib/mongoid/persistence/insertion.rb:25:in `block (2 levels) in prepare'
12. activesupport-3.1.3/lib/active_support/callbacks.rb:390:in `_run_save_callbacks'
13. activesupport-3.1.3/lib/active_support/callbacks.rb:81:in `run_callbacks'
14. mongoid-2.3.4/lib/mongoid/callbacks.rb:42:in `block in run_callbacks'
15. mongoid-2.3.4/lib/mongoid/callbacks.rb:67:in `call'
16. mongoid-2.3.4/lib/mongoid/callbacks.rb:67:in `run_cascading_callbacks'
17. mongoid-2.3.4/lib/mongoid/callbacks.rb:41:in `run_callbacks'
18. mongoid-2.3.4/lib/mongoid/persistence/insertion.rb:24:in `block in prepare'
19. mongoid-2.3.4/lib/mongoid/persistence/insertion.rb:22:in `tap'
20. mongoid-2.3.4/lib/mongoid/persistence/insertion.rb:22:in `prepare'
21. mongoid-2.3.4/lib/mongoid/persistence/operations/embedded/insert.rb:30:in `persist'
22. mongoid-2.3.4/lib/mongoid/persistence.rb:44:in `insert'
23. mongoid-2.3.4/lib/mongoid/persistence.rb:149:in `upsert'
24. mongoid-2.3.4/lib/mongoid/persistence.rb:70:in `save!'
25. ./spec/models/view_spec.rb:47:in `block (2 levels) in <top (required)>'
26. ... rspec cli/runner methods ...

Finished in 2.68 seconds
3 examples, 1 failure

Failed examples:

rspec ./spec/models/view_spec.rb:46 # View doesn't work