Problems with Factory girl and mongoid - ruby

Question

I have this to classes:
class User
...
has_and_belongs_to_many :posts
...
end
class Post
...
has_and_belongs_to_many :users
validates :user_ids, presence: true
...
end
Then I also have this factories:
FactoryGirl.define do
factory :user do
end
end
FactoryGirl.define do
factory :post do
end
end
before I have the validation validates :user_ids, presence: true, it worked well, but when I added that validation, it signals the error in the form so it is good. but rspec doesn't work:
Mongoid::Errors::Validations:
message:
Validation of User failed.
summary:
The following errors were found: Customer ids can't be blank
resolution:
Try persisting the document with valid data or remove the validations.
So I tryed this:
issue on github but for activerecord
blog post
and tried this things:
factory :post do
before_create do |post|
FactoryGirl.build(:user, post: post)
end
end
or
before(:create) do |post|
post.users << FactoryGirl.build(:user, post: post)
end
or
before(:create) do |post|
post.users << FactoryGirl.create(:user, post: post)
end
or
users { create_list(:user, 2) }
or
users { [create(:user, posts: [])] }
but then I get:
bundle exec rspec spec --colour --format documentation
/Users/toni/.rvm/gems/ruby#my-project/bin/ruby_executable_hooks:15: stack level too deep (SystemStackError)