On submitting new form second time action goes to update method

Sample Rails project for issue rails/rails#9746

Installation

Install the gems:

bundle install

Rake db setup task:

rake db:create
rake db:migrate

Steps to reproduce:

1. Go to new document form by clicking "New Document" link.
2. Input the form fields(fields are mandatory to test) and submit(after submit the form, it render with validation error message for user_id in paragraph model and now the form has primary id)
3. Click on submit(goes to update action)

Note:

The problem only occur when we passing parameters with content_ids.
Validation is running when we assinging parameter using attributes=.( But the document says validation doesn't trigger while assigning attributes to the object using attributes=)

for example:

params = {"document"=>{"name"=>"sadfds", "sections_attributes"=>{"0"=>{"name"=>"", "content_ids"=>["1", "2"]}}}}
@document = Document.new
@document.attributes =  params #validation is running in this line

jmaniv/nested_model

On submitting new form second time action goes to update method

Note: