feature suggestion for nested - nested attributes

[Benjamin Roth]

Hi,

I made a patch today to my code to solve a use case and wonder whether or not it would be useful for ActiveRecord users in general.

My setup is

class Form

  has_many :sections

  has_many :questions, through: :sections

  accepts_nested_attributes_for :sections

class 

class Section

  belongs_to :form

  has_many :questions

  accepts_nested_attributes_for :questions

class 

class Question

  belongs_to :section

end

My form lets the user edit sections and questions. Drag and drop is also allowed to change question order but also to change the section it belongs to.

With Rails code as is, it fails with a NotFound error, which is expected because each section looks for its own questions whenever a question has an `id` attribute:  https://github.com/rails/rails/blob/d9715fcde450cebec3aeff74d4d39db5542d6625/activerecord/lib/active_record/nested_attributes.rb#L453

I'd suggest something like:

class Section

  belongs_to :form

  has_many :questions

  accepts_nested_attributes_for :questions, scope: ->{ form.questions }

class 

to fetch existing records based on a custom scope.

Is this a desired feature? Should I submit a pull request?

Benjamin