“Cannot modify association because the source reflection class is associated via :has_many”

Answer 10/19/2018 Developer FAQ 2

I've looked at other questions with a similar title and issue, but their solutions do not seem to apply to me. Sorry for not being more descriptive, I was already reaching the 150 limit.

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I have the following Models:

class Event < ActiveRecord::Base
  has_many :weeks, dependent: :destroy
  has_many :tests, through: :weeks

  accepts_nested_attribues_for :weeks, allow_destroy: true
  accepts_nested_attribues_for :tests, allow_destroy: true
end

class Week < ActiveRecord::Base
  belongs_to :event
  has_many :tests, dependent: :destroy
end

class Test < ActiveRecord::Base
  belongs_to :week
end

And this view:

=form_for @event do |e|
  %h3 Event
  =render 'form', e: e # This partial contains the general data of the Event.

  %h3 Weeks

  %table
    %thead
      %th Start
      %th End
      %th X
    %tbody
      =e.fields_for :weeks do |w|
        =render 'week_fields', w: w
  =link_to_add_fields 'Add Week', e, :weeks # Custom helper that adds the fields of the week to the end of the table via JavaScript.

  %h3 Tests

  -@weeks.each do |week|
    %h5=week.start # Prints the 'start' of the Week.
    %table
      %thead
        %th Start
        %th End
        %th X
      %tbody
        = e.fields_for :tests, week.tests do |t|
          =render 'test_fields', t: t, s: week.id
    =link_to_add_fields 'Add Test', e, :tests, parent: week.id

This works fine for adding Events and Weeks, but when I try to add a new Test I get an error that says:

Cannot modify association 'Event#tests' because the source reflection class 'Test' is associated to 'Week' via :has_many.

I understand this is due to the way I got the associations in the view, but I'm not too sure how I should use them.

This is the code from the link_to_add_fields helper I'm using above:

def link_to_add_fields(name, f, association, parent: 0)
  new_object = f.object.send(association).klass.new
  id = new_object.object_id
  fields = f.fields_for(association, new_object, child_index: id) do |builder|
    render(association.to_s.singularize + "_fields", f: builder, s: parent)
  end
  link_to(name, '#', class: 'btn btn-default add_fields', id: "add_#{association}", data: {id: id, association: association, fields: fields.gsub("\n", "")})
end

The helper generates code that's later introduced onto the form via JavaScript. It's a modified version of the one used on a Railcasts episode.

This is what I'm using on my controller to limit the parameters:

def event_params
  params.require(:event).permit(:name, :start, :end, :place,
                                 tests_attributes: [:id, :name, :date, :time, :week_id],
                                 weeks_attributes: [:id, :start, :end, :_destroy])
end

This works perfectly for editing the data of an already existing Test, but not for new ones generated by the helper. I was reading that (in my case) tests_attributes should be within week_attributes, since the Test I'm trying to modify belongs to Week. like this:

def event_params
  params.require(:event).permit(:name, :start, :end, :place,
     weeks_attributes: [:id, :start, :end, :_destroy, test_attributes: [:id, :name, :date, :time, :week_id]])
end

I do not know how to make the form to follow this association (or if this is actually a solution). I know that Test cannot be modified cause it's being pulled as read-only, but I'm not sure how to arrange the association within the view, so that it can be saved when I update Event.

Any pointers are greatly appreciated.

Answer1:

This got a bit tricky, but I got it. It was a matter of modifying what was within what.

I got it working by using something like this:

%h3 Tests

-@weeks.each do |week|
  =e.fields_for :weeks, week do |w|
    %h5=week.start # Prints the 'start' of the Week.
    %table
      %thead
        %th Start
        %th End
        %th X
      %tbody
        = w.fields_for :tests, week.tests do |t|
          =render 'test_fields', t: t, s: week.id
    =link_to_add_fields 'Add Test', w, :tests, parent: week.id

The trick was to make the fields generated for Test to be within the fields generated for Week. So first I added e.fields_for :weeks, week do |w| which would indicate that the following fields belong to Weeks, then I limited the data to the current selected Week by using the previous each loop.

Next I used the new w association in the fields_for for Test. This would make every field generated by the render to be in the format of "event[weeks_attributes][week_id][tests_attributes][test_id]" which is what I was expecting for the parameters in my controller.

The same is done in link_to_add_fields where I change the e association that belonged to Event, for w which belongs to Weeks. This would also change the format of the new records generated via JavaScript, using the format I previously described.

Finally, I had to change the format expected in the controller. I was using the singular form test, it had to be changed to plural, as it is possible to add multiple records in the form. So I ended up with something like this:

def event_params
  params.require(:event).permit(:name, :start, :end, :place,
     weeks_attributes: [:id, :start, :end, :_destroy, tests_attributes: [:id, :name, :date, :time, :week_id]])
end

And it works!

Answer2:

I think you should use complex nested form structure here to add test with week. e.fields_for :tests should be inside the e.fields_for :weeks block for proper associations.

You can get idea from here Deep Nested Rails 4 Form