Cannot modify association 'Grandparent#children' because the source reflection class 'Child' is associated to 'Parent' via :has_many.
This affects versions 2.3.2 and 2.3.3 I can not speak for any other versions as I have not tried it on them.

This exception is raised on has_many :through relationships where the accepts nested attributes is on the grandchild

Steps to re-create the problem

scaffold child name:string, parent_id:integer
scaffold parent name:string, grandparent_id:integer
scaffold grandparent name:string

Obviously run the migrations.
Change the models to look like this

class Child < ActiveRecord::Base
belongs_to :parent end

class Parent < ActiveRecord::Base
has_many :children belongs_to :grand_parent

accepts_nested_attributes_for :children

end

class Grandparent < ActiveRecord::Base
has_many :parents has_many :children,

:through => :parents

accepts_nested_attributes_for :children end

Change the grandparent edit.html.erb to look like this

Editing grandparent

<% form_for(@grandparent) do |f| %>
<%= f.error_messages %>

<%= render :partial => 'form', :locals => {:f => f} %>

<%= f.submit 'Update' %>

<%= link_to 'Show', @grandparent %> |
<%= link_to 'Back', grandparents_path %>

Create the partial '_from.html.erb'

<%= f.label :name %><br />
<%= f.text_field :name %>

You might also need the _child_list.html.erb file in the grandparents view folder
<%= child_list.house %>, <%=link_to 'Edit'%>

Now create a _child_form.html.erb partial in the grandparents view folder

<%= f.label :name %><br />
<%= f.text_field :name %>

<%= link_to_function "Show/Hide children", {:id => "toggle_children"}%> <!--ignore this, It is not needed to reproduce the problem and is there to show/hide a list of children using jQuery code-->

In application_helper.rb add these 2 methods
module ApplicationHelper
def link_to_toggle_child_list(div_to_toggle, link_text)

link_to_function link_text do |page|
  page << "jQuery('#{div_to_toggle}).toggle('slow')"
end

end

def add_child_link(form_builder)

link_to_function 'New child' do |page|
  form_builder.fields_for :children,
                          Child.new,
                          :child_index => 'NEW_RECORD' do |f|
    html = render(:partial => 'child_form', :locals => { :f => f })
    page << "jQuery('#maintenance_form').replaceWith('#{escape_javascript(html)}'.replace(/NEW_RECORD/g, new Date().getTime()))"
  end
end

end

end

Create a new grandparent record. and give it a name of g1
Create a new parent record and give it a name of p1 and set the granparent_id to the value of the index for the grandparent record you just created

Once you have this then use the edit view from the grandparent index view for "g1".
Click the New child link and in the form that appears enter a name then click the update button.
You will get the error reported above.

To prove that the code should work

Copy the partials from.html.erb child_form.html.erb and _child_list.html.erb to the parents view folder.

Change the edit.html.erb to render the _form partial in the parents view folder

Then use the app to edit the parent "p1", add a child using the same link as before It will work and you will a new child record associated weith the parent record.

Please let me knopw if you need any further info and if you can think of an immediate work around for this I would love to hear it as this is rather an urgent problem for me right now

Thanks

James