How to maintain the ordering for nested attributes when using accepts_nested_attributes_for in a Rails application

Question

Here is the parent model:

class TypeWell < ActiveRecord::Base
   ...

  has_many :type_well_phases, :dependent => :destroy
  accepts_nested_attributes_for :type_well_phases, :reject_if => lambda { |a| a[:phase_id].blank? }, :allow_destroy => true

  ...
end

Here is the nested model:

class TypeWellPhase < ActiveRecord::Base

  belongs_to :type_well
  belongs_to :phase

end

Here is the Phase model:

class Phase < ActiveRecord::Base
  ... 
  has_many :type_well_phases
  ...
end

I add nested records in child table (TypeWellPhases) by copying ALL records from my phases (Phase model) table in the parent model's controller as shown below:

class TypeWellsController < ResourceController
   ...
  def new
    @new_heading = "New Type Well - Computed"
    @type_well   = TypeWell.new
    initialize_phase_fields
  end

  private

  def initialize_phase_fields
    Phase.order("id").all.each do |p|
      type_well_phase               = @type_well.type_well_phases.build
      type_well_phase.phase_id      = p.id
      type_well_phase.gw_heat_value = p.gw_heat_value
    end
  end
  ...
end

I do this because I want to maintain a specific order by the children fields that are added. The part of the code Phase.order("id") is for that since the phases table has these records in a specific order.

After this I use the simple_form_for and simple_fields_for helpers as shown below in my form partial:

= simple_form_for @type_well do |f|
    ...
    #type_well_phases
      = f.simple_fields_for :type_well_phases do |type_well_phase|
        = render "type_well_phase_fields", :f => type_well_phase

Everything works as desired; most of the times. However, sometimes the ordering of Child rows in the form gets messed up after it has been saved. The order is important in this application that is why I explicitly do this ordering in the private method in the controller.

I am using the "cocoon" gem for adding removing child records. I am not sure as to why this order gets messed up sometimes.

Sorry for such a long post, but I wanted to provide all the pertinent details up front.

Appreciate any pointers.

Bharat

Answer 1

I'll explain you in a more generic way. Say, you have Product and Order models:

= form_for @product do |f|
    ...
    = f.fields_for :orders do |order_fields|
        = order_fields.text_field :name

If you want your orders to be sorted by name then just sort them :)

Instead of:

    = f.fields_for :orders do |order_fields|

put:

    = f.fields_for :orders, f.object.orders.order(:name) do |order_fields|

As you see, the f variable that is a parameter of the block of form_for has method object. It's your @product, so you can fetch its orders via .orders and then apply needed sorting via .order(:name) (sorry for this little confusion: order/orders).

The key to your solution that you can pass sorted orders as the second parameter for fields_for.

P.S. Your using the simple_form gem doesn't affect my solution. It'll work if you add 'simple_' to helpers. Just wanted my answer to be more helpful for others and not too task-related.