Rails Edit/Update action not working with nested items

Question

I've got working new/create actions and a form for my @miniature model and it's nested model @scales. I can't get the update/edit actions right. Should be simple but I'm very stuck.

@miniature has_many @scales through @sizes.

In my @miniature model I have

    has_many :sizes, dependent: :destroy
    has_many :scales, :through => :sizes
    accepts_nested_attributes_for :sizes, allow_destroy: true

In the controller I have

    def new
        @miniature = Miniature.new 
        @all_scales = Scale.all
        @size = @miniature.sizes.build
    end

    def create
        @miniature = Miniature.new(miniature_params)
        params[:scales][:id].each do |scale|
          if !scale.empty?
            @miniature.sizes.build(:scale_id => scale)
          end
        end
        if @miniature.save
          redirect_to miniature
        else
          render 'new'
        end
    end

private
    def miniature_params
      params.require(:miniature).permit(:name, :release_date, :material, :pcode, :notes,  sizes_attributes: [:id, :scale_id, :miniset_id])
    end

That works but edit and update actions do not. I have my edit set up the same as my new.

def edit
    @miniature = Miniature.find(params[:id])
    @all_scales = Scale.all
    @size = @miniature.sizes.build
end

I had thought that updating miniature params would update the @sizes model but it doesn't

def update
    @miniature = Miniature.find(params[:id])
    if @miniature.update_attributes(miniature_params)
      flash[:success] = "Miniature updated"
      redirect_to @miniature
    else
      render 'edit'
    end
end

It currently updates the @miniature but not the @sizes info. Is my problem in the edit or in the update or in both?

The relevant bit of the form:

<%= f.fields_for(@size) do |sf| %>
      <%= sf.label simple_pluralize(@miniature.scales.count, 'Scale') %>

        <%= collection_select( :scales, :id, @all_scales, :id, :name, 
                   {:selected => @miniature.scales.map(&:id)}, 
                   {class: 'multiselect', multiple: true}) %>
<% end %>

Any help or pointers to further reading very much appreciated. Even if it's just to say "You're overlooking this obvious thing, go and do some more reading/work".

It seems likely I need to have an update action more similar to my create action's if statement?

UPDATE for JKen13579

This is the PATCH request from my server log when submitting an edit:

Started PATCH "/miniatures/21" for 127.0.0.1 at 2014-04-02 16:00:10 +0100
Processing by MiniaturesController#update as HTML
  Parameters: {"utf8"=>"✓", "authenticity_token"=>"jQ79L1Exx83C47jnCF3nsWQ2tV07tRwKfI8wNeLzojo=", "miniature"=>{"name"=>"Test Miniature", "material"=>"Metal", "pcode"=>"123123123123123", "release_date(1i)"=>"2013", "release_date(2i)"=>"2", "release_date(3i)"=>"2", "notes"=>""}, "scales"=>{"id"=>["", "2"]}, "commit"=>"Save changes", "id"=>"21"}
  User Load (0.7ms)  SELECT "users".* FROM "users" WHERE "users"."id" = 4 ORDER BY "users"."id" ASC LIMIT 1
  Miniature Load (0.2ms)  SELECT "miniatures".* FROM "miniatures" WHERE "miniatures"."id" = ? LIMIT 1  [["id", "21"]]
   (0.1ms)  begin transaction
   (0.2ms)  commit transaction
Redirected to http://localhost:3000/miniatures/21
Completed 302 Found in 12ms (ActiveRecord: 1.3ms)

UPDATE for Observer

I'm using fields_for because that's what I managed to get the new/create action to work with. I'm not tied to it if there is a better way, at least as far as I know.

My routes has

resources :miniatures do
    collection do
    get :scales
    get :collections
    get :lines
    get :contents
    post :import
    end
    member do
      get :minisets, :setminis
    end
  end

and further down

resources :sizes
resources :scales

I think my routes file is generally a bit cluttered and is due for a refactoring.

Answer 1

As per the server log entry(see below), I see that in params hash, scales (highlighted in bold) is being passed as a separate hash and not within miniature:

Parameters: {"utf8"=>"✓", "authenticity_token"=>"jQ79L1Exx83C47jnCF3nsWQ2tV07tRwKfI8wNeLzojo=", "miniature"=>{"name"=>"Test Miniature", "material"=>"Metal", "pcode"=>"123123123123123", "release_date(1i)"=>"2013", "release_date(2i)"=>"2", "release_date(3i)"=>"2", "notes"=>""}, "scales"=>{"id"=>["", "2"]}, "commit"=>"Save changes", "id"=>"21"}

Change the update action as:

def update
    @miniature = Miniature.find(params[:id])
    if params[:scales][:id]
      ## Convert ["", "1","2","4","8"] to [1,2,4,8]
      params[:scales][:id] = params[:scales][:id].reject(&:empty?).map(&:to_i) 
      ## Get the scale_id from sizes already present in database [1,2,5,6] 
      old_scales = @miniature.sizes.pluck(:scale_id)
      ## Find the new scales to be added [1,2,4,8] - [1,2,5,6] = [4,8]
      new_scales = params[:scales][:id] - old_scales 
      ## Find the old_scales to be deleted [1,2,5,6] - [1,2,4,8] = [5,6]
      old_scales = old_scales - params[:scales][:id] 
      ## Build new_scales [4,8]
      new_scales.each do |scale|
        @miniature.sizes.build(:scale_id => scale)
      end
      ## Delete old_scales [5,6]
      Size.delete_all(:scale_id => old_scales)
    end
    if @miniature.update_attributes(miniature_params)
      flash[:success] = "Miniature updated"
      redirect_to @miniature
    else
      render 'edit'
    end
end

Update the create action so scale_id is passed as Integer and not String:

def create
    @miniature = Miniature.new(miniature_params)
    if params[:scales][:id]
      ## Convert ["", "1","2","4","8"] to [1,2,4,8]
      params[:scales][:id] = params[:scales][:id].reject(&:empty?).map(&:to_i)
      params[:scales][:id].each do |scale|
        @miniature.sizes.build(:scale_id => scale)
      end
    end
    if @miniature.save
      redirect_to miniature
    else
      render 'new'
    end
end