Rails Edit/Update action not working with nested items - ruby-on-rails

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