Move distributor permissions onto order class, do not show links to change distributor if the action is invalid

app/controllers/spree/distributors_controller.rb

@@ -14,7 +14,7 @@ module Spree
 
       order = current_order(true)
 
-      if order.line_items.empty?
+      if order.can_change_distributor?
         order.distributor = distributor
         order.save!
       end
@@ -25,7 +25,7 @@ module Spree
     def deselect
       order = current_order(true)
 
-      if order.line_items.empty?
+      if order.can_change_distributor?
         order.distributor = nil
         order.save!
       end

app/models/spree/order_decorator.rb

@@ -1,6 +1,11 @@
 Spree::Order.class_eval do
   belongs_to :distributor
 
+  def can_change_distributor?
+    # Distributor may not be changed once an item has been added to the cart/order
+    line_items.empty?
+  end
+
   # before_validation :shipping_address_from_distributor
 
   private

app/views/spree/products/_source_sidebar.html.haml

@@ -6,7 +6,10 @@
 
   %h6.filter_name Shop by Distributor
   %ul.filter_choices
+    - order = current_order(false)
     - @distributors.each do |distributor|
-      %li.nowrap= link_to distributor.name, select_distributor_path(distributor)
-    - if current_distributor
+      %li.nowrap
+        - if order.nil? || order.can_change_distributor?
+          = link_to distributor.name, select_distributor_path(distributor)
+    - if current_distributor && order.can_change_distributor?
       %li.nowrap= link_to 'Leave distributor', deselect_distributors_path

spec/models/order_spec.rb

@@ -0,0 +1,11 @@
+require 'spec_helper'
+
+describe Spree::Order do
+  it "provides permissions for changing distributor" do
+    p = build(:product)
+
+    subject.can_change_distributor?.should be_true
+    subject.add_variant(p.master, 1)
+    subject.can_change_distributor?.should be_false
+  end
+end

spec/requests/consumer/add_to_cart_spec.rb

@@ -46,6 +46,7 @@ feature %q{
     visit spree.root_path
     page.should_not have_selector 'a', :text => d1.name
     page.should_not have_selector 'a', :text => d2.name
+    page.should_not have_selector 'a', :text => 'Leave distributor'
   end
 
   context "adding a subsequent product to the cart" do