Replace exception with DB uniqueness constraint

By forbidding more than a row per variant in the spree_stock_items we
can ensure all variants have a single stock_item associated.

db/migrate/20180906094641_add_uniqueness_of_variant_id_to_spree_stock_items.rb

@@ -0,0 +1,5 @@
+class AddUniquenessOfVariantIdToSpreeStockItems < ActiveRecord::Migration
+  def change
+    add_index :spree_stock_items, :variant_id, unique: true
+  end
+end

db/schema.rb

@@ -11,7 +11,7 @@
 #
 # It's strongly recommended to check this file into your version control system.
 
-ActiveRecord::Schema.define(:version => 20180812214434) do
+ActiveRecord::Schema.define(:version => 20180906094641) do
 
   create_table "account_invoices", :force => true do |t|
     t.integer  "user_id",    :null => false
@@ -961,6 +961,7 @@ ActiveRecord::Schema.define(:version => 20180812214434) do
 
   add_index "spree_stock_items", ["stock_location_id", "variant_id"], :name => "stock_item_by_loc_and_var_id"
   add_index "spree_stock_items", ["stock_location_id"], :name => "index_spree_stock_items_on_stock_location_id"
+  add_index "spree_stock_items", ["variant_id"], :name => "index_spree_stock_items_on_variant_id", :unique => true
 
   create_table "spree_stock_locations", :force => true do |t|
     t.string   "name"

lib/open_food_network/variant_stock.rb

@@ -98,7 +98,6 @@ module OpenFoodNetwork
       warn_deprecation(__method__, 'Spree::Config[:track_inventory_levels]')
 
       raise_error_if_no_stock_item_available
-      raise_error_if_multiple_stock_items
 
       # There should be only one at the default stock location.
       #
@@ -126,11 +125,6 @@ module OpenFoodNetwork
       raise message if stock_items.empty?
     end
 
-    def raise_error_if_multiple_stock_items
-      message = 'A variant cannot have more than a stock item.'
-      raise message if stock_items.size > 1
-    end
-
     # Backwards compatible setting of stock levels in Spree 2.0.
     # It would be better to use `Spree::StockItem.adjust_count_on_hand` which
     # takes a value to add to the current stock level and uses proper locking.