Fix the transfer line item quantity can be negative.

app/models/transfer.rb

@@ -33,6 +33,7 @@ class Transfer < ApplicationRecord
   validate :storage_locations_belong_to_organization
   validate :storage_locations_must_be_different
   validate :from_storage_quantities
+  validate :line_items_quantity_is_positive
 
   def self.csv_export_headers
     ["From", "To", "Comment", "Total Moved"]
@@ -83,4 +84,8 @@ def insufficient_items
     inventory = View::Inventory.new(organization_id)
     line_items.select { |i| i.quantity > inventory.quantity_for(item_id: i.item_id) }
   end
+
+  def line_items_quantity_is_positive
+    line_items_quantity_is_at_least(1)
+  end
 end

spec/controllers/transfers_controller_spec.rb

@@ -1,5 +1,5 @@
 RSpec.describe TransfersController, type: :controller do
-  let(:organization) { create(:organization) }
+  let(:organization) { create(:organization, :with_items) }
   let(:user) { create(:user, organization: organization) }
 
   context "While signed in" do
@@ -60,6 +60,21 @@
         expect(response).to render_template("new")
         expect(flash.keys).to match_array(['error'])
       end
+
+      it "should raise an exception when quantity is negative" do
+        attributes = attributes_for(
+          :transfer,
+          organization_id: organization.id,
+          to_id: create(:storage_location, organization: organization).id,
+          from_id: create(:storage_location, organization: organization).id,
+          line_items_attributes: [
+            item_id: organization.items.first.id,
+            quantity: -1
+          ]
+        )
+        post :create, params: { transfer: attributes }
+        expect(flash.keys).to match_array(['error'])
+      end
     end
 
     describe "GET #new" do