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/models/transfer_spec.rb

@@ -54,6 +54,12 @@
       })
       expect(transfer).not_to be_valid
     end
+
+    it "requires that the quantity must be positive" do
+      item = create(:item)
+      transfer = build(:transfer, :with_items, item: item, item_quantity: -1)
+      expect(transfer).not_to be_valid
+    end
   end
 
   context "Scopes >" do