Sort nil values last in Relation#order instead of raising

lib/perron/relation.rb

@@ -34,9 +34,14 @@ def order(attribute, direction = :asc)
         attribute, direction = attribute.first
       end
 
-      sorted = sort_by { it.public_send(attribute) }
-
-      Relation.new((direction == :desc) ? sorted.reverse : sorted, @model_class)
+      # Keep `nil` values out of the comparison (it raises `ArgumentError:
+      # comparison of NilClass with ... failed`) and place them last, in their
+      # original relative order, regardless of direction.
+      present, missing = partition { |resource| !resource.public_send(attribute).nil? }
+      present = present.sort_by { it.public_send(attribute) }
+      present = present.reverse if direction == :desc
+
+      Relation.new(present + missing, @model_class)
     end
 
     def pluck(*attributes)

test/perron/relation_test.rb

@@ -124,4 +124,13 @@ class Perron::RelationTest < ActiveSupport::TestCase
     assert_instance_of Perron::Relation, result
     assert_equal 0, result.size
   end
+
+  test "#order sorts nil values last instead of raising" do
+    record = Struct.new(:id, :position)
+    relation = Perron::Relation.new([record.new(1, 30), record.new(2, nil), record.new(3, 10), record.new(4, nil)])
+
+    assert_equal [3, 1, 2, 4], relation.order(:position).map(&:id)
+    assert_equal [1, 3, 2, 4], relation.order(:position, :desc).map(&:id)
+    assert_instance_of Perron::Relation, relation.order(:position)
+  end
 end