Fix `QueryMethods#in_order_of` to handle empty order list

Fix: https://github.com/rails/rails/issues/43903
Ref: https://github.com/rails/rails/pull/42061

Simply don't generate the case statement if the order list is empty
and fallback to default ordering.

activerecord/CHANGELOG.md

@@ -1,3 +1,14 @@
+*   Fix `QueryMethods#in_order_of` to handle empty order list.
+
+    ```ruby
+    Post.in_order_of(:id, []).to_a
+    ```
+
+    Also more explicitly set the column as secondary order, so that any other
+    value is still ordered.
+
+    *Jean Boussier*
+
 *   Fix quoting of column aliases generated by calculation methods.
 
     Since the alias is derived from the table name, we can't assume the result

activerecord/lib/active_record/relation/query_methods.rb

@@ -432,10 +432,14 @@ module ActiveRecord
       references = column_references([column])
       self.references_values |= references unless references.empty?
 
-      values = values.map { |value| type_caster.type_cast_for_database(column, value) }
-      column = order_column(column.to_s) if column.is_a?(Symbol)
+      if values.empty?
+        spawn.order!(column)
+      else
+        values = values.map { |value| type_caster.type_cast_for_database(column, value) }
 
-      spawn.order!(connection.field_ordered_value(column, values))
+        arel_column = column.is_a?(Symbol) ? order_column(column.to_s) : column
+        spawn.order!(connection.field_ordered_value(arel_column, values), column)
+      end
     end
 
     # Replaces any existing order defined on the relation with the specified order.

activerecord/test/cases/relation/field_ordered_values_test.rb

@@ -14,6 +14,18 @@ class FieldOrderedValuesTest < ActiveRecord::TestCase
     assert_equal(order, posts.map(&:id))
   end
 
+  def test_unspecified_order
+    order = [3, 4, 1]
+    post_ids = Post.in_order_of(:id, order).map(&:id)
+    expected_order = order + (post_ids - order).sort
+    assert_equal(expected_order, post_ids)
+  end
+
+  def test_in_order_of_empty
+    posts = Post.in_order_of(:id, [])
+    assert_equal(posts.map(&:id).sort, posts.map(&:id))
+  end
+
   def test_in_order_of_with_enums_values
     Book.destroy_all
     Book.create!(status: :proposed)