don't fetch relationships twice

lib/ja_serializer/builder/included.ex

@@ -41,16 +41,20 @@ defmodule JaSerializer.Builder.Included do
   end
 
   defp resource_objects_for(structs, conn, serializer, opts) do
+    structs = Enum.filter(structs, &is_map/1)
     %{data: structs, conn: conn, serializer: serializer, opts: opts}
     |> ResourceObject.build()
     |> List.wrap()
   end
 
   # Find relationships that should be included.
   defp relationships_with_include(context) do
-    context.data
-    |> context.serializer.relationships(context.conn)
-    |> Enum.filter(fn {rel_name, rel_definition} ->
+    context
+    |> case do
+         %{relationships: relationships} -> relationships
+         %{data: data} -> context.serializer.relationships(data, context.conn)
+       end
+    |> Enum.filter(fn({rel_name, rel_definition}) ->
       case context[:opts][:include] do
         # if `include` param is not present only return 'default' includes
         nil ->

test/ja_serializer/builder/included_test.exs

@@ -477,4 +477,13 @@ defmodule JaSerializer.Builder.IncludedTest do
     ids = Enum.map(json["included"], &Map.get(&1, "id"))
     assert "p1" in ids
   end
+
+  test "non-existent include that is serialized into resource identifier results in no includes being added" do
+    a1 = %TestModel.Article{id: "a1", title: "a1", author: "p1"}
+
+    json = JaSerializer.format(ArticleSerializer, a1, %{}, include: "author")
+    keys = Map.keys(json)
+    assert not "included" in keys
+    assert json["data"]["relationships"]["author"]["data"]["id"] == "p1"
+  end
 end