feat: make the serialization of nil relationships configurable

README.md

@@ -199,6 +199,7 @@ config :jsonapi,
   namespace: "/api",
   field_transformation: :underscore,
   remove_links: false,
+  serialize_nil_relationships: false,
   json_library: Jason,
   paginator: nil
 ```
@@ -219,6 +220,10 @@ config :jsonapi,
   `:camelize_shallow` or `:dasherize_shallow`.
 - **remove_links**. `links` data can optionally be removed from the payload via
   setting the configuration above to `true`. Defaults to `false`.
+- **serialize_nil_relationships**. By default, relationships on a resource that
+  are `nil` will be omitted during serialization. Setting this to `true` will
+  serialize these relationships, provided they are loaded on the resource.
+  Defaults to `false`.
 - **json_library**. Defaults to [Jason](https://hex.pm/packages/jason).
 - **paginator**. Module implementing pagination links generation. Defaults to `nil`.
 

lib/jsonapi/serializer.ex

@@ -270,7 +270,7 @@ defmodule JSONAPI.Serializer do
     |> Enum.uniq()
   end
 
-  defp assoc_loaded?(nil), do: false
+  defp assoc_loaded?(nil), do: serialize_nil_relationships?()
   defp assoc_loaded?(%{__struct__: Ecto.Association.NotLoaded}), do: false
   defp assoc_loaded?(_association), do: true
 
@@ -304,6 +304,9 @@ defmodule JSONAPI.Serializer do
 
   defp remove_links?, do: Application.get_env(:jsonapi, :remove_links, false)
 
+  @spec serialize_nil_relationships? :: boolean()
+  defp serialize_nil_relationships?, do: Application.get_env(:jsonapi, :serialize_nil_relationships, false)
+
   defp transform_fields(fields) do
     case Utils.String.field_transformation() do
       :camelize -> Utils.String.expand_fields(fields, &Utils.String.camelize/1)

test/jsonapi/serializer_test.exs

@@ -202,6 +202,9 @@ defmodule JSONAPI.SerializerTest do
 
         %PolymorphicDataTwo{} ->
           "polymorphic_data_one"
+
+        nil ->
+          nil
       end
     end
 
@@ -221,6 +224,7 @@ defmodule JSONAPI.SerializerTest do
 
     on_exit(fn ->
       Application.delete_env(:jsonapi, :field_transformation)
+      Application.delete_env(:jsonapi, :serialize_nil_relationships)
     end)
 
     {:ok, []}
@@ -360,7 +364,7 @@ defmodule JSONAPI.SerializerTest do
     assert Enum.count(encoded[:included]) == 1
   end
 
-  test "serialize handles a nil relationship" do
+  test "serialize excludes a nil relationship by default" do
     data = %{
       id: 1,
       text: "Hello",
@@ -380,11 +384,96 @@ defmodule JSONAPI.SerializerTest do
     assert attributes[:body] == data[:body]
 
     assert encoded_data[:links][:self] == PostView.url_for(data, nil)
+    refute Map.has_key?(encoded_data[:relationships], :best_comments)
+    assert map_size(encoded_data[:relationships]) == 1
+
+    assert Enum.count(encoded[:included]) == 1
+  end
+
+  test "serialize excludes unloaded ecto association by default" do
+    data = %{
+      id: 1,
+      text: "Hello",
+      body: "Hello world",
+      author: %{id: 2, username: "jason"},
+      best_comments: %{__struct__: Ecto.Association.NotLoaded}
+    }
+
+    encoded = Serializer.serialize(PostView, data, nil)
+
+    encoded_data = encoded[:data]
+    assert encoded_data[:id] == PostView.id(data)
+    assert encoded_data[:type] == PostView.type()
+
+    attributes = encoded_data[:attributes]
+    assert attributes[:text] == data[:text]
+    assert attributes[:body] == data[:body]
+
+    assert encoded_data[:links][:self] == PostView.url_for(data, nil)
+    refute Map.has_key?(encoded_data[:relationships], :best_comments)
     assert map_size(encoded_data[:relationships]) == 1
 
     assert Enum.count(encoded[:included]) == 1
   end
 
+  test "serialize includes nil relationships when configured" do
+    Application.put_env(:jsonapi, :serialize_nil_relationships, true)
+
+    data = %{
+      id: 1,
+      text: "Hello",
+      body: "Hello world",
+      author: %{id: 2, username: "jason"},
+      best_comments: nil
+    }
+
+    encoded = Serializer.serialize(PostView, data, nil)
+
+    encoded_data = encoded[:data]
+    assert encoded_data[:id] == PostView.id(data)
+    assert encoded_data[:type] == PostView.type()
+
+    attributes = encoded_data[:attributes]
+    assert attributes[:text] == data[:text]
+    assert attributes[:body] == data[:body]
+
+    assert encoded_data[:links][:self] == PostView.url_for(data, nil)
+    assert map_size(encoded_data[:relationships]) == 3
+    assert encoded_data[:relationships][:best_comments][:data] == nil
+    assert encoded_data[:relationships][:polymorphics][:data] == nil
+
+    assert Enum.count(encoded[:included]) == 1
+  end
+
+  test "serialize excludes unloaded ecto association when serialize nil relationships configured" do
+    Application.put_env(:jsonapi, :serialize_nil_relationships, true)
+
+    data = %{
+      id: 1,
+      text: "Hello",
+      body: "Hello world",
+      author: %{id: 2, username: "jason"},
+      best_comments: %{__struct__: Ecto.Association.NotLoaded}
+    }
+
+    encoded = Serializer.serialize(PostView, data, nil)
+
+    encoded_data = encoded[:data]
+    assert encoded_data[:id] == PostView.id(data)
+    assert encoded_data[:type] == PostView.type()
+
+    attributes = encoded_data[:attributes]
+    assert attributes[:text] == data[:text]
+    assert attributes[:body] == data[:body]
+
+    assert encoded_data[:links][:self] == PostView.url_for(data, nil)
+    assert map_size(encoded_data[:relationships]) == 2
+    assert Map.has_key?(encoded_data[:relationships], :polymorphics)
+    refute Map.has_key?(encoded_data[:relationships], :best_comments)
+
+    assert Enum.count(encoded[:included]) == 1
+  end
+
   test "serialize handles a relationship self link on a show request" do
     data = %{
       id: 1,