From 9d05d83a26814f15dac7e99926d7b0a62e0d1777 Mon Sep 17 00:00:00 2001
From: Nick <nick.duitz@9elements.com>
Date: Thu, 13 Mar 2025 11:25:13 +0100
Subject: [PATCH 1/9] add backwards compatible polymorphic resource support

---
 lib/jsonapi/serializer.ex        |  4 +-
 lib/jsonapi/view.ex              | 86 +++++++++++++++++++++++++-------
 test/jsonapi/serializer_test.exs | 42 +++++++++++++++-
 test/jsonapi/view_test.exs       | 55 ++++++++++++++++++++
 4 files changed, 165 insertions(+), 22 deletions(-)

diff --git a/lib/jsonapi/serializer.ex b/lib/jsonapi/serializer.ex
index 4ff2c5c4..395399aa 100644
--- a/lib/jsonapi/serializer.ex
+++ b/lib/jsonapi/serializer.ex
@@ -62,7 +62,7 @@ defmodule JSONAPI.Serializer do
 
     encoded_data = %{
       id: view.id(data),
-      type: view.type(),
+      type: view.resource_type(data),
       attributes: transform_fields(view.attributes(data, conn)),
       relationships: %{}
     }
@@ -255,7 +255,7 @@ defmodule JSONAPI.Serializer do
 
   def encode_rel_data(view, data) do
     %{
-      type: view.type(),
+      type: view.resource_type(data),
       id: view.id(data)
     }
   end
diff --git a/lib/jsonapi/view.ex b/lib/jsonapi/view.ex
index 4b007ba0..55f6fd66 100644
--- a/lib/jsonapi/view.ex
+++ b/lib/jsonapi/view.ex
@@ -140,10 +140,12 @@ defmodule JSONAPI.View do
   @type options :: keyword()
   @type resource_id :: String.t()
   @type resource_type :: String.t()
+  @type polymorphic_resource :: boolean()
 
   @callback attributes(data(), Conn.t() | nil) :: map()
   @callback id(data()) :: resource_id() | nil
   @callback fields() :: [field()]
+  @callback polymorphic_fields(data()) :: [field()]
   @callback get_field(field(), data(), Conn.t()) :: any()
   @callback hidden(data()) :: [field()]
   @callback links(data(), Conn.t()) :: links()
@@ -156,6 +158,7 @@ defmodule JSONAPI.View do
               {atom(), t() | {t(), :include} | {atom(), t()} | {atom(), t(), :include}}
             ]
   @callback type() :: resource_type()
+  @callback polymorphic_type(data()) :: resource_type()
   @callback url_for(data(), Conn.t() | nil) :: String.t()
   @callback url_for_pagination(data(), Conn.t(), Paginator.params()) :: String.t()
   @callback url_for_rel(term(), String.t(), Conn.t() | nil) :: String.t()
@@ -167,7 +170,8 @@ defmodule JSONAPI.View do
     {type, opts} = Keyword.pop(opts, :type)
     {namespace, opts} = Keyword.pop(opts, :namespace)
     {path, opts} = Keyword.pop(opts, :path)
-    {paginator, _opts} = Keyword.pop(opts, :paginator)
+    {paginator, opts} = Keyword.pop(opts, :paginator)
+    {polymorphic_resource, _opts} = Keyword.pop(opts, :polymorphic_resource, false)
 
     quote do
       alias JSONAPI.{Serializer, View}
@@ -178,6 +182,7 @@ defmodule JSONAPI.View do
       @namespace unquote(namespace)
       @path unquote(path)
       @paginator unquote(paginator)
+      @polymorphic_resource unquote(polymorphic_resource)
 
       @impl View
       def id(nil), do: nil
@@ -205,8 +210,21 @@ defmodule JSONAPI.View do
         end)
       end
 
-      @impl View
-      def fields, do: raise("Need to implement fields/0")
+      cond do
+        !@polymorphic_resource ->
+          @impl View
+          def fields, do: raise("Need to implement fields/0")
+
+          @impl View
+          def polymorphic_fields(_data), do: nil
+
+        @polymorphic_resource ->
+          @impl View
+          def fields, do: nil
+
+          @impl View
+          def polymorphic_fields(_data), do: raise("Need to implement polymorphic_fields/1")
+      end
 
       @impl View
       def hidden(_data), do: []
@@ -241,11 +259,27 @@ defmodule JSONAPI.View do
       @impl View
       def relationships, do: []
 
-      @impl View
-      if @resource_type do
-        def type, do: @resource_type
-      else
-        def type, do: raise("Need to implement type/0")
+      cond do
+        @resource_type ->
+          @impl View
+          def type, do: @resource_type
+
+          @impl View
+          def polymorphic_type(_data), do: nil
+
+        !@polymorphic_resource ->
+          @impl View
+          def type, do: raise("Need to implement type/0")
+
+          @impl View
+          def polymorphic_type(_data), do: nil
+
+        @polymorphic_resource ->
+          @impl View
+          def type, do: nil
+
+          @impl View
+          def polymorphic_type(_data), do: raise("Need to implement polymorphic_type/1")
       end
 
       @impl View
@@ -264,6 +298,22 @@ defmodule JSONAPI.View do
       def visible_fields(data, conn),
         do: View.visible_fields(__MODULE__, data, conn)
 
+      def resource_fields(data) do
+        if @polymorphic_resource do
+          polymorphic_fields(data)
+        else
+          fields()
+        end
+      end
+
+      def resource_type(data) do
+        if @polymorphic_resource do
+          polymorphic_type(data)
+        else
+          type()
+        end
+      end
+
       defoverridable View
 
       def index(models, conn, _params, meta \\ nil, options \\ []),
@@ -336,11 +386,11 @@ defmodule JSONAPI.View do
 
   @spec url_for(t(), term(), Conn.t() | nil) :: String.t()
   def url_for(view, data, nil = _conn) when is_nil(data) or is_list(data),
-    do: URI.to_string(%URI{path: Enum.join([view.namespace(), path_for(view)], "/")})
+    do: URI.to_string(%URI{path: Enum.join([view.namespace(), path_for(view, data)], "/")})
 
   def url_for(view, data, nil = _conn) do
     URI.to_string(%URI{
-      path: Enum.join([view.namespace(), path_for(view), view.id(data)], "/")
+      path: Enum.join([view.namespace(), path_for(view, data), view.id(data)], "/")
     })
   end
 
@@ -349,7 +399,7 @@ defmodule JSONAPI.View do
       scheme: scheme(conn),
       host: host(conn),
       port: port(conn),
-      path: Enum.join([view.namespace(), path_for(view)], "/")
+      path: Enum.join([view.namespace(), path_for(view, data)], "/")
     })
   end
 
@@ -358,7 +408,7 @@ defmodule JSONAPI.View do
       scheme: scheme(conn),
       host: host(conn),
       port: port(conn),
-      path: Enum.join([view.namespace(), path_for(view), view.id(data)], "/")
+      path: Enum.join([view.namespace(), path_for(view, data), view.id(data)], "/")
     })
   end
 
@@ -392,8 +442,8 @@ defmodule JSONAPI.View do
   def visible_fields(view, data, conn) do
     all_fields =
       view
-      |> requested_fields_for_type(conn)
-      |> net_fields_for_type(view.fields())
+      |> requested_fields_for_type(data, conn)
+      |> net_fields_for_type(view.resource_fields(data))
 
     hidden_fields = view.hidden(data)
 
@@ -420,11 +470,11 @@ defmodule JSONAPI.View do
     |> URI.to_string()
   end
 
-  defp requested_fields_for_type(view, %Conn{assigns: %{jsonapi_query: %{fields: fields}}}) do
-    fields[view.type()]
+  defp requested_fields_for_type(view, data, %Conn{assigns: %{jsonapi_query: %{fields: fields}}}) do
+    fields[view.resource_type(data)]
   end
 
-  defp requested_fields_for_type(_view, _conn), do: nil
+  defp requested_fields_for_type(_view, _data, _conn), do: nil
 
   defp host(%Conn{host: host}),
     do: Application.get_env(:jsonapi, :host, host)
@@ -438,5 +488,5 @@ defmodule JSONAPI.View do
   defp scheme(%Conn{scheme: scheme}),
     do: Application.get_env(:jsonapi, :scheme, to_string(scheme))
 
-  defp path_for(view), do: view.path() || view.type()
+  defp path_for(view, data), do: view.path() || view.resource_type(data)
 end
diff --git a/test/jsonapi/serializer_test.exs b/test/jsonapi/serializer_test.exs
index 54f4d803..7f02e2a3 100644
--- a/test/jsonapi/serializer_test.exs
+++ b/test/jsonapi/serializer_test.exs
@@ -13,7 +13,8 @@ defmodule JSONAPI.SerializerTest do
     def relationships do
       [
         author: {JSONAPI.SerializerTest.UserView, :include},
-        best_comments: {JSONAPI.SerializerTest.CommentView, :include}
+        best_comments: {JSONAPI.SerializerTest.CommentView, :include},
+        polymorphics: {JSONAPI.SerializerTest.PolymorphicView, :include}
       ]
     end
   end
@@ -183,6 +184,38 @@ defmodule JSONAPI.SerializerTest do
     end
   end
 
+  defmodule PolymorphicDataOne do
+    defstruct [:id, :some_field]
+  end
+
+  defmodule PolymorphicDataTwo do
+    defstruct [:id, :some_other_field]
+  end
+
+  defmodule PolymorphicView do
+    use JSONAPI.View, polymorphic_resource: true
+
+    def polymorphic_type(data) do
+      case data do
+        %PolymorphicDataOne{} ->
+          "polymorphic_data_one"
+
+        %PolymorphicDataTwo{} ->
+          "polymorphic_data_one"
+      end
+    end
+
+    def polymorphic_fields(data) do
+      case data do
+        %PolymorphicDataOne{} ->
+          [:some_field]
+
+        %PolymorphicDataTwo{} ->
+          [:some_other_field]
+      end
+    end
+  end
+
   setup do
     Application.put_env(:jsonapi, :field_transformation, :underscore)
 
@@ -219,6 +252,10 @@ defmodule JSONAPI.SerializerTest do
       best_comments: [
         %{id: 5, text: "greatest comment ever", user: %{id: 4, username: "jack"}},
         %{id: 6, text: "not so great", user: %{id: 2, username: "jason"}}
+      ],
+      polymorphic: [
+        %PolymorphicDataOne{id: 1, some_field: "foo"},
+        %PolymorphicDataTwo{id: 2, some_other_field: "foo"}
       ]
     }
 
@@ -842,7 +879,8 @@ defmodule JSONAPI.SerializerTest do
 
     assert configs == [
              {:author, :author, JSONAPI.SerializerTest.UserView, true},
-             {:best_comments, :best_comments, JSONAPI.SerializerTest.CommentView, true}
+             {:best_comments, :best_comments, JSONAPI.SerializerTest.CommentView, true},
+             {:polymorphics, :polymorphics, JSONAPI.SerializerTest.PolymorphicView, true}
            ]
   end
 
diff --git a/test/jsonapi/view_test.exs b/test/jsonapi/view_test.exs
index 8ea8af01..aecf70a1 100644
--- a/test/jsonapi/view_test.exs
+++ b/test/jsonapi/view_test.exs
@@ -55,6 +55,38 @@ defmodule JSONAPI.ViewTest do
     def get_field(field, _data, _conn), do: "#{field}!"
   end
 
+  defmodule PolymorphicDataOne do
+    defstruct [:some_field]
+  end
+
+  defmodule PolymorphicDataTwo do
+    defstruct [:some_other_field]
+  end
+
+  defmodule PolymorphicView do
+    use JSONAPI.View, polymorphic_resource: true
+
+    def polymorphic_type(data) do
+      case data do
+        %PolymorphicDataOne{} ->
+          "polymorphic_data_one"
+
+        %PolymorphicDataTwo{} ->
+          "polymorphic_data_one"
+      end
+    end
+
+    def polymorphic_fields(data) do
+      case data do
+        %PolymorphicDataOne{} ->
+          [:some_field]
+
+        %PolymorphicDataTwo{} ->
+          [:some_other_field]
+      end
+    end
+  end
+
   setup do
     Application.put_env(:jsonapi, :field_transformation, :underscore)
     Application.put_env(:jsonapi, :namespace, "/other-api")
@@ -71,6 +103,20 @@ defmodule JSONAPI.ViewTest do
     assert PostView.type() == "posts"
   end
 
+  describe "resource_type/1" do
+    test "equals result of type/0 if resource is not polymorphic" do
+      assert PostView.type() == PostView.resource_type(%{})
+    end
+
+    test "equals result of polymorphic_type/1 if resource is polymorphic" do
+      assert PolymorphicView.polymorphic_type(%PolymorphicDataOne{}) ==
+               PolymorphicView.resource_type(%PolymorphicDataOne{})
+
+      assert PolymorphicView.polymorphic_type(%PolymorphicDataTwo{}) ==
+               PolymorphicView.resource_type(%PolymorphicDataTwo{})
+    end
+  end
+
   describe "namespace/0" do
     setup do
       Application.put_env(:jsonapi, :namespace, "/cake")
@@ -314,4 +360,13 @@ defmodule JSONAPI.ViewTest do
              static_fun: "static_fun/2"
            } == DynamicView.attributes(data, conn)
   end
+
+  test "attributes/2 can return polymorphic fields" do
+    data = %PolymorphicDataTwo{some_other_field: "foo"}
+    conn = %Plug.Conn{assigns: %{jsonapi_query: %JSONAPI.Config{}}}
+
+    assert %{
+             some_other_field: "foo"
+           } == PolymorphicView.attributes(data, conn)
+  end
 end

From fa2d8809d29b034c1b85ba0957d2f212d1c909ae Mon Sep 17 00:00:00 2001
From: Nick <nick.duitz@9elements.com>
Date: Thu, 10 Apr 2025 11:14:04 +0200
Subject: [PATCH 2/9] polymorphic_relationships

---
 lib/jsonapi/serializer.ex | 16 ++++++++--------
 lib/jsonapi/view.ex       | 14 ++++++++++++++
 2 files changed, 22 insertions(+), 8 deletions(-)

diff --git a/lib/jsonapi/serializer.ex b/lib/jsonapi/serializer.ex
index 395399aa..f22505cb 100644
--- a/lib/jsonapi/serializer.ex
+++ b/lib/jsonapi/serializer.ex
@@ -58,7 +58,7 @@ defmodule JSONAPI.Serializer do
   end
 
   def encode_data(view, data, conn, query_includes, options) do
-    valid_includes = get_includes(view, query_includes)
+    valid_includes = get_includes(view, query_includes, data)
 
     encoded_data = %{
       id: view.id(data),
@@ -80,7 +80,7 @@ defmodule JSONAPI.Serializer do
 
   @spec encode_relationships(Conn.t(), document(), tuple(), list()) :: tuple()
   def encode_relationships(conn, doc, {view, data, _, _} = view_info, options) do
-    view.relationships()
+    view.resource_relationships(data)
     |> Enum.filter(&assoc_loaded?(Map.get(data, get_data_key(&1))))
     |> Enum.map_reduce(doc, &build_relationships(conn, view_info, &1, &2, options))
   end
@@ -273,13 +273,13 @@ defmodule JSONAPI.Serializer do
   defp assoc_loaded?(%{__struct__: Ecto.Association.NotLoaded}), do: false
   defp assoc_loaded?(_association), do: true
 
-  defp get_includes(view, query_includes) do
-    includes = get_default_includes(view) ++ get_query_includes(view, query_includes)
+  defp get_includes(view, query_includes, data) do
+    includes = get_default_includes(view, data) ++ get_query_includes(view, query_includes, data)
     Enum.uniq(includes)
   end
 
-  defp get_default_includes(view) do
-    rels = view.relationships()
+  defp get_default_includes(view, data) do
+    rels = view.resource_relationships(data)
 
     Enum.filter(rels, &include_rel_by_default/1)
   end
@@ -290,8 +290,8 @@ defmodule JSONAPI.Serializer do
     include_by_default
   end
 
-  defp get_query_includes(view, query_includes) do
-    rels = view.relationships()
+  defp get_query_includes(view, query_includes, data) do
+    rels = view.resource_relationships(data)
 
     query_includes
     |> Enum.map(fn
diff --git a/lib/jsonapi/view.ex b/lib/jsonapi/view.ex
index 55f6fd66..8454ce7e 100644
--- a/lib/jsonapi/view.ex
+++ b/lib/jsonapi/view.ex
@@ -157,6 +157,9 @@ defmodule JSONAPI.View do
   @callback relationships() :: [
               {atom(), t() | {t(), :include} | {atom(), t()} | {atom(), t(), :include}}
             ]
+  @callback polymorphic_relationships(data()) :: [
+              {atom(), t() | {t(), :include} | {atom(), t()} | {atom(), t(), :include}}
+            ]
   @callback type() :: resource_type()
   @callback polymorphic_type(data()) :: resource_type()
   @callback url_for(data(), Conn.t() | nil) :: String.t()
@@ -259,6 +262,9 @@ defmodule JSONAPI.View do
       @impl View
       def relationships, do: []
 
+      @impl View
+      def polymorphic_relationships(_data), do: []
+
       cond do
         @resource_type ->
           @impl View
@@ -314,6 +320,14 @@ defmodule JSONAPI.View do
         end
       end
 
+      def resource_relationships(data) do
+        if @polymorphic_resource do
+          polymorphic_relationships(data)
+        else
+          relationships()
+        end
+      end
+
       defoverridable View
 
       def index(models, conn, _params, meta \\ nil, options \\ []),

From 1a40b441d3f8d569344476c49f1627126dcd672f Mon Sep 17 00:00:00 2001
From: Nick <nick.duitz@9elements.com>
Date: Tue, 6 May 2025 14:06:53 +0200
Subject: [PATCH 3/9] remove polymorphic_resource type, rename with suffix ? to
 indicate it is a boolean

---
 lib/jsonapi/view.ex | 19 +++++++++----------
 1 file changed, 9 insertions(+), 10 deletions(-)

diff --git a/lib/jsonapi/view.ex b/lib/jsonapi/view.ex
index 8454ce7e..987ef160 100644
--- a/lib/jsonapi/view.ex
+++ b/lib/jsonapi/view.ex
@@ -140,7 +140,6 @@ defmodule JSONAPI.View do
   @type options :: keyword()
   @type resource_id :: String.t()
   @type resource_type :: String.t()
-  @type polymorphic_resource :: boolean()
 
   @callback attributes(data(), Conn.t() | nil) :: map()
   @callback id(data()) :: resource_id() | nil
@@ -174,7 +173,7 @@ defmodule JSONAPI.View do
     {namespace, opts} = Keyword.pop(opts, :namespace)
     {path, opts} = Keyword.pop(opts, :path)
     {paginator, opts} = Keyword.pop(opts, :paginator)
-    {polymorphic_resource, _opts} = Keyword.pop(opts, :polymorphic_resource, false)
+    {polymorphic_resource?, _opts} = Keyword.pop(opts, :polymorphic_resource?, false)
 
     quote do
       alias JSONAPI.{Serializer, View}
@@ -185,7 +184,7 @@ defmodule JSONAPI.View do
       @namespace unquote(namespace)
       @path unquote(path)
       @paginator unquote(paginator)
-      @polymorphic_resource unquote(polymorphic_resource)
+      @polymorphic_resource? unquote(polymorphic_resource?)
 
       @impl View
       def id(nil), do: nil
@@ -214,14 +213,14 @@ defmodule JSONAPI.View do
       end
 
       cond do
-        !@polymorphic_resource ->
+        !@polymorphic_resource? ->
           @impl View
           def fields, do: raise("Need to implement fields/0")
 
           @impl View
           def polymorphic_fields(_data), do: nil
 
-        @polymorphic_resource ->
+        @polymorphic_resource? ->
           @impl View
           def fields, do: nil
 
@@ -273,14 +272,14 @@ defmodule JSONAPI.View do
           @impl View
           def polymorphic_type(_data), do: nil
 
-        !@polymorphic_resource ->
+        !@polymorphic_resource? ->
           @impl View
           def type, do: raise("Need to implement type/0")
 
           @impl View
           def polymorphic_type(_data), do: nil
 
-        @polymorphic_resource ->
+        @polymorphic_resource? ->
           @impl View
           def type, do: nil
 
@@ -305,7 +304,7 @@ defmodule JSONAPI.View do
         do: View.visible_fields(__MODULE__, data, conn)
 
       def resource_fields(data) do
-        if @polymorphic_resource do
+        if @polymorphic_resource? do
           polymorphic_fields(data)
         else
           fields()
@@ -313,7 +312,7 @@ defmodule JSONAPI.View do
       end
 
       def resource_type(data) do
-        if @polymorphic_resource do
+        if @polymorphic_resource? do
           polymorphic_type(data)
         else
           type()
@@ -321,7 +320,7 @@ defmodule JSONAPI.View do
       end
 
       def resource_relationships(data) do
-        if @polymorphic_resource do
+        if @polymorphic_resource? do
           polymorphic_relationships(data)
         else
           relationships()

From 9448be001f4387702a8087af46a1cd536ab0b579 Mon Sep 17 00:00:00 2001
From: Nick <nick.duitz@9elements.com>
Date: Tue, 6 May 2025 14:29:02 +0200
Subject: [PATCH 4/9] match type spec; refactor types

---
 lib/jsonapi/view.ex              | 22 ++++++++++------------
 test/jsonapi/serializer_test.exs |  2 +-
 test/jsonapi/view_test.exs       |  2 +-
 3 files changed, 12 insertions(+), 14 deletions(-)

diff --git a/lib/jsonapi/view.ex b/lib/jsonapi/view.ex
index 987ef160..943e97ff 100644
--- a/lib/jsonapi/view.ex
+++ b/lib/jsonapi/view.ex
@@ -140,11 +140,13 @@ defmodule JSONAPI.View do
   @type options :: keyword()
   @type resource_id :: String.t()
   @type resource_type :: String.t()
+  @type resource_relationship :: [{atom(), t() | {t(), :include} | {atom(), t()} | {atom(), t(), :include}}]
+  @type resource_fields :: [field()]
 
   @callback attributes(data(), Conn.t() | nil) :: map()
   @callback id(data()) :: resource_id() | nil
-  @callback fields() :: [field()]
-  @callback polymorphic_fields(data()) :: [field()]
+  @callback fields() :: resource_fields()
+  @callback polymorphic_fields(data()) :: resource_fields()
   @callback get_field(field(), data(), Conn.t()) :: any()
   @callback hidden(data()) :: [field()]
   @callback links(data(), Conn.t()) :: links()
@@ -153,14 +155,10 @@ defmodule JSONAPI.View do
   @callback pagination_links(data(), Conn.t(), Paginator.page(), Paginator.options()) ::
               Paginator.links()
   @callback path() :: String.t() | nil
-  @callback relationships() :: [
-              {atom(), t() | {t(), :include} | {atom(), t()} | {atom(), t(), :include}}
-            ]
-  @callback polymorphic_relationships(data()) :: [
-              {atom(), t() | {t(), :include} | {atom(), t()} | {atom(), t(), :include}}
-            ]
-  @callback type() :: resource_type()
-  @callback polymorphic_type(data()) :: resource_type()
+  @callback relationships() :: resource_relationship()
+  @callback polymorphic_relationships(data()) :: resource_relationship()
+  @callback type() :: resource_type() | nil
+  @callback polymorphic_type(data()) :: resource_type() | nil
   @callback url_for(data(), Conn.t() | nil) :: String.t()
   @callback url_for_pagination(data(), Conn.t(), Paginator.params()) :: String.t()
   @callback url_for_rel(term(), String.t(), Conn.t() | nil) :: String.t()
@@ -218,11 +216,11 @@ defmodule JSONAPI.View do
           def fields, do: raise("Need to implement fields/0")
 
           @impl View
-          def polymorphic_fields(_data), do: nil
+          def polymorphic_fields(_data), do: []
 
         @polymorphic_resource? ->
           @impl View
-          def fields, do: nil
+          def fields, do: []
 
           @impl View
           def polymorphic_fields(_data), do: raise("Need to implement polymorphic_fields/1")
diff --git a/test/jsonapi/serializer_test.exs b/test/jsonapi/serializer_test.exs
index 7f02e2a3..c3f28e0a 100644
--- a/test/jsonapi/serializer_test.exs
+++ b/test/jsonapi/serializer_test.exs
@@ -193,7 +193,7 @@ defmodule JSONAPI.SerializerTest do
   end
 
   defmodule PolymorphicView do
-    use JSONAPI.View, polymorphic_resource: true
+    use JSONAPI.View, polymorphic_resource?: true
 
     def polymorphic_type(data) do
       case data do
diff --git a/test/jsonapi/view_test.exs b/test/jsonapi/view_test.exs
index aecf70a1..85e74c8e 100644
--- a/test/jsonapi/view_test.exs
+++ b/test/jsonapi/view_test.exs
@@ -64,7 +64,7 @@ defmodule JSONAPI.ViewTest do
   end
 
   defmodule PolymorphicView do
-    use JSONAPI.View, polymorphic_resource: true
+    use JSONAPI.View, polymorphic_resource?: true
 
     def polymorphic_type(data) do
       case data do

From cf2b3747d76cb34b898fb7a1db15497bc67b5a6f Mon Sep 17 00:00:00 2001
From: Nick <nick.duitz@9elements.com>
Date: Tue, 6 May 2025 15:03:51 +0200
Subject: [PATCH 5/9] add as callbacks

---
 lib/jsonapi/view.ex | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/lib/jsonapi/view.ex b/lib/jsonapi/view.ex
index 943e97ff..a912587e 100644
--- a/lib/jsonapi/view.ex
+++ b/lib/jsonapi/view.ex
@@ -147,6 +147,7 @@ defmodule JSONAPI.View do
   @callback id(data()) :: resource_id() | nil
   @callback fields() :: resource_fields()
   @callback polymorphic_fields(data()) :: resource_fields()
+  @callback resource_fields(data()) :: resource_fields()
   @callback get_field(field(), data(), Conn.t()) :: any()
   @callback hidden(data()) :: [field()]
   @callback links(data(), Conn.t()) :: links()
@@ -157,8 +158,10 @@ defmodule JSONAPI.View do
   @callback path() :: String.t() | nil
   @callback relationships() :: resource_relationship()
   @callback polymorphic_relationships(data()) :: resource_relationship()
+  @callback resource_relationship(data()) :: resource_relationship()
   @callback type() :: resource_type() | nil
   @callback polymorphic_type(data()) :: resource_type() | nil
+  @callback resource_type(data()) :: resource_type()
   @callback url_for(data(), Conn.t() | nil) :: String.t()
   @callback url_for_pagination(data(), Conn.t(), Paginator.params()) :: String.t()
   @callback url_for_rel(term(), String.t(), Conn.t() | nil) :: String.t()
@@ -301,6 +304,7 @@ defmodule JSONAPI.View do
       def visible_fields(data, conn),
         do: View.visible_fields(__MODULE__, data, conn)
 
+      @impl View
       def resource_fields(data) do
         if @polymorphic_resource? do
           polymorphic_fields(data)
@@ -309,6 +313,7 @@ defmodule JSONAPI.View do
         end
       end
 
+      @impl View
       def resource_type(data) do
         if @polymorphic_resource? do
           polymorphic_type(data)
@@ -317,6 +322,7 @@ defmodule JSONAPI.View do
         end
       end
 
+      @impl View
       def resource_relationships(data) do
         if @polymorphic_resource? do
           polymorphic_relationships(data)

From 8d4b0661d510bd6f94d95d4a422ff94daed733cf Mon Sep 17 00:00:00 2001
From: Nick <nick.duitz@9elements.com>
Date: Tue, 6 May 2025 15:08:31 +0200
Subject: [PATCH 6/9] fix names

---
 lib/jsonapi/view.ex | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/lib/jsonapi/view.ex b/lib/jsonapi/view.ex
index a912587e..44ba3e5d 100644
--- a/lib/jsonapi/view.ex
+++ b/lib/jsonapi/view.ex
@@ -140,7 +140,7 @@ defmodule JSONAPI.View do
   @type options :: keyword()
   @type resource_id :: String.t()
   @type resource_type :: String.t()
-  @type resource_relationship :: [{atom(), t() | {t(), :include} | {atom(), t()} | {atom(), t(), :include}}]
+  @type resource_relationships :: [{atom(), t() | {t(), :include} | {atom(), t()} | {atom(), t(), :include}}]
   @type resource_fields :: [field()]
 
   @callback attributes(data(), Conn.t() | nil) :: map()
@@ -156,9 +156,9 @@ defmodule JSONAPI.View do
   @callback pagination_links(data(), Conn.t(), Paginator.page(), Paginator.options()) ::
               Paginator.links()
   @callback path() :: String.t() | nil
-  @callback relationships() :: resource_relationship()
-  @callback polymorphic_relationships(data()) :: resource_relationship()
-  @callback resource_relationship(data()) :: resource_relationship()
+  @callback relationships() :: resource_relationships()
+  @callback polymorphic_relationships(data()) :: resource_relationships()
+  @callback resource_relationships(data()) :: resource_relationships()
   @callback type() :: resource_type() | nil
   @callback polymorphic_type(data()) :: resource_type() | nil
   @callback resource_type(data()) :: resource_type()

From ec2cca543858e468ea37334454a6b955beb7fcb3 Mon Sep 17 00:00:00 2001
From: Nick <nick.duitz@9elements.com>
Date: Thu, 5 Jun 2025 17:42:17 +0200
Subject: [PATCH 7/9] fix credo linting

---
 lib/jsonapi/serializer.ex | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/lib/jsonapi/serializer.ex b/lib/jsonapi/serializer.ex
index f22505cb..353bdebe 100644
--- a/lib/jsonapi/serializer.ex
+++ b/lib/jsonapi/serializer.ex
@@ -80,7 +80,8 @@ defmodule JSONAPI.Serializer do
 
   @spec encode_relationships(Conn.t(), document(), tuple(), list()) :: tuple()
   def encode_relationships(conn, doc, {view, data, _, _} = view_info, options) do
-    view.resource_relationships(data)
+    data
+    |> view.resource_relationships()
     |> Enum.filter(&assoc_loaded?(Map.get(data, get_data_key(&1))))
     |> Enum.map_reduce(doc, &build_relationships(conn, view_info, &1, &2, options))
   end

From 53d343f8627c867d655dafcd8ad4a9964dcf43c7 Mon Sep 17 00:00:00 2001
From: Nick <nick.duitz@9elements.com>
Date: Thu, 5 Jun 2025 17:51:44 +0200
Subject: [PATCH 8/9] update docs on how to use polymorphic resources

---
 lib/jsonapi/view.ex | 76 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 76 insertions(+)

diff --git a/lib/jsonapi/view.ex b/lib/jsonapi/view.ex
index 44ba3e5d..2b265d1c 100644
--- a/lib/jsonapi/view.ex
+++ b/lib/jsonapi/view.ex
@@ -114,6 +114,82 @@ defmodule JSONAPI.View do
   and then use the `[user: {UserView, :include}]` syntax in your `includes` function. This tells
   the serializer to *always* include if its loaded.
 
+  ## Polymorphic Resources
+
+  Polymorphic resources allow you to serialize different types of data with the same view module.
+  This is useful when you have a collection of resources that share some common attributes but
+  have different types, fields, or relationships based on the specific data being serialized.
+
+  To enable polymorphic resources, set `polymorphic_resource?: true` when using the JSONAPI.View:
+
+      defmodule MediaView do
+        use JSONAPI.View, polymorphic_resource?: true
+
+        def polymorphic_type(%{type: "image"}), do: "image"
+        def polymorphic_type(%{type: "video"}), do: "video"
+        def polymorphic_type(%{type: "audio"}), do: "audio"
+
+        def polymorphic_fields(%{type: "image"}), do: [:id, :url, :width, :height, :alt_text]
+        def polymorphic_fields(%{type: "video"}), do: [:id, :url, :duration, :thumbnail]
+        def polymorphic_fields(%{type: "audio"}), do: [:id, :url, :duration, :bitrate]
+
+        def polymorphic_relationships(%{type: "image"}), do: [album: AlbumView]
+        def polymorphic_relationships(%{type: "video"}), do: [playlist: PlaylistView, author: UserView]
+        def polymorphic_relationships(%{type: "audio"}), do: [album: AlbumView, artist: ArtistView]
+      end
+
+  ### Required Callbacks for Polymorphic Resources
+
+  When using polymorphic resources, you must implement these callbacks instead of their non-polymorphic counterparts:
+
+  - `polymorphic_type/1` - Returns the JSONAPI type string based on the data
+  - `polymorphic_fields/1` - Returns the list of fields to serialize based on the data
+
+  ### Optional Callbacks for Polymorphic Resources
+
+  - `polymorphic_relationships/1` - Returns relationships specific to the data type (defaults to empty list)
+
+  ### Example Usage
+
+  With the above `MediaView`, you can serialize different media types:
+
+      # Image data
+      image = %{id: 1, type: "image", url: "/image.jpg", width: 800, height: 600, alt_text: "A photo"}
+      MediaView.show(image, conn)
+      # => %{data: %{id: "1", type: "image", attributes: %{url: "/image.jpg", width: 800, height: 600, alt_text: "A photo"}}}
+
+      # Video data
+      video = %{id: 2, type: "video", url: "/video.mp4", duration: 120, thumbnail: "/thumb.jpg"}
+      MediaView.show(video, conn)
+      # => %{data: %{id: "2", type: "video", attributes: %{url: "/video.mp4", duration: 120, thumbnail: "/thumb.jpg"}}}
+
+  ### Custom Field Functions
+
+  You can still define custom field functions that work across all polymorphic types:
+
+      defmodule MediaView do
+        use JSONAPI.View, polymorphic_resource?: true
+
+        def file_size(data, _conn) do
+          # Custom logic to calculate file size
+          calculate_file_size(data.url)
+        end
+
+        def polymorphic_fields(%{type: "image"}), do: [:id, :url, :file_size, :width, :height]
+        def polymorphic_fields(%{type: "video"}), do: [:id, :url, :file_size, :duration]
+        # ... other polymorphic implementations
+      end
+
+  ### Notes
+
+  - When `polymorphic_resource?: true` is set, the regular `type/0`, `fields/0`, and `relationships/0`
+    functions are not used and will return default values (nil or empty list)
+  - The polymorphic callbacks receive the actual data as their first argument, allowing you to
+    determine the appropriate type, fields, and relationships dynamically
+  - All other view functionality (links, meta, hidden fields, etc.) works the same way
+  - **Important**: Polymorphic resources currently do not work for deserializing data from POST
+    requests yet. They are only supported for serialization (rendering responses)
+
   ## Options
     * `:host` (binary) - Allows the `host` to be overridden for generated URLs. Defaults to `host` of the supplied `conn`.
 

From 63c43ab56580ef6f380ad394e6c48be4fce860f8 Mon Sep 17 00:00:00 2001
From: Nick <nick.duitz@9elements.com>
Date: Sat, 9 Aug 2025 00:18:48 +0200
Subject: [PATCH 9/9] no need for these to be callbacks

---
 lib/jsonapi/view.ex | 6 ------
 1 file changed, 6 deletions(-)

diff --git a/lib/jsonapi/view.ex b/lib/jsonapi/view.ex
index 2b265d1c..12fd8e73 100644
--- a/lib/jsonapi/view.ex
+++ b/lib/jsonapi/view.ex
@@ -223,7 +223,6 @@ defmodule JSONAPI.View do
   @callback id(data()) :: resource_id() | nil
   @callback fields() :: resource_fields()
   @callback polymorphic_fields(data()) :: resource_fields()
-  @callback resource_fields(data()) :: resource_fields()
   @callback get_field(field(), data(), Conn.t()) :: any()
   @callback hidden(data()) :: [field()]
   @callback links(data(), Conn.t()) :: links()
@@ -234,10 +233,8 @@ defmodule JSONAPI.View do
   @callback path() :: String.t() | nil
   @callback relationships() :: resource_relationships()
   @callback polymorphic_relationships(data()) :: resource_relationships()
-  @callback resource_relationships(data()) :: resource_relationships()
   @callback type() :: resource_type() | nil
   @callback polymorphic_type(data()) :: resource_type() | nil
-  @callback resource_type(data()) :: resource_type()
   @callback url_for(data(), Conn.t() | nil) :: String.t()
   @callback url_for_pagination(data(), Conn.t(), Paginator.params()) :: String.t()
   @callback url_for_rel(term(), String.t(), Conn.t() | nil) :: String.t()
@@ -380,7 +377,6 @@ defmodule JSONAPI.View do
       def visible_fields(data, conn),
         do: View.visible_fields(__MODULE__, data, conn)
 
-      @impl View
       def resource_fields(data) do
         if @polymorphic_resource? do
           polymorphic_fields(data)
@@ -389,7 +385,6 @@ defmodule JSONAPI.View do
         end
       end
 
-      @impl View
       def resource_type(data) do
         if @polymorphic_resource? do
           polymorphic_type(data)
@@ -398,7 +393,6 @@ defmodule JSONAPI.View do
         end
       end
 
-      @impl View
       def resource_relationships(data) do
         if @polymorphic_resource? do
           polymorphic_relationships(data)