Fully Support Sparse Fieldsets

Whilst the `QueryParser` was correctly identifying requested fieldsets,
nothing was done to actually support this.

This change prunes returned fields to those requested should it be the
case.

Note that this change also includes a few more typespecs for functions I
touched or read.

Resolves #120
Closes #156

lib/jsonapi/plugs/query_parser.ex

@@ -67,10 +67,12 @@ defmodule JSONAPI.QueryParser do
   For more details please see `JSONAPI.UnderscoreParameters`.
   """
 
+  @impl Plug
   def init(opts) do
     build_config(opts)
   end
 
+  @impl Plug
   def call(conn, opts) do
     query_params_config_struct =
       conn
@@ -111,7 +113,8 @@ defmodule JSONAPI.QueryParser do
     end
   end
 
-  def parse_fields(config, map) when map_size(map) == 0, do: config
+  @spec parse_fields(Config.t(), map()) :: Config.t() | no_return()
+  def parse_fields(%Config{} = config, fields) when fields == %{}, do: config
 
   def parse_fields(%Config{} = config, fields) do
     Enum.reduce(fields, config, fn {type, value}, acc ->

lib/jsonapi/view.ex

@@ -37,9 +37,9 @@ defmodule JSONAPI.View do
 
   ## Fields
 
-  By default, the resulting JSON document consists of fields, defined in fields/0
+  By default, the resulting JSON document consists of fields, defined in `fields/0`
   function. You can define custom fields or override current fields by defining
-  inside the view function `field_name/2` that takes data and conn as arguments.
+  inside the view function `field_name/2` that takes `data` and `conn` as arguments.
 
       defmodule UserView do
         use JSONAPI.View
@@ -51,6 +51,32 @@ defmodule JSONAPI.View do
         def relationships, do: []
       end
 
+  Fields may be omitted manually using the `hidden/1` function.
+
+      defmodule UserView do
+        use JSONAPI.View
+
+        def fields, do: [:id, :username, :email]
+
+        def type, do: "user"
+
+        def hidden(_data) do
+          [:email] # will be removed from the response
+        end
+      end
+
+  In order to use [sparse fieldsets](https://jsonapi.org/format/#fetching-sparse-fieldsets)
+  you must include the `JSONAPI.QueryParser` plug.
+
+  ### Advanced Fields Usage
+
+  Note that the overriddable `attributes/2` method can be used for more control
+  over which fields are serialized from your view. It is **strongly** recommended
+  that you exhaustively try a combination of `fields/0`, and `hidden/1` first as
+  this method will likely be deprecated in a future release. Should you desire to
+  do so, you may wish to call `visisble_fields/2` within your implementation in
+  order to get the list of fields that will be sent to the client.
+
   ## Relationships
 
   Currently the relationships callback expects that a map is returned
@@ -84,6 +110,9 @@ defmodule JSONAPI.View do
   The default behaviour for `host` and `scheme` is to derive it from the `conn` provided, while the
   default style for presentation in names is to be underscored and not dashed.
   """
+
+  alias Plug.Conn
+
   defmacro __using__(opts \\ []) do
     {type, opts} = Keyword.pop(opts, :type)
     {namespace, _opts} = Keyword.pop(opts, :namespace, "")
@@ -104,10 +133,37 @@ defmodule JSONAPI.View do
         def type, do: raise("Need to implement type/0")
       end
 
-      def attributes(data, conn) do
-        hidden = hidden(data)
+      defp requested_fields_for_type(%Conn{assigns: %{jsonapi_query: %{fields: fields}}} = conn) do
+        fields[type()]
+      end
 
-        visible_fields = fields() -- hidden
+      defp requested_fields_for_type(_conn), do: nil
+
+      defp fields_for_type(requested_fields, fields) when requested_fields in [nil, %{}],
+        do: fields
+
+      defp fields_for_type(requested_fields, fields) do
+        fields
+        |> MapSet.new()
+        |> MapSet.intersection(MapSet.new(requested_fields))
+        |> MapSet.to_list()
+      end
+
+      @spec visible_fields(map(), conn :: nil | Conn.t()) :: list(atom)
+      def visible_fields(data, conn) do
+        all_fields =
+          conn
+          |> requested_fields_for_type()
+          |> fields_for_type(fields())
+
+        hidden_fields = hidden(data)
+
+        all_fields -- hidden_fields
+      end
+
+      @spec attributes(map(), conn :: nil | Conn.t()) :: map()
+      def attributes(data, conn) do
+        visible_fields = visible_fields(data, conn)
 
         Enum.reduce(visible_fields, %{}, fn field, intermediate_map ->
           value =

test/jsonapi/view_test.exs

@@ -135,7 +135,28 @@ defmodule JSONAPI.ViewTest do
     assert data.meta.total_pages == 100
   end
 
-  test "attributes/2 does not display hidden fields with deprecated hidden/0" do
+  test "visible_fields/2 returns all field names by default" do
+    data = %{age: 100, first_name: "Jason", last_name: "S", password: "securepw"}
+
+    assert [:age, :first_name, :last_name, :full_name] ==
+             UserView.visible_fields(data, %Plug.Conn{})
+  end
+
+  test "visible_fields/2 removes any hidden field names" do
+    data = %{title: "Hidden body", body: "Something"}
+
+    assert [:title] == PostView.visible_fields(data, %Plug.Conn{})
+  end
+
+  test "visible_fields/2 trims returned field names to only those requested" do
+    data = %{body: "Chunky", title: "Bacon"}
+    config = %JSONAPI.Config{fields: %{PostView.type() => [:body]}}
+    conn = %Plug.Conn{assigns: %{jsonapi_query: config}}
+
+    assert [:body] == PostView.visible_fields(data, conn)
+  end
+
+  test "attributes/2 does not display hidden fields" do
     expected_map = %{age: 100, first_name: "Jason", last_name: "S", full_name: "Jason S"}
 
     assert expected_map ==
@@ -161,4 +182,12 @@ defmodule JSONAPI.ViewTest do
                nil
              )
   end
+
+  test "attributes/2 can return only requested fields" do
+    data = %{body: "Chunky", title: "Bacon"}
+    config = %JSONAPI.Config{fields: %{PostView.type() => [:body]}}
+    conn = %Plug.Conn{assigns: %{jsonapi_query: config}}
+
+    assert %{body: "Chunky"} == PostView.attributes(data, conn)
+  end
 end

test/jsonapi_test.exs

@@ -275,6 +275,25 @@ defmodule JSONAPITest do
     assert Map.has_key?(json, "links")
   end
 
+  test "handles sparse fields properly" do
+    conn =
+      :get
+      |> conn("/posts?include=other_user.company&fields[mytype]=text,excerpt")
+      |> Plug.Conn.assign(:data, [@default_data])
+      |> MyPostPlug.call([])
+
+    assert %{
+             "data" => [
+               %{
+                 "attributes" => %{
+                   "text" => "Hello",
+                   "excerpt" => "He"
+                 }
+               }
+             ]
+           } = Jason.decode!(conn.resp_body)
+  end
+
   test "omits explicit nil meta values as per http://jsonapi.org/format/#document-meta" do
     conn =
       :get