Verify fields given to @derive (michalmuskala#123)

Author: thiamsantos

lib/encoder.ex

@@ -135,15 +135,35 @@ defimpl Jason.Encoder, for: Any do
   end
 
   defp fields_to_encode(struct, opts) do
+    fields = Map.keys(struct)
+
     cond do
       only = Keyword.get(opts, :only) ->
-        only
+        case only -- fields do
+          [] ->
+            only
+
+          error_keys ->
+            raise ArgumentError,
+              "`:only` specified keys (#{inspect(error_keys)}) that are not defined in defstruct: " <>
+                "#{inspect(fields -- [:__struct__])}"
+
+        end
 
       except = Keyword.get(opts, :except) ->
-        Map.keys(struct) -- [:__struct__ | except]
+        case except -- fields do
+          [] ->
+            fields -- [:__struct__ | except]
+
+          error_keys ->
+            raise ArgumentError,
+              "`:except` specified keys (#{inspect(error_keys)}) that are not defined in defstruct: " <>
+                "#{inspect(fields -- [:__struct__])}"
+
+        end
 
       true ->
-        Map.keys(struct) -- [:__struct__]
+        fields -- [:__struct__]
     end
   end
 end

test/encode_test.exs

@@ -134,6 +134,32 @@ defmodule Jason.EncoderTest do
     assert to_json(derived_using_except) == ~s({"size":10})
   end
 
+  test "@derive validate `except:`" do
+    message = "`:except` specified keys ([:invalid]) that are not defined in defstruct: [:name]"
+
+    assert_raise ArgumentError, message, fn ->
+      Code.eval_string("""
+      defmodule InvalidExceptField do
+        @derive {Jason.Encoder, except: [:invalid]}
+        defstruct name: ""
+      end
+      """)
+    end
+  end
+
+  test "@derive validate `only:`" do
+    message = "`:only` specified keys ([:invalid]) that are not defined in defstruct: [:name]"
+
+    assert_raise ArgumentError, message, fn ->
+      Code.eval_string("""
+      defmodule InvalidOnlyField do
+        @derive {Jason.Encoder, only: [:invalid]}
+        defstruct name: ""
+      end
+      """)
+    end
+  end
+
   defmodule KeywordTester do
     defstruct [:baz, :foo, :quux]
   end