Pass only the minimum set of fields needed to resolvers (#12)

* Pass only the minimum set of fields needed to resolvers

* Add a test that ensures only the minimum set of values needed for resolvers are being passed

Author: zkayser

lib/workflow.ex

@@ -1005,18 +1005,32 @@ defmodule Pacer.Workflow do
     case module.__graph__(:resolver, vertex) do
       {:field, resolver} when is_function(resolver, 1) ->
         metadata = %{field: vertex, workflow: workflow.__struct__}
+        resolver_args = Map.take(workflow, [vertex | module.__graph__(:dependencies, vertex)])
 
         :telemetry.span([:pacer, :execute_vertex], metadata, fn ->
-          result = Map.put(workflow, vertex, resolver.(workflow))
+          result =
+            Map.put(
+              workflow,
+              vertex,
+              resolver.(resolver_args)
+            )
+
           {result, metadata}
         end)
 
       {:batch, [{field, resolver}]} ->
         metadata = %{field: field, workflow: workflow.__struct__}
 
         try do
+          resolver_args =
+            Map.take(workflow, [field | module.__graph__(:batched_field_dependencies, field)])
+
           :telemetry.span([:pacer, :execute_vertex], metadata, fn ->
-            {Map.put(workflow, field, resolver.(workflow)), metadata}
+            {Map.put(
+               workflow,
+               field,
+               resolver.(resolver_args)
+             ), metadata}
           end)
         catch
           _kind, _error ->

test/workflow_test.exs

@@ -290,6 +290,33 @@ defmodule Pacer.WorkflowTest do
     refute Map.has_key?(result, :field_with_default)
   end
 
+  test "resolver functions only receive their explicit dependencies and the current field when invoked" do
+    defmodule ResolverInputSizeTest do
+      use Pacer.Workflow
+
+      graph do
+        field(:a, default: 1)
+        field(:b, resolver: &__MODULE__.resolve_b/1, dependencies: [:a])
+        field(:c, resolver: &__MODULE__.resolve_c/1, dependencies: [:a, :b])
+      end
+
+      def resolve_b(args) do
+        assert map_size(args) == 2
+        assert Map.has_key?(args, :a)
+        assert Map.has_key?(args, :b)
+      end
+
+      def resolve_c(args) do
+        assert map_size(args) == 3
+        assert Map.has_key?(args, :a)
+        assert Map.has_key?(args, :b)
+        assert Map.has_key?(args, :c)
+      end
+    end
+
+    Pacer.Workflow.execute(ResolverInputSizeTest)
+  end
+
   test "field defaults" do
     assert %TestGraph{
              field_a: %FieldNotSet{},