Raise when primary keys in Ecto are nils

lib/dataloader/ecto.ex

@@ -519,7 +519,18 @@ if Code.ensure_loaded?(Ecto) do
             |> :erlang.term_to_iovec()
             |> :erlang.md5()
           else
-            Enum.map(primary_keys, &Map.get(record, &1))
+            primary_key_values = Enum.map(primary_keys, &Map.get(record, &1))
+
+            Enum.each(Enum.zip(primary_keys, primary_key_values), fn {key, value} ->
+              if is_nil(value) do
+                raise """
+                Key #{inspect(key)} is nil for record #{inspect(record)}.
+                Make sure to query all the fields that make up the primary key.
+                """
+              end
+            end)
+
+            primary_key_values
           end
 
         queryable = chase_down_queryable([assoc_field], schema)

test/dataloader/ecto_test.exs

@@ -477,6 +477,20 @@ defmodule Dataloader.EctoTest do
     end
   end
 
+  test "raises error when a primary key value is nil", %{loader: loader} do
+    %User{username: "Robert Lewandowski"} |> Repo.insert!()
+
+    user_with_nil_id =
+      from(u in User, select: [:username], where: u.username == "Robert Lewandowski")
+      |> Repo.one()
+
+    assert_raise RuntimeError, ~r/Key :id is nil for record/, fn ->
+      loader
+      |> Dataloader.load(Test, :posts, user_with_nil_id)
+      |> Dataloader.run()
+    end
+  end
+
   test "when dataloader times out it raises an error" do
     user =
       %User{username: "Ben Wilson"}