Only drop autogenerated ids and not domain ids

lib/ex_machina/ecto.ex

@@ -121,9 +121,18 @@ defmodule ExMachina.Ecto do
     |> Map.from_struct
     |> Map.delete(:__meta__)
     |> Map.drop(struct.__schema__(:associations))
-    |> Map.drop(struct.__schema__(:primary_key))
+    |> drop_autogenerated_ids(struct)
   end
   defp drop_ecto_fields(record) do
     raise ArgumentError, "#{inspect record} is not an Ecto model. Use `build` instead."
   end
+
+  # Don't want to drop non-autogenrated ids because those are meaningful to the
+  # domain in some way and will be expected to be set explicitly
+  defp drop_autogenerated_ids(map, struct) do
+    case struct.__schema__(:autogenerate_id) do
+      nil -> map
+      {name, _type} -> Map.delete(map, name)
+    end
+  end
 end

test/ex_machina/ecto_test.exs

@@ -37,6 +37,13 @@ defmodule ExMachina.EctoTest do
     }
   end
 
+  test "params_for/2 leaves ids that are not auto-generated" do
+    assert TestFactory.params_for(:custom) == %{
+      custom_id: 1,
+      name: "Testing"
+    }
+  end
+
   test "params_for/2 raises when passed a map" do
     assert_raise ArgumentError, fn ->
       TestFactory.params_for(:user_map)

test/support/models/custom.ex

@@ -0,0 +1,9 @@
+defmodule ExMachina.Custom do
+  use Ecto.Schema
+
+  @primary_key {:custom_id, :integer, []}
+  schema "customs" do
+    field :name, :string
+  end
+end
+

test/support/test_factory.ex

@@ -1,6 +1,12 @@
 defmodule ExMachina.TestFactory do
   use ExMachina.Ecto, repo: ExMachina.TestRepo
 
+  def custom_factory do
+    %ExMachina.Custom{
+      custom_id: 1,
+      name: "Testing"
+    }
+  end
   def user_factory do
     %ExMachina.User{
       name: "John Doe",