Re-use implementation of diff-association for has_many_through

lib/ecto_diff.ex

@@ -367,33 +367,26 @@ defmodule EctoDiff do
     end
   end
 
-  defp diff_association(previous, current, %{cardinality: :many, through: [through, field]}, acc, opts) do
-    assoc_diff =
-      acc
-      |> Keyword.get_values(through)
-      |> List.flatten()
-      |> Enum.map(& &1[:changes][field])
-
-    diff =
-      case assoc_diff do
-        [_ | _] = diff ->
-          List.flatten(diff)
-
-        _ ->
-          {previous, current} =
-            case length(previous) - length(current) do
-              0 -> {previous, current}
-              n when n > 0 -> {previous, current ++ List.duplicate(nil, n)}
-              n when n < 0 -> {previous ++ List.duplicate(nil, -n), current}
-            end
-
-          Enum.zip_with([previous, current], fn [p, c] -> do_diff(p, c, opts) end)
-      end
-
-    if no_changes?(diff), do: acc, else: [{field, diff} | acc]
+  defp diff_association(
+         previous,
+         current,
+         %{cardinality: :many, through: [through, assoc_field]} = has_through,
+         acc,
+         opts
+       ) do
+    %{field: field, owner: owner} = has_through
+
+    association = owner.__schema__(:association, through).related
+    struct = association.__schema__(:association, assoc_field).related
+
+    do_diff_association(previous, current, field, struct, acc, opts)
   end
 
   defp diff_association(previous, current, %{cardinality: :many, field: field, related: struct}, acc, opts) do
+    do_diff_association(previous, current, field, struct, acc, opts)
+  end
+
+  defp do_diff_association(previous, current, field, struct, acc, opts) do
     primary_key_fields = get_primary_key_fields(struct, opts)
 
     if primary_key_fields == [],

test/ecto_diff_test.exs

@@ -813,6 +813,8 @@ defmodule EctoDiffTest do
         })
         |> Repo.update()
 
+      updated_pet = Repo.preload(updated_pet, [:toys], force: true)
+
       {:ok, diff} = EctoDiff.diff(pet, updated_pet)
 
       assert %EctoDiff{
@@ -861,6 +863,7 @@ defmodule EctoDiffTest do
       [ball_id] = Enum.map(pet.toys, & &1.id)
 
       {:ok, updated_pet} = pet |> Pet.update(%{resources: [%{id: resource_id, toys: []}]}) |> Repo.update()
+      updated_pet = Repo.preload(updated_pet, [:toys], force: true)
 
       {:ok, diff} = EctoDiff.diff(pet, updated_pet)