From 99d9588c357b1b7e3e6c7d299cd6496cbfea5da9 Mon Sep 17 00:00:00 2001
From: Derek Kraan <derek@kraan.dev>
Date: Tue, 1 Jul 2025 10:54:59 +0200
Subject: [PATCH 1/2] Propagate opts configured in `prepare_query/3` to
 preloads

This ensures that when `:on_preloader_spawn` is set in the
`prepare_query/3` callback, it is propagated to the preloader.
---
 lib/ecto/repo/queryable.ex | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/lib/ecto/repo/queryable.ex b/lib/ecto/repo/queryable.ex
index 43e8d07797..3264671d32 100644
--- a/lib/ecto/repo/queryable.ex
+++ b/lib/ecto/repo/queryable.ex
@@ -213,7 +213,7 @@ defmodule Ecto.Repo.Queryable do
 
   ## Helpers
 
-  defp execute(operation, name, query, {adapter_meta, opts} = tuplet) do
+  defp execute(operation, name, query, {adapter_meta, opts} = _tuplet) do
     %{adapter: adapter, cache: cache, repo: repo} = adapter_meta
 
     {query, opts} = repo.prepare_query(operation, query, opts)
@@ -244,7 +244,14 @@ defmodule Ecto.Repo.Queryable do
         {count,
          rows
          |> Ecto.Repo.Assoc.query(assocs, sources, preprocessor)
-         |> Ecto.Repo.Preloader.query(name, preloads, take, assocs, postprocessor, tuplet)}
+         |> Ecto.Repo.Preloader.query(
+           name,
+           preloads,
+           take,
+           assocs,
+           postprocessor,
+           {adapter_meta, opts}
+         )}
     end
   end
 

From c92d8a808e8c7cdaa0bd57cd534561c0dc71b11b Mon Sep 17 00:00:00 2001
From: Derek Kraan <derek@kraan.dev>
Date: Tue, 1 Jul 2025 14:46:19 +0200
Subject: [PATCH 2/2] WIP tests

---
 lib/ecto/repo.ex        |  8 +++++++-
 test/ecto/repo_test.exs | 35 ++++++++++++++++++++++++++++++++++-
 2 files changed, 41 insertions(+), 2 deletions(-)

diff --git a/lib/ecto/repo.ex b/lib/ecto/repo.ex
index e570e0b038..7ad6320016 100644
--- a/lib/ecto/repo.ex
+++ b/lib/ecto/repo.ex
@@ -644,11 +644,17 @@ defmodule Ecto.Repo do
         def preload(struct_or_structs_or_nil, preloads, opts \\ []) do
           repo = get_dynamic_repo()
 
+          {adapter_meta, opts} =
+            _tuplet =
+            Ecto.Repo.Supervisor.tuplet(repo, prepare_opts(:preload, opts))
+
+          {_query, opts} = repo.prepare_query(:preload, struct_or_structs_or_nil, opts)
+
           Ecto.Repo.Preloader.preload(
             struct_or_structs_or_nil,
             repo,
             preloads,
-            Ecto.Repo.Supervisor.tuplet(repo, prepare_opts(:preload, opts))
+            {adapter_meta, opts}
           )
         end
 
diff --git a/test/ecto/repo_test.exs b/test/ecto/repo_test.exs
index bd10e72f11..01d9c420aa 100644
--- a/test/ecto/repo_test.exs
+++ b/test/ecto/repo_test.exs
@@ -182,7 +182,7 @@ defmodule Ecto.RepoTest do
   end
 
   defmodule MySchemaOneField do
-     use Ecto.Schema
+    use Ecto.Schema
 
     @primary_key false
     schema "my_schema" do
@@ -2211,6 +2211,13 @@ defmodule Ecto.RepoTest do
 
       def prepare_query(op, query, opts) do
         send(self(), {op, query, opts})
+
+        opts =
+          case Keyword.fetch(opts, :for_on_preloader_spawn) do
+            {:ok, fun} -> [{:on_preloader_spawn, fun} | opts]
+            _ -> opts
+          end
+
         {%{query | prefix: "rewritten"}, opts}
       end
     end
@@ -2270,6 +2277,32 @@ defmodule Ecto.RepoTest do
       assert_received {:callback_ran, pid2} when pid2 != self()
       assert pid1 != pid2
     end
+
+    test "preload with :on_preloader_spawn in prepare_query/3 callback" do
+      test_process = self()
+      fun = fn -> send(test_process, {:callback_ran, self()}) end
+
+      %MySchemaWithMultiAssoc{parent_id: 1, mother_id: 2}
+      |> PrepareRepo.preload([:parent, :mother], for_on_preloader_spawn: fun)
+
+      assert_received {:callback_ran, pid1} when pid1 != self()
+      assert_received {:callback_ran, pid2} when pid2 != self()
+      assert pid1 != pid2
+    end
+
+    test "all with preload with :on_preloader_spawn in prepare_query/3 callback" do
+      test_process = self()
+      fun = fn -> send(test_process, {:callback_ran, self()}) end
+
+      from(p in MyParent,
+        preload: [:parent, :mother]
+      )
+      |> PrepareRepo.all(for_on_preloader_spawn: fun)
+
+      assert_received {:callback_ran, pid1} when pid1 != self()
+      assert_received {:callback_ran, pid2} when pid2 != self()
+      assert pid1 != pid2
+    end
   end
 
   describe "prepare_transaction" do