feat: add preload_order for has_one (todo: add documentation)

integration_test/cases/preload.exs

@@ -605,6 +605,21 @@ defmodule Ecto.Integration.PreloadTest do
     assert [%{name: "foo"}, %{name: "bar"}] = post.ordered_users
   end
 
+  test "custom preload_order (one)" do
+    post = TestRepo.insert!(%Post{users: [%User{name: "bar"}, %User{name: "foo"}], title: "1"})
+
+    TestRepo.insert!(%Comment{text: "1", post_id: post.id})
+    TestRepo.insert!(%Comment{text: "2", post_id: post.id})
+    TestRepo.insert!(%Comment{text: "3", post_id: post.id})
+
+    post = TestRepo.preload(post, [:first_comment, :last_comment])
+
+    # asc
+    assert %{text: "1"} = post.first_comment
+    # desc
+    assert %{text: "3"} = post.last_comment
+  end
+
   test "custom preload_order with mfa" do
     post1 = TestRepo.insert!(%Post{users: [%User{name: "bar"}, %User{name: "foo"}], title: "1"})
     post2 = TestRepo.insert!(%Post{users: [%User{name: "baz"}, %User{name: "foz"}], title: "2"})

integration_test/support/schemas.exs

@@ -49,6 +49,8 @@ defmodule Ecto.Integration.Post do
     has_many :comments, Ecto.Integration.Comment, on_delete: :delete_all, on_replace: :delete
     has_many :force_comments, Ecto.Integration.Comment, on_replace: :delete_if_exists
     has_many :ordered_comments, Ecto.Integration.Comment, preload_order: [:text]
+    has_one :first_comment, Ecto.Integration.Comment, preload_order: [asc: :id]
+    has_one :last_comment, Ecto.Integration.Comment, preload_order: [desc: :id]
     # The post<->permalink relationship should be marked as uniq
     has_one :permalink, Ecto.Integration.Permalink, on_delete: :delete_all, on_replace: :delete
     has_one :force_permalink, Ecto.Integration.Permalink, on_replace: :delete_if_exists

lib/ecto/repo/preloader.ex

@@ -326,8 +326,11 @@ defmodule Ecto.Repo.Preloader do
           end
 
         {:one, _} ->
-          query
-      end
+          case assoc.relationship do
+            :parent -> query
+            _ -> add_preload_order(assoc.preload_order, query)
+          end
+        end
 
     unzip_ids Ecto.Repo.Queryable.all(repo_name, query, tuplet), [], []
   end