feat: support offset option in lateral join queries (#700)

Applies relationship.offset to the lateral join base query so that
has_one/has_many relationships with offset work correctly in PostgreSQL.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: nallwhy

lib/data_layer.ex

@@ -1116,6 +1116,13 @@ defmodule AshPostgres.DataLayer do
         base_query
       end
 
+    base_query =
+      if Map.get(relationship, :offset) do
+        from(row in base_query, offset: ^relationship.offset)
+      else
+        base_query
+      end
+
     base_query =
       cond do
         Map.get(relationship, :manual) ->

test/load_test.exs

@@ -65,6 +65,33 @@ defmodule AshPostgres.Test.LoadTest do
              |> Map.get(:latest_comment)
   end
 
+  test "has_one with offset returns the correct record" do
+    post =
+      Post
+      |> Ash.Changeset.for_create(:create, %{title: "title"})
+      |> Ash.create!()
+
+    %{id: first_comment_id} =
+      Comment
+      |> Ash.Changeset.for_create(:create, %{title: "first"})
+      |> Ash.Changeset.manage_relationship(:post, post, type: :append_and_remove)
+      |> Ash.create!()
+
+    :timer.sleep(1)
+
+    Comment
+    |> Ash.Changeset.for_create(:create, %{title: "second"})
+    |> Ash.Changeset.manage_relationship(:post, post, type: :append_and_remove)
+    |> Ash.create!()
+
+    # second_latest_comment: sort(created_at: :desc), offset(1) => first comment
+    assert %{id: ^first_comment_id} =
+             Post
+             |> Ash.Query.load(:second_latest_comment)
+             |> Ash.read_one!()
+             |> Map.get(:second_latest_comment)
+  end
+
   test "belongs_to relationships can be loaded" do
     assert %Comment{post: %Ash.NotLoaded{type: :relationship}} =
              comment =

test/support/resources/post.ex

@@ -762,6 +762,12 @@ defmodule AshPostgres.Test.Post do
       public?(true)
     end
 
+    has_one :second_latest_comment, AshPostgres.Test.Comment do
+      sort(created_at: :desc)
+      offset(1)
+      public?(true)
+    end
+
     has_many :comments_matching_post_title, AshPostgres.Test.Comment do
       public?(true)
       filter(expr(title == parent_expr(title)))