test: Add failing test for aggregate with parent() + select() + limit() (#644)

Reproduces a bug where combining select() + limit() with an aggregate
that uses parent() in its filter causes SQL error.

This bug was found in ash_graphql where GraphQL list queries would
automatically trigger this scenario.

Author: jechol

priv/resource_snapshots/test_repo/chats/20251030040520.json

@@ -0,0 +1,74 @@
+{
+  "attributes": [
+    {
+      "allow_nil?": false,
+      "default": "fragment(\"gen_random_uuid()\")",
+      "generated?": false,
+      "precision": null,
+      "primary_key?": true,
+      "references": null,
+      "scale": null,
+      "size": null,
+      "source": "id",
+      "type": "uuid"
+    },
+    {
+      "allow_nil?": true,
+      "default": "nil",
+      "generated?": false,
+      "precision": null,
+      "primary_key?": false,
+      "references": null,
+      "scale": null,
+      "size": null,
+      "source": "name",
+      "type": "text"
+    },
+    {
+      "allow_nil?": true,
+      "default": "nil",
+      "generated?": false,
+      "precision": null,
+      "primary_key?": false,
+      "references": {
+        "deferrable": false,
+        "destination_attribute": "id",
+        "destination_attribute_default": null,
+        "destination_attribute_generated": null,
+        "index?": false,
+        "match_type": null,
+        "match_with": null,
+        "multitenancy": {
+          "attribute": null,
+          "global": null,
+          "strategy": null
+        },
+        "name": "chats_last_read_message_id_fkey",
+        "on_delete": null,
+        "on_update": null,
+        "primary_key?": true,
+        "schema": "public",
+        "table": "messages"
+      },
+      "scale": null,
+      "size": null,
+      "source": "last_read_message_id",
+      "type": "uuid"
+    }
+  ],
+  "base_filter": null,
+  "check_constraints": [],
+  "custom_indexes": [],
+  "custom_statements": [],
+  "has_create_action": true,
+  "hash": "78989357AB74584B9A18249E307C842D17AA96B7DD790EAADC9FA1C6422388B5",
+  "identities": [],
+  "multitenancy": {
+    "attribute": null,
+    "global": null,
+    "strategy": null
+  },
+  "repo": "Elixir.AshPostgres.TestRepo",
+  "schema": null,
+  "table": "chats"
+}

priv/test_repo/migrations/20251030040520_add_last_read_message_to_chats.exs

@@ -0,0 +1,31 @@
+defmodule AshPostgres.TestRepo.Migrations.AddLastReadMessageToChats do
+  @moduledoc """
+  Updates resources based on their most recent snapshots.
+
+  This file was autogenerated with `mix ash_postgres.generate_migrations`
+  """
+
+  use Ecto.Migration
+
+  def up do
+    alter table(:chats) do
+      add(
+        :last_read_message_id,
+        references(:messages,
+          column: :id,
+          name: "chats_last_read_message_id_fkey",
+          type: :uuid,
+          prefix: "public"
+        )
+      )
+    end
+  end
+
+  def down do
+    drop(constraint(:chats, "chats_last_read_message_id_fkey"))
+
+    alter table(:chats) do
+      remove(:last_read_message_id)
+    end
+  end
+end

test/aggregate_test.exs

@@ -5,7 +5,7 @@
 defmodule AshSql.AggregateTest do
   use AshPostgres.RepoCase, async: false
   import ExUnit.CaptureIO
-  alias AshPostgres.Test.{Author, Comment, Organization, Post, Rating, User}
+  alias AshPostgres.Test.{Author, Chat, Comment, Organization, Post, Rating, User}
 
   require Ash.Query
   import Ash.Expr
@@ -1860,4 +1860,63 @@ defmodule AshSql.AggregateTest do
     assert Enum.at(results, 0).count_of_comments == 3
     assert Enum.at(results, 1).count_of_comments == 2
   end
+
+  describe "aggregate with parent filter and limited select" do
+    test "FAILS when combining select() + limit() with aggregate using parent() in filter" do
+      # BUG: When using select() + limit() with an aggregate that uses parent()
+      # in its filter, the query generation creates a subquery that's missing the parent
+      # fields, causing a SQL error.
+      #
+      # This bug was found in ash_graphql where GraphQL list queries with pagination
+      # would fail when loading aggregates that use parent() in filters.
+      #
+      # The bug requires BOTH conditions:
+      # 1. select() limits which fields are included (e.g., only :id)
+      # 2. limit() causes a subquery to be generated
+      # 3. An aggregate filter references parent() fields that aren't in select()
+      #
+      # Without BOTH select() and limit(), the query works fine (see tests below).
+      #
+      # Current error:
+      # ERROR 42703 (undefined_column) column s0.last_read_message_id does not exist
+      #
+      # Generated query:
+      # SELECT s0."id", coalesce(s1."unread_message_count"::bigint, ...)
+      # FROM (SELECT sc0."id" AS "id" FROM "chats" AS sc0 LIMIT 10) AS s0
+      # LEFT OUTER JOIN LATERAL (
+      #   SELECT ... FROM "messages" WHERE ... s0."last_read_message_id" ...  # <- field not in subquery!
+      # ) AS s1 ON TRUE
+      #
+      # Expected fix: Ash should automatically include parent() referenced fields
+      # (like last_read_message_id) in the subquery even if not explicitly selected.
+
+      Chat
+      |> Ash.Query.select(:id)
+      |> Ash.Query.load(:unread_message_count)
+      |> Ash.Query.limit(10)
+      |> Ash.read!()
+    end
+
+    test "works WITHOUT select() - limit alone doesn't cause the bug" do
+      Chat
+      |> Ash.Query.load(:unread_message_count)
+      |> Ash.Query.limit(10)
+      |> Ash.read!()
+    end
+
+    test "works WITHOUT limit() - select alone doesn't cause the bug" do
+      Chat
+      |> Ash.Query.select(:id)
+      |> Ash.Query.load(:unread_message_count)
+      |> Ash.read!()
+    end
+
+    test "works when selecting the parent() referenced field explicitly (workaround)" do
+      Chat
+      |> Ash.Query.select([:id, :last_read_message_id])
+      |> Ash.Query.load(:unread_message_count)
+      |> Ash.Query.limit(10)
+      |> Ash.read!()
+    end
+  end
 end

test/support/resources/chat.ex

@@ -24,6 +24,12 @@ defmodule AshPostgres.Test.Chat do
   end
 
   relationships do
+    belongs_to :last_read_message, AshPostgres.Test.Message do
+      allow_nil?(true)
+      public?(true)
+      attribute_writable?(true)
+    end
+
     has_many :messages, AshPostgres.Test.Message do
       public?(true)
     end
@@ -41,4 +47,11 @@ defmodule AshPostgres.Test.Chat do
       sort(sent_at: :desc)
     end
   end
+
+  aggregates do
+    count :unread_message_count, :messages do
+      public?(true)
+      filter(expr(is_nil(parent(last_read_message_id)) or id > parent(last_read_message_id)))
+    end
+  end
 end