refactor: prune relationship projections before query execution

lib/data_layer.ex

@@ -942,6 +942,7 @@ defmodule AshPostgres.DataLayer do
   @impl true
   def run_query(query, resource) do
     query = AshSql.Bindings.default_bindings(query, resource, AshPostgres.SqlImplementation)
+    query = AshSql.Query.Projection.prune(query)
 
     if AshPostgres.DataLayer.Info.polymorphic?(resource) && no_table?(query) do
       raise_table_error!(resource, :read)
@@ -1165,7 +1166,7 @@ defmodule AshPostgres.DataLayer do
                     AshSql.dynamic_repo(source_resource, AshPostgres.SqlImplementation, query)
 
                   repo.one(
-                    query,
+                    AshSql.Query.Projection.prune(query),
                     AshSql.repo_opts(
                       repo,
                       AshPostgres.SqlImplementation,
@@ -1226,6 +1227,8 @@ defmodule AshPostgres.DataLayer do
             _ -> lateral_join_query
           end
 
+        lateral_join_query = AshSql.Query.Projection.prune(lateral_join_query)
+
         results =
           repo.all(
             lateral_join_query,
@@ -2630,7 +2633,12 @@ defmodule AshPostgres.DataLayer do
 
       with_savepoint(repo, query, fn ->
         {:ok,
-         repo.one(query) |> Enum.sort_by(&elem(&1, 0)) |> Enum.map(&elem(&1, 1)) |> Enum.reverse()}
+         query
+         |> AshSql.Query.Projection.prune()
+         |> repo.one()
+         |> Enum.sort_by(&elem(&1, 0))
+         |> Enum.map(&elem(&1, 1))
+         |> Enum.reverse()}
       end)
     rescue
       e ->

mix.exs

@@ -190,7 +190,7 @@ defmodule AshPostgres.MixProject do
     [
       {:ash, ash_version("~> 3.37 and >= 3.37.1")},
       {:spark, "~> 2.3 and >= 2.3.4"},
-      {:ash_sql, ash_sql_version("~> 0.8 and >= 0.8.8")},
+      {:ash_sql, ash_sql_version("~> 0.8 and >= 0.8.9")},
       {:igniter, "~> 0.6 and >= 0.6.29", optional: true},
       {:ecto_sql, "~> 3.13"},
       {:ecto, "~> 3.13"},

test/relationship_projection_test.exs

@@ -0,0 +1,88 @@
+# SPDX-FileCopyrightText: 2019 ash_postgres contributors <https://github.com/ash-project/ash_postgres/graphs/contributors>
+#
+# SPDX-License-Identifier: MIT
+
+defmodule AshPostgres.RelationshipProjectionTest do
+  use AshPostgres.RepoCase, async: false
+  alias AshPostgres.Test.{Comment, Post}
+  require Ash.Query
+  require Ash.Sort
+  import Ash.Expr
+
+  test "calculations, filtering and sorting keep the same latest related row" do
+    posts =
+      for title <- ["first", "second", "empty"],
+          do: Ash.create!(Ash.Changeset.for_create(Post, :create, %{title: title}))
+
+    [first, second, empty] = posts
+
+    for {post, title, likes, created_at} <- [
+          {first, "old", 10, ~U[2026-01-01 00:00:00.000000Z]},
+          {first, "new", 2, ~U[2026-01-02 00:00:00.000000Z]},
+          {second, "other", 8, ~U[2026-01-02 00:00:00.000000Z]}
+        ] do
+      Comment
+      |> Ash.Changeset.for_create(:create, %{
+        post_id: post.id,
+        title: title,
+        likes: likes,
+        created_at: created_at
+      })
+      |> Ash.create!()
+    end
+
+    ids = Enum.map(posts, & &1.id)
+
+    query =
+      Post
+      |> Ash.Query.filter(id in ^ids)
+      |> Ash.Query.select([:id])
+      |> Ash.Query.calculate(:latest_title, :string, expr(latest_comment.title))
+      |> Ash.Query.calculate(
+        :latest_created_at,
+        :utc_datetime_usec,
+        expr(latest_comment.created_at)
+      )
+
+    original = query |> Ash.data_layer_query!() |> Map.fetch!(:query)
+    {before_sql, before_params} = TestRepo.to_sql(:all, original)
+    {after_sql, after_params} = TestRepo.to_sql(:all, AshSql.Query.Projection.prune(original))
+    assert before_sql =~ ~s("edited_duration" AS "edited_duration")
+    refute after_sql =~ ~s("edited_duration" AS "edited_duration")
+    assert before_params == after_params
+
+    rows = Map.new(Ash.read!(query), &{&1.id, &1.calculations})
+    assert rows[first.id].latest_title == "new"
+    assert rows[second.id].latest_created_at == ~U[2026-01-02 00:00:00.000000Z]
+    assert rows[empty.id].latest_title == nil
+
+    assert [%{id: id}] = query |> Ash.Query.filter(latest_comment.likes > 5) |> Ash.read!()
+    assert id == second.id
+
+    sorted =
+      query
+      |> Ash.Query.sort([{Ash.Sort.expr_sort(latest_comment.likes), :desc_nils_last}])
+      |> Ash.read!()
+
+    assert Enum.map(sorted, & &1.id) == [second.id, first.id, empty.id]
+  end
+
+  test "loading the whole relationship still includes fields unused by calculations" do
+    post = Post |> Ash.Changeset.for_create(:create, %{title: "post"}) |> Ash.create!()
+
+    comment =
+      Comment
+      |> Ash.Changeset.for_create(:create, %{
+        post_id: post.id,
+        title: "comment",
+        label: "label",
+        likes: 4
+      })
+      |> Ash.create!()
+
+    assert %{latest_comment: %{id: id, label: "label", likes: 4}} =
+             Ash.load!(post, :latest_comment)
+
+    assert id == comment.id
+  end
+end