test: reproduce parent() scoping bug in nested exists with calculation (#717)

When a calculation on resource B uses `parent()` inside an unrelated
`exists()`, and resource A references that calculation via
`exists(relationship_to_b, b_calculation)`, the `parent()` references
incorrectly resolve to resource A instead of resource B.

Example: Post has `exists(Author, first_name == parent(title))` where
parent(title) should refer to Post.title. When Author uses
`exists(posts, that_post_calculation)`, the generated SQL produces
`authors.title` instead of `posts.title`.

Author: barnabasJ

test/nested_exists_parent_scope_test.exs

@@ -0,0 +1,54 @@
+# SPDX-FileCopyrightText: 2019 ash_postgres contributors <https://github.com/ash-project/ash_postgres/graphs/contributors>
+#
+# SPDX-License-Identifier: MIT
+
+defmodule AshPostgres.Test.NestedExistsParentScopeTest do
+  use AshPostgres.RepoCase, async: false
+
+  alias AshPostgres.Test.{Author, Post}
+
+  require Ash.Query
+
+  describe "nested exists with calculation containing parent() inside unrelated exists" do
+    test "parent() references in a calculation are scoped to the calculation's own resource, not the outer exists" do
+      # Setup: Author "Alice" with a post titled "Alice"
+      # The post's `has_matching_author_by_unrelated_exists` calculation checks:
+      #   exists(Author, first_name == parent(title))
+      # When used standalone on the post, parent(title) refers to Post.title — works fine.
+      #
+      # Author's `has_post_matching_author_via_nested_exists` calculation checks:
+      #   exists(posts, has_matching_author_by_unrelated_exists)
+      # This inlines the Post calculation. The bug: parent(title) gets scoped to
+      # Author instead of Post, generating SQL like `authors.title` which doesn't exist.
+
+      author =
+        Author
+        |> Ash.Changeset.for_create(:create, %{first_name: "Alice"})
+        |> Ash.create!()
+
+      Post
+      |> Ash.Changeset.for_create(:create, %{title: "Alice", author_id: author.id})
+      |> Ash.create!()
+
+      # This should work: the post titled "Alice" matches the author named "Alice"
+      assert %{has_post_matching_author_via_nested_exists: true} =
+               Ash.load!(author, [:has_post_matching_author_via_nested_exists])
+    end
+
+    test "the inner calculation works correctly when loaded directly on the child resource" do
+      # Sanity check: the Post calculation works fine when not nested
+      author =
+        Author
+        |> Ash.Changeset.for_create(:create, %{first_name: "Bob"})
+        |> Ash.create!()
+
+      post =
+        Post
+        |> Ash.Changeset.for_create(:create, %{title: "Bob", author_id: author.id})
+        |> Ash.create!()
+
+      assert %{has_matching_author_by_unrelated_exists: true} =
+               Ash.load!(post, [:has_matching_author_by_unrelated_exists])
+    end
+  end
+end

test/support/resources/author.ex

@@ -206,6 +206,15 @@ defmodule AshPostgres.Test.Author do
       expr(exists(posts, title == parent(first_name)))
     )
 
+    # When used standalone on a Post, parent(title) correctly refers to the Post.
+    # But when used inside exists(posts, ...) from Author, parent(title) incorrectly
+    # resolves to Author (which doesn't have a title column).
+    calculate(
+      :has_post_matching_author_via_nested_exists,
+      :boolean,
+      expr(exists(posts, has_matching_author_by_unrelated_exists))
+    )
+
     calculate(:profile_description_calc, :string, expr(profile.description), allow_nil?: true)
 
     calculate(:true_if_actor_in_context, :boolean, TrueIfActorInContext)

test/support/resources/post.ex

@@ -1036,6 +1036,18 @@ defmodule AshPostgres.Test.Post do
       expr(exists(followers, name == "fred"))
     )
 
+    # Used to test nested exists with parent() — see nested_exists_test.exs
+    calculate(
+      :has_matching_author_by_unrelated_exists,
+      :boolean,
+      expr(
+        exists(
+          AshPostgres.Test.Author,
+          first_name == parent(title)
+        )
+      )
+    )
+
     calculate(
       :composite_origin,
       AshPostgres.Test.CompositePoint,