wip: first draft of a unit test to reproduce this type of issue:

1) test Thing.create! create works without associating other things
(ThingTest)
     test/thing_test.exs:20
     ** (Ash.Error.Unknown)
     Bread Crumbs:
       > Exception raised in: Things.get_by_id

     Unknown Error

     * ** (RuntimeError) Error while building reference:
all_other_things.associated_at
       (ash_sql 0.2.38) lib/expr.ex:1796:
AshSql.Expr.default_dynamic_expr/6
       (ash_sql 0.2.38) lib/expr.ex:88:
AshSql.Expr.default_dynamic_expr/6

priv/resource_snapshots/test_repo/co_authored_posts/20241208221219.json

@@ -0,0 +1,97 @@
+{
+  "attributes": [
+    {
+      "allow_nil?": false,
+      "default": "nil",
+      "generated?": false,
+      "primary_key?": false,
+      "references": null,
+      "size": null,
+      "source": "role",
+      "type": "text"
+    },
+    {
+      "allow_nil?": true,
+      "default": "nil",
+      "generated?": false,
+      "primary_key?": false,
+      "references": null,
+      "size": null,
+      "source": "was_cancelled_at",
+      "type": "utc_datetime"
+    },
+    {
+      "allow_nil?": false,
+      "default": "nil",
+      "generated?": false,
+      "primary_key?": true,
+      "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": "co_authored_posts_author_id_fkey",
+        "on_delete": null,
+        "on_update": null,
+        "primary_key?": true,
+        "schema": "public",
+        "table": "authors"
+      },
+      "size": null,
+      "source": "author_id",
+      "type": "uuid"
+    },
+    {
+      "allow_nil?": false,
+      "default": "nil",
+      "generated?": false,
+      "primary_key?": true,
+      "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": "co_authored_posts_post_id_fkey",
+        "on_delete": null,
+        "on_update": null,
+        "primary_key?": true,
+        "schema": "public",
+        "table": "posts"
+      },
+      "size": null,
+      "source": "post_id",
+      "type": "uuid"
+    }
+  ],
+  "base_filter": null,
+  "check_constraints": [],
+  "custom_indexes": [],
+  "custom_statements": [],
+  "has_create_action": true,
+  "hash": "E1F2AC3AED1987928E3A2446584C268EC54D0BCA616D81A495F4AB26E3999444",
+  "identities": [],
+  "multitenancy": {
+    "attribute": null,
+    "global": null,
+    "strategy": null
+  },
+  "repo": "Elixir.AshPostgres.TestRepo",
+  "schema": null,
+  "table": "co_authored_posts"
+}

priv/test_repo/migrations/20241208221219_migrate_resources44.exs

@@ -0,0 +1,48 @@
+defmodule AshPostgres.TestRepo.Migrations.MigrateResources44 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
+    create table(:co_authored_posts, primary_key: false) do
+      add(:role, :text, null: false)
+      add(:was_cancelled_at, :utc_datetime)
+
+      add(
+        :author_id,
+        references(:authors,
+          column: :id,
+          name: "co_authored_posts_author_id_fkey",
+          type: :uuid,
+          prefix: "public"
+        ),
+        primary_key: true,
+        null: false
+      )
+
+      add(
+        :post_id,
+        references(:posts,
+          column: :id,
+          name: "co_authored_posts_post_id_fkey",
+          type: :uuid,
+          prefix: "public"
+        ),
+        primary_key: true,
+        null: false
+      )
+    end
+  end
+
+  def down do
+    drop(constraint(:co_authored_posts, "co_authored_posts_author_id_fkey"))
+
+    drop(constraint(:co_authored_posts, "co_authored_posts_post_id_fkey"))
+
+    drop(table(:co_authored_posts))
+  end
+end

test/many_to_many_expr_test.exs

@@ -0,0 +1,212 @@
+defmodule AshPostgres.ManyToManyExprTest do
+  use AshPostgres.RepoCase, async: false
+
+  alias AshPostgres.Test.Author
+  alias AshPostgres.Test.CoAuthorPost
+  alias AshPostgres.Test.Post
+
+  require Ash.Query
+
+  setup ctx do
+    main_author =
+      if ctx[:main_author?],
+        do: create_author(),
+        else: nil
+
+    co_authors =
+      if ctx[:co_authors],
+        do: 1..ctx[:co_authors]
+            |> Stream.map(&
+              Author
+              |> Ash.Changeset.for_create(:create, %{first_name: "John #{&1}", last_name: "Doe"})
+              |> Ash.create!()
+            )
+            |> Enum.into([]),
+      else: []
+
+    %{
+      main_author: main_author,
+      co_authors: co_authors,
+    }
+  end
+
+  def create_author(params \\ %{first_name: "John", last_name: "Doe"}) do
+    Author
+    |> Ash.Changeset.for_create(:create, params)
+    |> Ash.create!()
+  end
+
+  def create_post(author) do
+    Post
+    |> Ash.Changeset.for_create(:create, %{title: "Post by #{author.first_name}"})
+    |> Ash.create!()
+  end
+
+  def create_co_author_post(author, post, role) do
+    CoAuthorPost
+    |> Ash.Changeset.for_create(:create, %{author_id: author.id, post_id: post.id, role: role})
+    |> Ash.create!()
+  end
+
+  def get_author!(author_id) do
+    Author
+    |> Ash.Query.new()
+    |> Ash.Query.filter(id == ^author_id)
+    |> Ash.Query.load([:all_co_authored_posts, :cancelled_co_authored_posts, :editor_of, :writer_of])
+    |> Ash.read_one!()
+  end
+
+  def get_co_author_post!(a_id, p_id) do
+    CoAuthorPost
+    |> Ash.Query.new()
+    |> Ash.Query.filter(author_id == ^a_id and post_id == ^p_id)
+    |> Ash.read_one!()
+  end
+
+  def get_post!(post_id) do
+    Post.get_by_id!(post_id, load: [:co_author_posts, :co_authors_unfiltered, :co_authors])
+  end
+
+  def cancel(author, post) do
+    get_co_author_post!(author.id, post.id)
+    |> CoAuthorPost.cancel()
+  end
+
+  def uncancel(author, post) do
+    get_co_author_post!(author.id, post.id)
+    |> CoAuthorPost.uncancel()
+  end
+
+  describe "manual join-resource insertion" do
+    @tag main_author?: true
+    @tag co_authors: 3
+    test "filter on many_to_many relationship using parent works as expected - basic",
+      %{
+        main_author: main_author,
+        co_authors: co_authors,
+      } do
+      post = create_post(main_author)
+
+      [first_ca, second_ca, third_ca] = co_authors
+
+      # Add first co-author
+      create_co_author_post(first_ca, post, :editor)
+
+      first_ca = get_author!(first_ca.id)
+      post = get_post!(post.id)
+
+      assert Enum.count(post.co_authors) == 1
+      assert Enum.count(first_ca.all_co_authored_posts) == 1
+      assert Enum.count(first_ca.editor_of) == 1
+      assert Enum.count(first_ca.writer_of) == 0
+      assert Enum.count(first_ca.cancelled_co_authored_posts) == 0
+
+      # Add second co-author
+      create_co_author_post(second_ca, post, :writer)
+
+      second_ca = get_author!(second_ca.id)
+      post = get_post!(post.id)
+
+      assert Enum.count(post.co_authors) == 2
+      assert Enum.count(second_ca.all_co_authored_posts) == 1
+      assert Enum.count(second_ca.editor_of) == 0
+      assert Enum.count(second_ca.writer_of) == 1
+      assert Enum.count(second_ca.cancelled_co_authored_posts) == 0
+
+      # Add third co-author
+      create_co_author_post(third_ca, post, :proof_reader)
+
+      third_ca = get_author!(third_ca.id)
+      post = get_post!(post.id)
+
+      assert Enum.count(post.co_authors) == 3
+      assert Enum.count(third_ca.all_co_authored_posts) == 1
+      assert Enum.count(third_ca.editor_of) == 0
+      assert Enum.count(third_ca.writer_of) == 0
+      assert Enum.count(third_ca.cancelled_co_authored_posts) == 0
+    end
+
+    @tag main_author?: true
+    @tag co_authors: 4
+    test "filter on many_to_many relationship using parent works as expected - cancelled",
+      %{
+        main_author: main_author,
+        co_authors: co_authors,
+      } do
+      first_post = create_post(main_author)
+      second_post = create_post(main_author)
+
+      [first_ca, second_ca, third_ca, fourth_ca] = co_authors
+
+      # Add first co-author
+      create_co_author_post(first_ca, first_post, :editor)
+      create_co_author_post(first_ca, second_post, :writer)
+
+      first_ca = get_author!(first_ca.id)
+      first_post = get_post!(first_post.id)
+
+      assert Enum.count(first_post.co_authors) == 1
+      assert Enum.count(first_post.co_authors_unfiltered) == 1
+
+      assert Enum.count(first_ca.all_co_authored_posts) == 2
+      assert Enum.count(first_ca.editor_of) == 1
+      assert Enum.count(first_ca.writer_of) == 1
+      assert Enum.count(first_ca.cancelled_co_authored_posts) == 0
+
+      # Add second co-author
+      create_co_author_post(second_ca, first_post, :proof_reader)
+      create_co_author_post(second_ca, second_post, :writer)
+
+      second_ca = get_author!(second_ca.id)
+      first_post = get_post!(first_post.id)
+      second_post = get_post!(second_post.id)
+
+      assert Enum.count(second_post.co_authors) == 2
+      assert Enum.count(second_post.co_authors_unfiltered) == 2
+
+      assert Enum.count(second_ca.all_co_authored_posts) == 2
+      assert Enum.count(second_ca.editor_of) == 0
+      assert Enum.count(second_ca.writer_of) == 1
+      assert Enum.count(second_ca.cancelled_co_authored_posts) == 0
+
+      # Add third co-author
+      create_co_author_post(third_ca, first_post, :proof_reader)
+      create_co_author_post(third_ca, second_post, :proof_reader)
+      cancel(third_ca, second_post)
+
+      third_ca = get_author!(third_ca.id)
+      first_post = get_post!(first_post.id)
+      second_post = get_post!(second_post.id)
+
+      assert Enum.count(first_post.co_authors) == 3
+      assert Enum.count(first_post.co_authors_unfiltered) == 3
+      assert Enum.count(second_post.co_authors) == 2
+      assert Enum.count(second_post.co_authors_unfiltered) == 3
+
+      assert Enum.count(third_ca.all_co_authored_posts) == 2
+      assert Enum.count(third_ca.editor_of) == 0
+      assert Enum.count(third_ca.writer_of) == 0
+      assert Enum.count(third_ca.cancelled_co_authored_posts) == 1
+
+      # Add fourth co-author
+      create_co_author_post(fourth_ca, first_post, :proof_reader)
+      create_co_author_post(fourth_ca, second_post, :editor)
+      cancel(fourth_ca, first_post)
+      cancel(fourth_ca, second_post)
+
+      fourth_ca = get_author!(fourth_ca.id)
+      first_post = get_post!(first_post.id)
+      second_post = get_post!(second_post.id)
+
+      assert Enum.count(first_post.co_authors) == 3
+      assert Enum.count(first_post.co_authors_unfiltered) == 4
+      assert Enum.count(second_post.co_authors) == 2
+      assert Enum.count(second_post.co_authors_unfiltered) == 4
+
+      assert Enum.count(fourth_ca.all_co_authored_posts) == 2
+      assert Enum.count(fourth_ca.editor_of) == 1
+      assert Enum.count(fourth_ca.writer_of) == 0
+      assert Enum.count(fourth_ca.cancelled_co_authored_posts) == 2
+    end
+  end
+end

test/support/domain.ex

@@ -3,6 +3,7 @@ defmodule AshPostgres.Test.Domain do
   use Ash.Domain
 
   resources do
+    resource(AshPostgres.Test.CoAuthorPost)
     resource(AshPostgres.Test.Post)
     resource(AshPostgres.Test.Comment)
     resource(AshPostgres.Test.IntegerPost)