Chats: Add cascading delete on both referenced users.

Also remove the now-superfluous join in the chat controller,
which was only used to filter out these cases.

lib/pleroma/web/pleroma_api/controllers/chat_controller.ex

@@ -149,9 +149,7 @@ def index(%{assigns: %{user: %{id: user_id} = user}} = conn, _params) do
       from(c in Chat,
         where: c.user_id == ^user_id,
         where: c.recipient not in ^blocked_ap_ids,
-        order_by: [desc: c.updated_at],
+        order_by: [desc: c.updated_at]
-        inner_join: u in User,
-        on: u.ap_id == c.recipient
       )
       |> Repo.all()
 

priv/repo/migrations/20200831142509_chat_constraints.exs

@@ -0,0 +1,22 @@
+defmodule Pleroma.Repo.Migrations.ChatConstraints do
+  use Ecto.Migration
+
+  def change do
+    remove_orphans = """
+    delete from chats where not exists(select id from users where ap_id = chats.recipient);
+    """
+
+    execute(remove_orphans)
+
+    drop(constraint(:chats, "chats_user_id_fkey"))
+
+    alter table(:chats) do
+      modify(:user_id, references(:users, type: :uuid, on_delete: :delete_all))
+
+      modify(
+        :recipient,
+        references(:users, column: :ap_id, type: :string, on_delete: :delete_all)
+      )
+    end
+  end
+end

test/chat_test.exs

@@ -26,6 +26,28 @@ test "it creates a chat for a user and recipient" do
       assert chat.id
     end
 
+    test "deleting the user deletes the chat" do
+      user = insert(:user)
+      other_user = insert(:user)
+
+      {:ok, chat} = Chat.bump_or_create(user.id, other_user.ap_id)
+
+      Repo.delete(user)
+
+      refute Chat.get_by_id(chat.id)
+    end
+
+    test "deleting the recipient deletes the chat" do
+      user = insert(:user)
+      other_user = insert(:user)
+
+      {:ok, chat} = Chat.bump_or_create(user.id, other_user.ap_id)
+
+      Repo.delete(other_user)
+
+      refute Chat.get_by_id(chat.id)
+    end
+
     test "it returns and bumps a chat for a user and recipient if it already exists" do
       user = insert(:user)
       other_user = insert(:user)