refactor: fix review issues - member_count aggregate, migration down, docs, actor handling

lib/membership/group.ex

@@ -67,12 +67,12 @@ defmodule Mv.Membership.Group do
     validate present(:name)
 
     # Case-insensitive name uniqueness validation
-    validate fn changeset, _context ->
+    validate fn changeset, context ->
       name = Ash.Changeset.get_attribute(changeset, :name)
       current_id = Ash.Changeset.get_attribute(changeset, :id)
 
       if name do
-        check_name_uniqueness(name, current_id)
+        check_name_uniqueness(name, current_id, context)
       else
         :ok
       end
@@ -115,24 +115,8 @@ defmodule Mv.Membership.Group do
     many_to_many :members, Mv.Membership.Member, through: Mv.Membership.MemberGroup
   end
 
-  calculations do
-    calculate :member_count, :integer do
-      description "Number of members in this group"
-
-      calculation fn [group], _context ->
-        system_actor = SystemActor.get_system_actor()
-        opts = Helpers.ash_actor_opts(system_actor)
-
-        query =
-          Mv.Membership.MemberGroup
-          |> Ash.Query.filter(group_id == ^group.id)
-
-        case Ash.read(query, opts) do
-          {:ok, member_groups} -> [length(member_groups)]
-          {:error, _} -> [0]
-        end
-      end
-    end
+  aggregates do
+    count :member_count, :member_groups
   end
 
   identities do
@@ -140,14 +124,21 @@ defmodule Mv.Membership.Group do
   end
 
   # Private helper function for case-insensitive name uniqueness check
-  defp check_name_uniqueness(name, exclude_id) do
+  # Uses context actor if available (respects policies), falls back to system actor
+  defp check_name_uniqueness(name, exclude_id, context) do
+    # Use context actor if available (respects user permissions), otherwise fall back to system actor
+    actor =
+      case context do
+        %{actor: actor} when not is_nil(actor) -> actor
+        _ -> SystemActor.get_system_actor()
+      end
+
     query =
       Mv.Membership.Group
       |> Ash.Query.filter(fragment("LOWER(?) = LOWER(?)", name, ^name))
       |> maybe_exclude_id(exclude_id)
 
-    system_actor = SystemActor.get_system_actor()
-    opts = Helpers.ash_actor_opts(system_actor)
+    opts = Helpers.ash_actor_opts(actor)
 
     case Ash.read(query, opts) do
       {:ok, []} ->