don't error out if the featured collection has a string ID

lib/pleroma/web/activity_pub/activity_pub.ex

@@ -1657,7 +1657,13 @@ defmodule Pleroma.Web.ActivityPub.ActivityPub do
       )
       when type in ["OrderedCollection", "Collection"] do
     {:ok, objects} = Collections.Fetcher.fetch_collection(collection)
-    Map.new(objects, fn %{"id" => object_ap_id} -> {object_ap_id, NaiveDateTime.utc_now()} end)
+
+    # Items can either be a map _or_ a string
+    objects
+    |> Map.new(fn
+      ap_id when is_binary(ap_id) -> {ap_id, NaiveDateTime.utc_now()}
+      %{"id" => object_ap_id} -> {object_ap_id, NaiveDateTime.utc_now()}
+    end)
   end
 
   def fetch_and_prepare_featured_from_ap_id(nil) do

test/fixtures/mastodon/featured_collection.json

@@ -0,0 +1,20 @@
+{
+  "@context": [
+    "https://www.w3.org/ns/activitystreams",
+    {
+      "ostatus": "http://ostatus.org#",
+      "atomUri": "ostatus:atomUri",
+      "inReplyToAtomUri": "ostatus:inReplyToAtomUri",
+      "conversation": "ostatus:conversation",
+      "sensitive": "as:sensitive",
+      "toot": "http://joinmastodon.org/ns#",
+      "votersCount": "toot:votersCount"
+    }
+  ],
+  "id": "https://example.com/users/alisaie/collections/featured",
+  "type": "OrderedCollection",
+  "totalItems": 5,
+  "orderedItems": [
+    "https://example.com/users/alisaie/statuses/108311386746229284"
+  ]
+}

test/pleroma/web/activity_pub/activity_pub_test.exs

@@ -347,6 +347,39 @@ defmodule Pleroma.Web.ActivityPub.ActivityPubTest do
     assert Map.has_key?(data, "http://inserted")
   end
 
+  test "fetches user featured when it has string IDs" do
+    featured_url = "https://example.com/alisaie/collections/featured"
+    dead_url = "https://example.com/users/alisaie/statuses/108311386746229284"
+
+    featured_data =
+      "test/fixtures/mastodon/featured_collection.json"
+      |> File.read!()
+
+    Tesla.Mock.mock(fn
+      %{
+        method: :get,
+        url: ^featured_url
+      } ->
+        %Tesla.Env{
+          status: 200,
+          body: featured_data,
+          headers: [{"content-type", "application/activity+json"}]
+        }
+
+      %{
+        method: :get,
+        url: ^dead_url
+      } ->
+        %Tesla.Env{
+          status: 404,
+          body: "{}",
+          headers: [{"content-type", "application/activity+json"}]
+        }
+    end)
+
+    {:ok, %{}} = ActivityPub.fetch_and_prepare_featured_from_ap_id(featured_url)
+  end
+
   test "it fetches the appropriate tag-restricted posts" do
     user = insert(:user)