Add utility functions for objects and activities.

test/activity_test.exs

@@ -0,0 +1,18 @@
+defmodule Pleroma.ActivityTest do
+  use Pleroma.DataCase
+  import Pleroma.Factory
+
+  test "returns an activity by it's AP id" do
+    activity = insert(:note_activity)
+    found_activity = Pleroma.Activity.get_by_ap_id(activity.data["id"])
+
+    assert activity == found_activity
+  end
+
+  test "returns activities by it's objects AP ids" do
+    activity = insert(:note_activity)
+    [found_activity] = Pleroma.Activity.all_by_object_ap_id(activity.data["object"]["id"])
+
+    assert activity == found_activity
+  end
+end