activity: add helpers for updating activities in the database

lib/pleroma/activity.ex

@@ -1,7 +1,7 @@
 defmodule Pleroma.Activity do
   use Ecto.Schema
   alias Pleroma.{Repo, Activity, Notification, Object}
-  import Ecto.Query
+  import Ecto.{Query, Changeset}
 
   schema "activities" do
     field(:data, :map)
@@ -22,6 +22,13 @@ def get_by_ap_id(ap_id) do
     )
   end
 
+  def change(struct, params \\ %{}) do
+    struct
+    |> cast(params, [:data])
+    |> validate_required([:data])
+    |> unique_constraint(:ap_id, name: :activities_unique_apid_index)
+  end
+
   # TODO:
   # Go through these and fix them everywhere.
   # Wrong name, only returns create activities