akkoma/priv/repo/migrations/20190414125034_migrate_old_bookmarks.exs

defmodule Pleroma.Repo.Migrations.MigrateOldBookmarks do
  use Ecto.Migration
  import Ecto.Query
  alias Pleroma.Activity
  alias Pleroma.Bookmark
  alias Pleroma.User
  alias Pleroma.Repo

  def change do
    query =
      from(u in User,
        where: u.local == true,
        where: fragment("array_length(bookmarks, 1)") > 0,
        select: %{id: u.id, bookmarks: fragment("bookmarks")}
      )

    Repo.stream(query)
    |> Enum.each(fn %{id: user_id, bookmarks: bookmarks} ->
      Enum.each(bookmarks, fn ap_id ->
        activity = Activity.get_create_by_object_ap_id(ap_id)
	unless is_nil(activity), do: {:ok, _} = Bookmark.create(user_id, activity.id)
      end)
    end)

    alter table(:users) do
      remove(:bookmarks)
    end
  end
end