How to update field value dynamically in Ecto migration?

I have a Users table like:

     email     | username
---------------+----------
 123@321.com   |
 123@123.com   |
 haha@haha.com |

and I want to update username field by email field, just slice the email before @.

     email     | username
---------------+----------
 123@321.com   | 123
 123@123.com   | 123
 haha@haha.com | haha

I have try to use the following migration:

defmodule MyApp.Repo.Migrations.AddDefaultUsernameForUsers do
  use Ecto.Migration
  import Ecto.Query

  def up do
      from(u in MyApp.User, update: [set: [username: String.split(u.email, "@") |> List.first ]])
        |> MyApp.Repo.update_all([])
  end

  def down do
      MyApp.Repo.update_all(MyApp.User, set: [username: nil])
  end
end

But when runing the migration, I got the following error:

$ mix ecto.migrate
** (Ecto.Query.CompileError) `List.first(String.split(u.email(), "@"))` is not a valid query expression

How can I solve this?