Add a where to a query depending on condition

Hi, me again.

I got the following code:

updated_at = Map.get(params, "updated_since", "1970-01-01T12:00:00")
query =
  from m in ChatMessage,
  join: u in assoc(m, :chat_user),
  order_by: [asc: m.updated_at],
  where: u.chat_id ==^ id,
  where: m.updated_at > ^updated_at

I added the parameter updated_since as optional parameter to some api calls.
I don’t want to use Map.get in all controllers and added a methode for this:

def check_updated(query, params) do
    case Map.get(params, "updated_since") do
        nil -> query 
        updated_since -> query |> where(updated_at: > ^updated_since)
    end
end

I get a SyntaxError and think this is wrong anyway.
What would be the right way to implement a method to extract
where: xyz.updated_at > ^updated_at
to a central method?

Thanks.