title: Ecto query unique through a belongs_to association?
body: <p>I have two models <code>Event</code> and <code>Customer</code>. The relationship is that an Event <code>belongs_to</code> a Customer, which can <code>has_many</code> Events.</p>
  
  <p>Now, I need to return the count of unique Customers by way of Events, which fall in a particular date range. Does Ecto have any query method that can do what I need?</p>