title: dynamic select field in ecto query
body: <p>I have a trouble with ecto query situation, that I Have a ORDER table with basic fields: <code>current_status, picked_up_at, delivered_at, returned_at</code>. where: <code>picked_up_at, delivered_at, returned_at</code> is Date type. and data struct i want to query back is: <code>%{ current_status, date }</code>, where date is one of the value of <code>[picked_up_at, delivered_at, returned_at]</code></p>
  
  <p>it depends on value of <code>current_status</code> if <code>current_status</code> == "picked_up" so date in result will be <code>picked_up_at</code> field and so on. the problem is i can create a dynamic query field in <code>Select</code> query</p>
  
  <p>Hope you guys can help! thank you!</p>