I haven’t played with ecto much yet so what I say take it with a grain of salt.

My guess is that you try to get various columns from all the rows in your table based on some conditional logic. i.e.

if (user_requests == username)
   bring username
if (user_requests == email)
  bring email also

bring_all_that_requested()

In this case you can take advantage the fact that ecto queries are composable. So you can mix and mach your queries as you wish. Take a look at this Ecto.Query — Ecto v3.14.0

Update: I tried this myself in iex and indeed can’t compose select

iex(18)> query = from u in User, select: %{username: u.username}
query = from u in User, select: %{username: u.username}
#Ecto.Query<from u in Qssite.User, select: %{username: u.username}>
iex(19)> Repo.all(query)
Repo.all(query)
...
iex(20)> query = from u in query, select: %{email: u.email}
query = from u in query, select: %{email: u.email}
** (Ecto.Query.CompileError) only one select expression is allowed in query

but one can use struct or map for that.

P.S.: If you really want to delete that key then I think Map.delete/2 and Map.drop/2 should suffice. No need to convert to map. Or you could simply set the keys to nil.

  user = Repo.get(User, 1)
  user = user = %{user| email: nil, username: nil}