How to dynamically or conditionally select a value from a binding or nil?

def fetch_stuff(query, include_some_value?) do
  # SomeModel embeds SomeOtherModel which contains lots of stuff.
  # SomeModel.embedded is not required (default nil).
  preferably_nil_sometimes = (
    # binding.id (:integer) works but nil would be much better.
    if include_some_value?, do: :lots_of_stuff, else: :id
  )

  from [binding] in query,
  select: %SomeModel{
    id: binding.id,
    embedded: field(binding, ^preferably_nil_sometimes)
  }
end