gistfile1.rb

users = User.where('name IS NULL')
users.each do |user|
  # Some api call which will retrive name of the user based on user email
  name = API.getUserName user.email
  if name.blank?
    next
  else
    user.name = name
  end
end

# Does this work?
users.each(&:save!)

# Has to save each user individually!