User.count(:username=>params[:username])

class User
  include DataMapper::Resource

  property :id,  Serial
  property :username,  String, unique_index: true, required: true, length: 3..32
  property :password,  String, required: true, length: 5..64
  property :email,  String, unique_index: true, required: true, format: :email_address

end