Согласно документации datamapper и тому, как я это делаю, вы можете сделать:
@user = User.first(:email => params[email]) #strictly the same as find_by_email in Activerecord
@user = User.all(:email => params[email]) #strictly the same as find_all_by_email in Activerecord
@user = User.last(:email => params[email]) #no real equivalent in Active Record