We have this code:

    validate :mac_uniqueness,
             :if => Proc.new { |nic| nic.managed? && nic.host && nic.host.managed? && !nic.host.compute? && !nic.virtual? && nic.mac.present? }

    def interface_attribute_uniqueness(attr, base = Nic::Base.where(nil))
      in_memory_candidates = self.host.present? ? self.host.interfaces.select { |i| i.persisted? && !i.marked_for_destruction? } : [self]
      db_candidates = base.where(attr => self.public_send(attr))
      db_candidates = db_candidates.select { |c| c.id != self.id && in_memory_candidates.map(&:id).include?(c.id) }
      errors.add(attr, :taken) if db_candidates.present?
    end

It does not seem to work, one can create two hosts with two managed NICs with same MACs.

This is not an issue, but it surprised me and we should explicitly either allow creating same MAC addresses everywhere and inform the user about consequences (preferred) or disallow same MAC addresses everywhere (in database).