Create an entity object in the domain layer when the ID is unknown

Question

I am trying to learn the Rules Design Pattern as described here: http://www.michael-whelan.net/rules-design-pattern/ (towards the bottom of the webpage). I have written a class like this:

public class OfferCalculator : IOfferCalculator
    {
        public List<IOfferRule> _rules;

        public RulesOfferCalculator(List<IOfferRule> rules)
        {
            _rules = rules;
            _rules.Add(new AgeRule());
        }

        public virtual List<string> CalculateOffers(Application application)
        {
            List<string> offers = new List<string>();
            foreach (var rule in _rules)
            {
                offers.Add(rule.getOffer(application));
            }
            return offers;
        }
    }

and a class like this:

public interface IOfferRule 
    {
        string getOffer(Application application); 
    }

and a class like this:

public class AgeRule : IOfferRule
    {
        public String getOffer(Application application)
        {
            if (application.Age >= 50)
            {
                return "PremierCard";
            }
            else
            {
                return "NormalCard";
            }
        }
    }

If the applicants age is 50 or greater then they are offered a Premier Card and if it is less than 50 then they are offered a normal card.

I need to record what offer they are entitled to in the database, which looks like this:

CREATE TABLE Card (id int identity not null, name varchar(100))
insert into card (name) values ('PremierCard');
insert into card (name) values ('NormalCard');

CREATE TABLE CardPerson (PersonID int not null, FOREIGN KEY (PersonID) REFERENCES Person(ID) cardid,FOREIGN KEY (CardID) REFERENCES Card(ID) int not null, primary key (personid,cardid))
CREATE TABLE Person (id int identity not null, name varchar(100))

I believe AgeRule should return a card entity rather than a string? How do I create a card object in the Business Layer? Do I just inject a repository into AgeRule and run the following SQL statement (to populate the object):

select * from card where name = 'Premiercard'

This does not look "correct" to me. How do I get a Card object in the domain layer? Is there a "better" pattern for this?

Answer 1

If you modeled cards with an ID and a name, where ID is the primary key and name a secondary key, the latter can be used for selecting the right object as well as the ID. In your example, name is probably the only sensible criterion from which you can determine which of the two possible card entities is the correct one. So there is nothing inherently wrong with getting a card object by using a statement like select * from card where name = 'Premiercard'.

However, if the card table is prepopulated with all available card entities, and the cards do not change during the lifetime of your application, an alternative might be to select all available card entities from the database once the program starts, store them in a list somewhere, and reuse these objects whenever AgeRule.getOffer is called. So no extra database call is triggered in AgeRule.getOffer, just as now where it returns strings.