DDD Entities making use of Services

Question

I have an application that I'm trying to build with at least a nominally DDD-type domain model, and am struggling with a certain piece.

My entity has some business logic that uses some financial calculations and rate calculations that I currently have inside some domain services, as well as some constant values I'm putting in a value object.

I'm struggling with how to have the entity use the logic inside the domain services, or whether the logic inside those services even belongs there. This is what I have so far:

public class Ticket
{
    public Ticket(int id, ConstantRates constantRates, FinancialCalculationService f, RateCalculationService r)
    {
        Id = id;
        ConstantRates = constantRates;
        FinancialCalculator = f;
        RateCalculator = r;
    }

    private FinancialCalculationService FinancialCalculator { get; set; }

    private RateCalculationService RateCalculator { get; set; }

    private ConstantRates ConstantRates { get; set; }

    public int Id { get; private set; }

    public double ProjectedCosts { get; set; }

    public double ProjectedBenefits { get; set; }

    public double CalculateFinancialGain()
    {
        var discountRate = RateCalculator.CalculateDiscountRate(ConstantRates.Rate1, ConstantRates.Rate2,
                                                                ConstantRates.Rate3);

        return FinancialCalculator.CalculateNetPresentValue(discountRate,
                                                            new[] {ProjectedCosts*-1, ProjectedBenefits});
    }
}


public class ConstantRates
{
    public double Rate1 { get; set; }
    public double Rate2 { get; set; }
    public double Rate3 { get; set; }
}

public class RateCalculationService
{
    public double CalculateDiscountRate(double rate1, double rate2, double rate3 )
    {
        //do some jibba jabba
        return 8.0;
    }
}

public class FinancialCalculationService
{
    public double CalculateNetPresentValue(double rate, params double[] values)
    {
        return Microsoft.VisualBasic.Financial.NPV(rate, ref values);
    }

}

I feel like some of that calculation logic does belong in those domain services, but don't really like that I'll have to manually inject those dependencies from my Repository. Is there an alternate way that this should be modeled? Am I wrong in not liking that?

Having read the Blue Book but not really built anything in this style before, I'm looking for guidance.

EDIT

Thanks all for the feedback! Based on what I'm hearing, it sounds like my model should look more like the following. This look better?

public class Ticket
{
    public Ticket(int id)
    {
        Id = id;
    }

    private ConstantRates ConstantRates { get; set; }

    public int Id { get; private set; }

    public double ProjectedCosts { get; set; }

    public double ProjectedBenefits { get; set; }

    public double FinancialGain { get; set; }
}



public class ConstantRates
{
    public double Rate1 { get; set; }
    public double Rate2 { get; set; }
    public double Rate3 { get; set; }
}

public class FinancialGainCalculationService
{
    public FinancialGainCalculationService(RateCalculationService rateCalculator, 
        FinancialCalculationService financialCalculator,
        ConstantRateFactory rateFactory)
    {
        RateCalculator = rateCalculator;
        FinancialCalculator = financialCalculator;
        RateFactory = rateFactory;
    }

    private RateCalculationService RateCalculator { get; set; }
    private FinancialCalculationService FinancialCalculator { get; set; }
    private ConstantRateFactory RateFactory { get; set; }

    public void CalculateFinancialGainFor(Ticket ticket)
    {
        var constantRates = RateFactory.Create();
        var discountRate = RateCalculator.CalculateDiscountRate(constantRates.Rate1, constantRates.Rate2,
                                                                constantRates.Rate3);

        ticket.FinancialGain = FinancialCalculator.CalculateNetPresentValue(discountRate,
                                                            new[] {ticket.ProjectedCosts*-1, ticket.ProjectedBenefits});
    }
}

public class ConstantRateFactory
{
    public ConstantRates Create()
    {
        return new ConstantRates();
    }
}

public class RateCalculationService
{
    public double CalculateDiscountRate(double rate1, double rate2, double rate3 )
    {
        //do some jibba jabba
        return 8.0;
    }
}

public class FinancialCalculationService
{
    public double CalculateNetPresentValue(double rate, params double[] values)
    {
        return Microsoft.VisualBasic.Financial.NPV(rate, ref values);
    }

}

The domain model ends up being fairly anemic at this point, but as I add features maybe it'll have more to it.

EDIT 2

Okay, I got some more feedback that perhaps my 'calculation' services are more like strategy objects that it's okay for my Entity to depend on. Here's another take at it with more of the logic back in the Entity, and making use of those strategy objects. Thoughts on this? Any issues with instantiating those helpers directly in the Entity? I don't think I'll want to mock those out in my tests, but OTOH I can't test the CalculateFinancialGain method without testing those strategy objects, either.

public class Ticket
{
    public Ticket(int id, ConstantRates constantRates)
    {
        Id = id;
        ConstantRates = constantRates;
    }

    private ConstantRates ConstantRates { get; set; }

    public int Id { get; private set; }

    public double ProjectedCosts { get; set; }

    public double ProjectedBenefits { get; set; }

    public double CalculateFinancialGain()
    {
        var rateCalculator = new RateCalculator();
        var financeCalculator = new FinanceCalculator();
        var discountRate = rateCalculator.CalculateDiscountRate(ConstantRates.Rate1, ConstantRates.Rate2,
                                                                ConstantRates.Rate3);

        return financeCalculator.CalculateNetPresentValue(discountRate,
                                                            ProjectedCosts*-1, 
                                                            ProjectedBenefits); 
    }
}

public class ConstantRates
{
    public double Rate1 { get; set; }
    public double Rate2 { get; set; }
    public double Rate3 { get; set; }
}

public class RateCalculator
{
    public double CalculateDiscountRate(double rate1, double rate2, double rate3 )
    {
        //do some jibba jabba
        return 8.0;
    }
}

public class FinanceCalculator
{
    public double CalculateNetPresentValue(double rate, params double[] values)
    {
        return Microsoft.VisualBasic.Financial.NPV(rate, ref values);
    }

}

Answer 1

Have your service accept the Ticket entity as a parameter. Services should be stateless and the same service should be able to provide its services to any number of entities.

In your situation I would pull the FinancialCalculatorService and RateCalculatorService out of your entity and make the methods on each service accept the Ticket entity as a parameter.

Take a second and read pg. 105 of Domain-Driven Design by Eric Evans