Nhibernate mapping: Encapsulating a list within an object.

Question

I have two tables Deal and Cost. Costs can be of a number of types, eg Planned, Unplanned. Every deal can have one an only one of each type of cost. This is easy enough to map as a many-to-one on deal, reulting in an IList of Costs. But what I want is to create a costs object that has named properties for each cost. So that for a database that looks like this:

Deal:               Cost
ID   Name           ID   DealID   Type        Value
---------           -------------------------------
1    Test           1    1        Planned     10
                    2    1        Unplanned   5

it is accessible like this

Deal.Costs.Planned = 10m;
Deal.Costs.Unplanned = 5m;

How is the best way to go about mapping this? Should I even be mapping it or should I just write the properties by hand to query the underlying collection?

The difficulty as i see it is mapping the properties of cost so that they map to different rows in teh same table. Using one of the columns as a discriminator.

Answer 1

This doesn't really sound to me like Costs is a list.

I would map the Cost as a property on Deal. If you want to keep the Deal.Costs.Planned syntax you can use a Component mapping.

Answer 2

Write the properties methods by hand:

public class Deal
{
    public virtual Cost GetPlannedCost() 
    {
        return _costs.FirstOrDefault(x => x.IsPlanned()); 
    }

    public virtual Cost GetUnplannedCost() {}
    {
        return _costs.FirstOrDefault(x => x.IsUnplanned()); 
    }

    public virtual Cost[] GetAllCosts() 
    {
        return _costs.ToArray(); 
    }

    private ISet<Cost> _costs = new HashedSet<Cost>();
}

Answer 3

I would map Costs as a private collection and expose Planned and Unplanned costs on Deal. This would allow you to enforce "Every deal can have one an only one of each type of cost" as a business rule. It would look something like this:

public class Deal
{
    private IList<Cost> _costs = new List<Cost>(2); // map this collection

    public Cost PlannedCost
    {
        get { return _costs.FirstOrDefault(c => c.Type == CostType.Planned); }
        set
        {
            var plannedCost = _costs.FirstOrDefault(c => c.Type == CostType.Planned);
            if (plannedCost != null)
            {
                _costs.Remove(plannedCost);
            }
            _costs.Add(value);
        }
    }

    public Cost UnplannedCost { // similarly }
}

Answer 4

You can have formula properties in your class using Formula tag of NHibernate mapping.A formula is simply a SQL statement that will be inserted as a subquery in a mapping query. Obviously Formula properties are read only and can't be updated.

Since you want to calculate these properties for each Deal you should add them as Deal properties (if you'd like to use formula) thus you will have:

Deal.UnPlannedCosts
Deal.PlannedCosts

Here's an article that explains formula properties.