MVC3 Validate total for list of elements

I have a View Model that represents a total bill and list of parties that are splitting the bill. The parties splitting the bill each take a certain percentage of the total (example: Party A takes 50%, Party B takes 20%, Party C takes 30%).

The model looks something like this:

public class BillModel
{
    public int Total { get; set; }
    开发者_C百科public List<BillPartyModel> Parties { get; set; }
}

public class BillPartyModel
{
    public string Name { get; set; }
    public int SplitPercentage { get; set; }
}

Basically, I want to validate that the SplitPercentage for all parties totals to exactly 100. I was initially tempted to add custom validation attribute to SplitPercentage, but I'm not sure how to sum all the values in the validator. Could I perhaps add a custom validation attribute to Parties in the main model (and how would that work)? Or does someone know a better way to do this?

EDIT: I did as suggested below and ended up with this (using DataAnnotationsExtensions for the EqualTo tag):

Model

[EqualTo("TotalPercent", ErrorMessage = "Percentages do not add up to 100%.")]
public int SplitPercentage { get { return this.Allocations.Sum(a => a.Allocation); } }
public int TotalPercent { get { return 100; } }

View

@Html.HiddenFor(m => m.SplitPercentage)
@Html.ValidationMessageFor(m => m.SplitPercentage)
@Html.EditorFor(m => m.BillPartyModel)

...And then I just use a JavaScript OnChange event to update the AllocationTotal in order to get Client-Side validation.

public int Total { get { return Parties.Sum(p=>p.SplitPercentage); } }

Then when you are validating the model make sure Total = 100

You could look at using the custom validation attribute and create a editor template for your List<BillPartyModel> see here for a demo

In your custom validation attribute you can then check the model easily and loop over the split percentage properties to see they total 100