How update an entity inside Aggregate

Question

I have an aggregate named Campaigns every with a root entity named campaign, this root entity has a list of attempts (entity)

    public class Attempts: IEntity<Attempts>
    {
        private int id;
        public AttempNumber AttemptNumber {get;}
        //other fields 
    }

    public class Campaign: IEntity<Campaign> //root
    {
        private int id;
        public IList<Attempt> {get;}
        //other fields 
    }

Im using a method to add a campaign attempt

 public virtual void AssignAttempts(Attempts att)
        {
            Validate.NotNull(att, "attemps are required for assignment");

            this.attempts.add(att);
        }

Problem comes when i try to edit a specific item in attempts list. I get Attempt by AttempNumber and pass it to editAttempt method but i dont know how to set the attempt without deleting whole list and recreate it again

 public virtual void EditAttempts(Attempts att)
 {
        Validate.NotNull(att, "attemps are required for assignment");
 }

Any help will be appreciated!

Thanks, Pedro de la Cruz

Karbala

Answer 1

First, I think there may be a slight problem with your domain model. It seems to me like 'Campaign' should be an aggregate root entity having a collection of 'Attempt' value objects (or entities). There is no 'Campaigns' aggregate unless you have a parent concept to a campaign which would contain a collection of campaigns. Also, there is no 'Attempts' entity. Instead a collection of 'Attempt' entities or values on the 'Campaign' entity. 'Attempt' may be an entity if it has identity outside of a 'Campaign', otherwise it is a value object. The code could be something like this:

class Campaign {
    
    public string Id { get; set; }
    
    public ICollection<Attempt> Attempts { get; private set; }
    
    public Attempt GetAttempt(string id) {
        return this.Attempts.FirstOrDefault(x => x.Number == id);
    } 
    
}
    
class Attempt {
    public string Number { get; set; }
    public string Attribute1 { get; set; }
}

If you retrieve an Attempt from the Campaign entity and then change some of the properties, you should not have to insert it back into the campaign entity, it is already there. This is how the code would look if you were using NHibernate (similar for other ORMs):

var campaign = this.Session.Get<Campaign>("some-id");
    
var attempt = campaign.GetAttempt("some-attempt-id");
    
attempt.Attribute1 = "some new value";
    
this.Session.Flush(); // will commit changes made to Attempt 

Cristencristi

Modifying the attempt outside of the campaign aggregate root (AR) would break the rule that all modifications must go through the aggregate root. Otherwise the AR wouldn't have any control over the internal state and the whole design pattern would be useless. – Bracketing

@Bracketing you said that if we want to change an entity property we have to add a "SetAttemptAttribute1" method to aggregate, is it right? – Gauss

@Gauss Yes, that is right, although I'd try to find a more descriptive name than set.... – Bracketing

Answer 2

You don't need an Edit method. Your code can modify the Attempts in-place, like so:

Attempt toModify = MyRepository.GetAttemptById(id);
toModify.Counter++;
toModify.Location = "Paris";
MyRepository.SaveChanges(); // to actually persist to the DB

Of course how you name the SaveChanges() is up to you, this is the way Entity Framework names its general Save method.

Ophthalmia

It breaks the rules of an aggregate root to hold a reference to something from inside and modify it directly from the outside. All modifications have to be performed by methods of the aggregate root. – Bracketing