In Domain Driven Design should you Add/Update Entities through the AggregateRoot? - .net-core

Question

If you have a Vendor with a list of Contacts, in DDD which is the better approach for adding a contact to a Vendor?
Here's some sample C# code using a CQRS command.
Given the following command, how should we implement adding a Contact to a Vendor
AddVendorContactCommand()
{
string vendorId;
string contactName;
}
Should we add a contact through the Vendor:
AddVendorContactHandler(AddVendorContactCommand command)
{
var vendor = await dbContext.Vendors.FindAsync(command.vendorId);
vendor.AddContact(command.contactName);
dbContext.Save();
//doesn't require a dbSet for VendorContacts???
}
Or should we reference the VendorContact and bypass the Vendor entirely.
AddVendorContactHandler(AddVendorContactCommand command)
{
//handler
var newVendorContact = new VendorContact(command.vendorId, command.contactName);
dbContext.VendorContacts.Add(newVendorContact);
dbContext.Save();
//requires a dbSet for VendorContacts;
}
I feel like the better approach is to go through the Vendor, but that requires our AddVendorContactCommand to read from the database first. In CQRS Commands, it generally suggests avoid reads. The second approach to use VendorContacts directly will have higher performance than if we go through Vendor.
Argument to go through the Vendor are the following:
What if the Vendor doesn't exist
What if the Vendor isn't allowed any more contacts.
What if the Vendor is deleted, disabled or otherwise readonly
What's the correct DDD approach?

Answer 1

First, as a developer, I'm obligated to say there is no single correct approach to anything.
Now that is out of the way, given the information you have provided, I'm going to assume that the Vendor entity you described can (and in my opinion should) be the Aggregate Root. With that in mind, I would definitely go with the first option you described.
I think you have a misconception about CQRS Commands. It is perfectly fine to get data from the database inside commands. The thing you have to avoid is fetching the data from the query side, which could be a totally different database.
You are also correct, you won't need a DbSet<> for VendorContact entity, and you should keep it that way on the Command side, as you want to protect the invariants inside your Vendor Aggregate Root.