DDD: Modeling simple domain with two aggregate roots - domain-driven-design

Question

Let's say I want to create action web site where members would be able to bid for items. To model this domain I have three classes: Member, Item and Bid.
My brainstorming would go something like this:
Item can contain multiple bids
Bid is associated with one Item and one Member
Member can contain multiple bids
Member and Item can exist without bid instance
Bid instance can't exist without both Member and Item
Considering all this it is obvious that since Member and Item objects are independent we can consider them aggregate roots. Bid will be part of one of these aggregate. That is clear but what is confusing to me right now is which aggregate root should I choose? Item or Member?
This is example from Pro ASP.NET MVC 3 Framework book by Apress, and the way they did is like following:
Which gives following code:
public class Member
{
public string LoginName { get; set; } // The unique key
public int ReputationPoints { get; set; }
}
public class Item
{
public int ItemID { get; private set; } // The unique key
public string Title { get; set; }
public string Description { get; set; }
public DateTime AuctionEndDate { get; set; }
public IList<Bid> Bids { get; set; }
}
public class Bid
{
public Member Member { get; set; }
public DateTime DatePlaced { get; set; }
public decimal BidAmount { get; set; }
}
Member and Item are aggregate roots here and Bid is contained within Item.
Now let's say that I have application use case: "Get all bids posted by specific member". Does that mean that I would have to first get all Items (eg. from data base via repository interface) and then enumerate all bids for each Item trying to find matching Member? Isn't that a bit inefficient? So a better way would be then to aggregate Bid objects inside of Member. But in that case consider the new use case: "Get all bids for specific item". Now again we need to go other way around to get all bids...
So taking into account that I need to implement both of these use cases in my application, what is the right and efficient way to model this domain then?

Answer 1

Your domain should really reflect only Command (CQRS) requirements (update/change data). I presume that you need Queries (read data, no update/change of data): "Get all bids for specific item" and "Get all bids posted by specific member". So, this "querying" has nothing to do with the domain, as the query implementation is independent on the command implementation (command is calling a domain method). This gives you a freedom to implement each query in an efficient way. My approach is to implement an efficient DB view getting you only data you want to display in UI. Then you create a new class called BidForItemDto (DTO = data transfer object) and you map data from DB view into a collection of BidForItemDto (you can do it manually via ADO.NET or use NHibernate (preferred, does everything for you)). The same for the second query, create a new class called BidPostedByMemberDto.
So, if it is queries you need, just forget about domain, realize that it's just data you want to display in UI, and query them efficiently from the DB. Only when you do some action in UI (click a button to place a bid for instance), it's executing a command "place a bid", which would at the end call domain method Item.PlaceBid(Member member, DateTime date, decimal amount). And btw, IMHO is it an Item which "has many bids", and the domain method "place bid" would surely need to access previous bids to implement the whole logic correctly. Placing bids collection into Member does not make much sense to me...
From the top of my head some examples of DB views and sql queries:
Get all bids for specific item:
create view BidForItemDto
as
select
i.ItemId,
b.BidId,
b.MemberId,
b.DatePlaced,
b.BidAmount
from Item i
join Bid b ON b.ItemId = i.ItemId
query:
SELECT *
from BidFormItemDto
where ItemId = <provide item id>
Get all bids posted by specific member:
create view BidPostedByMemberDto
as
select
m.MemberId,
b.BidId,
b.MemberId,
b.DatePlaced,
b.BidAmount
from Member m
join Bid b ON b.MemberId = i.MemberId
query:
SELECT *
from BidPostedByMemberDto
where MemberId = <provide member id>