DDD modeling for a User Voice-like system

Question

I've taken on the challenge of modeling a simple User Voice-like system. High-level description:

• It's a portal for some SaaS users;
• They come and leave feature requests, suggestions, etc.;
• They should be able to vote/unvote for any suggestions;
• They can leave as many comments as they like on suggestions;
• Comments may be removed by the owner, but may not be edited.

I've modeled the domain as follows, using a DDD approach. Please advise about mistakes, warnings, improvements, etc.

I've also applied the advices from these posts:

• Don't create aggregate roots
• Creating new aggregates in DDD
• Link to an aggregate: reference or Id?

public abstract class Entity
{
 public Guid Id { get; protected set; } = Guid.NewGuid();
}
public class User : Entity // Aggregate Root
{
 public string Key => $"{Email}:{MarketplaceUrl}";
 public string Name { get; }
 public string Email { get; }
 public string MarketplaceName { get; }
 public Uri MarketplaceUrl { get; }
 internal User(string name, string email, string marketplaceName, Uri marketplaceUrl)
 {
 Name = name;
 Email = email;
 MarketplaceName = marketplaceName;
 MarketplaceUrl = marketplaceUrl;
 }
 public Suggestion MakeSuggestion(string text)
 {
 return new Suggestion(this, text);
 }
}
public class Suggestion : Entity // Aggregate Root
{
 public string Text { get; /* a suggestion cannot be altered */ }
 public User ByUser { get; }
 public DateTime SuggestedAt { get; }
 public ICollection<Comment> Comments { get; } = new List<Comment>();
 public ICollection<Vote> Votes { get; } = new List<Vote>();
 internal Suggestion(User byUser, string text)
 {
 ByUser = byUser;
 Text = text;
 SuggestedAt = DateTime.UtcNow;
 }
 public Comment AddComment(User byUser, string text)
 {
 var comment = new Comment(byUser, text);
 Comments.Add(comment);
 return comment;
 }
 public void RemoveComment(Comment comment, User userRemovingComment)
 {
 Comments.Remove(comment);
 }
 public void Unvote(User byUser)
 {
 var vote = Votes.SingleOrDefault(v => v.ByUser == byUser));
 if (vote != null)
 Votes.Remove(vote);
 }
 public Vote Vote(User byUser)
 {
 if (Votes.Any(v => v.ByUser == byUser))
 throw new CannotVoteTwiceOnSameSuggestionException();
 var vote = new Vote(byUser);
 Votes.Add(vote);
 return vote;
 }
}
public class Comment : Entity
{
 public string Text { get; /* a comment cannot be changed */ }
 public User ByUser { get; }
 public DateTime CommentedAt { get; }
 internal Comment(User byUser, string text)
 {
 ByUser = byUser;
 Text = text;
 CommentedAt = DateTime.UtcNow;
 }
}
public class Vote : Entity
{
 public User ByUser { get; }
 public DateTime VotedAt { get; }
 internal Vote(User byUser)
 {
 ByUser = byUser;
 VotedAt = DateTime.UtcNow;
 }
}
public interface IUserVoiceStore
{
 Task AddUserAsync(User user);
 Task AddSuggestionAsync(Suggestion suggestion);
 Task<Suggestion> GetSuggestionAsync(Guid id);
 Task<User> GetUserAsync(Guid id);
 // For when comments and votes are added/removed to/from a suggestion.
 Task UpdateSuggestionAsync(Suggestion suggestion);
}
public class UserVoiceService
{
 private readonly IUserVoiceStore store;
 public UserVoiceService(IUserVoiceStore store)
 {
 this.store = store;
 }
 public async Task<User> RegisterUserAsync(string name, string email, string marketplaceName, Uri marketplaceUrl)
 {
 var user = new User(name, email, marketplaceName, marketplaceUrl);
 await store.AddUserAsync(user);
 return user;
 }
}
public class CannotVoteTwiceOnSameSuggestionException : Exception { }
public class CannotRemoveCommentFromAnotherUserExcetion : Exception { }

• t3chb0t: You are not using the CannotRemoveComment...Exception anywhere?
• Phillippe Santana: @t3chb0t my mistake; I edited the code at Suggestion.RemoveComment to validate the user and throw if needed. Thanks.
• Mast: Please do not update the code in your question to incorporate feedback from answers, doing so goes against the Question + Answer style of Code Review. This is not a forum where you should keep the most updated version in your question. Please see what you may and may not do after receiving answers .
• Phillippe Santana: @Mast sorry about that. I'll revert the code to its original form and then post a link to a Gist with the updated code.

Answer 1

The design looks solid, a few thoughts though:

• I would split IUserVoiceStore into more granular UserRepository and SuggestionRepository. Also, UpdateSuggestionAsync() seems to indicate you can only update a suggestion and nothing else at a time, which can be limiting. It also IMO goes out of a repository's jurisdiction to flush a specific object to persistent storage. Using some kind of separate unit of work class where you can put multiple objects to be updated as part of a business transaction might be a better idea.

• Not always feasible, but maybe change the type of link between Suggestion and User from a full reference to just an ID to avoid the temptation of manipulating 2 aggregate roots at the same time. (I don't agree with the article you linked to in that regard)

• Keep an eye on suggestions with a large number of comments - depending on concurrent access, they can clog up your system and cause locking problems, especially if comments become more sophisticated, with images and so on.

• Phillippe Santana: So split the repo by Aggregate Root; nice, code updated. About your 3rd suggestion, do you think I should promote Comment to an AR and have a dedicated Repo? It does make some sense.
• Phillippe Santana: Also, about your 2nd suggestion, it is according to Vernon's advice, see informit.com/articles/article.aspx?p=2020371&seqNum=4 - Rule: Reference Other Aggregates by Identity. I'll update the code accordingly
• guillaume31: About promoting Comment, it would be premature optimization to do it without some transactional analysis based on real or expected numbers.