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POHow to avoid anemic domain models, or when to move methods from the entities into services
 

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  1. POHow to avoid anemic domain models, or when to move methods from the entities into services
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    <p>I have a common scenario that I am looking for some guidance from people more experienced with DDD and Domain Modeling in general.</p> <p>Say I start out building a blog engine, and the first requirement is that after an Article is posted, users can start posting Comments on it. This starts fine, and leads to the following design:</p> <pre><code>public class Article { public int Id { get; set; } public void AddComment(Comment comment) { // Add Comment } } </code></pre> <p>My MVC Controller is designed like this:</p> <pre><code>public class ArticleController { private readonly IRepository _repository; public ArticleController(IRepository repository) { _repository = repository; } public void AddComment(int articleId, Comment comment) { var article = _repository.Get&lt;Article&gt;(articleId); article.AddComment(comment); _repository.Save(article); return RedirectToAction("Index"); } } </code></pre> <p>Now everything works fine, and it meets the requirement. Next iteration we get a requirement that every time a Comment is posted, the blog author should get an email notifying him.</p> <p>At this point, I have 2 choices that I can think of. 1) Modify Article to require an IEmailService (in the ctor?) or obtain an EmailService from a static reference to my DI container</p> <p>1a) Seems pretty ugly. I believe it breaks some Domain model rules that my entities are aware of services?</p> <pre><code>public class Article { private readonly IEmailService _emailService; public Article(IEmailService emailService) { _emailService = emailService; } public void AddComment(Comment comment) { // Add Comment // Email admin _emailService.SendEmail(App.Config.AdminEmail, "New comment posted!"); } } </code></pre> <p>1b) Also seems ugly, I now require a configured DI container which is accessed statically.</p> <pre><code>public class Article { public void AddComment(Comment comment) { // Add Comment // Email admin var emailService = App.DIContainer.Resolve&lt;IEmailService&gt;(); emailService.SendEmail(App.Config.AdminEmail, "New comment posted!"); } } </code></pre> <p>2) Create an IArticleService and move the AddComment() method to this service instead of on the Article Entity itself.</p> <p>This solution is cleaner I believe, but Adding a comment is now less discoverable and requires an ArticleService to perform the work. It seems like AddComment should belong on the Article class itself.</p> <pre><code>public class ArticleService { private readonly IEmailService _emailService; public ArticleService(IEmailService emailService) { _emailService = emailService; } public void AddComment(Article article, Comment comment) { // Add comment // Email admin _emailService.SendEmail(App.Config.AdminEmail, "New comment posted!"); } } public class ArticleController { private readonly IRepository _repository; private readonly IArticleService _articleService; public ArticleController(IRepository repository, IArticleService articleService) { _repository = repository; _articleService = articleService; } public void AddComment(int articleId, Comment comment) { var article = _repository.Get&lt;Article&gt;(articleId); _articleService.AddComment(article, comment); _repository.Save(article); return RedirectToAction("Index"); } } </code></pre> <p>So I am basically look for advice from people more experienced in domain modeling. If I am missing a more obvious solution please let me know :)</p> <p>I generally dislike both solutions to be honest, because the Service option is less discoverable. I can no longer add a Comment to an instance of an Article without having an ArticleService available. It also feels less natural, since AddComment seems like such an obvious method on the Article type. </p> <p>Anyway I look forward to reading the input. Thanks in advance.</p>
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