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POWhere to put global rules validation in DDD
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copied!<p>I'm new to DDD, and I'm trying to apply it in real life. There is no questions about such validation logic, as null check, empty strings check, etc - that goes directly to entity constructor/property. But where to put validation of some global rules like 'Unique user name'?</p> <p>So, we have entity User</p> <pre><code>public class User : IAggregateRoot { private string _name; public string Name { get { return _name; } set { _name = value; } } // other data and behavior } </code></pre> <p>And repository for users</p> <pre><code>public interface IUserRepository : IRepository<User> { User FindByName(string name); } </code></pre> <p>Options are:</p> <ol> <li><em>Inject repository to entity</em></li> <li><em>Inject repository to factory</em></li> <li><em>Create operation on domain service</em></li> <li><em>???</em></li> </ol> <p>And each option more detailed:</p> <p><strong>1 .Inject repository to entity</strong></p> <p>I can query repository in entities constructor/property. But I think that keeping reference to repository in entity is a bad smell.</p> <pre><code>public User(IUserRepository repository) { _repository = repository; } public string Name { get { return _name; } set { if (_repository.FindByName(value) != null) throw new UserAlreadyExistsException(); _name = value; } } </code></pre> <p>Update: We can use DI to hide dependency between User and IUserRepository via Specification object.</p> <p><strong>2. Inject repository to factory</strong></p> <p>I can put this verification logic in UserFactory. But what if we want to change name of already existing user?</p> <p><strong>3. Create operation on domain service</strong></p> <p>I can create domain service for creating and editing users. But someone can directly edit name of user without calling that service...</p> <pre><code>public class AdministrationService { private IUserRepository _userRepository; public AdministrationService(IUserRepository userRepository) { _userRepository = userRepository; } public void RenameUser(string oldName, string newName) { if (_userRepository.FindByName(newName) != null) throw new UserAlreadyExistException(); User user = _userRepository.FindByName(oldName); user.Name = newName; _userRepository.Save(user); } } </code></pre> <p><strong>4. ???</strong></p> <p><strong>Where do you put global validation logic for entities?</strong></p> <p>Thanks!</p>

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