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    <p>In an ASP.NET MVC architecture, you should try to do most of your validation in the Model, but with low-level validation rules like these, it's sometimes impossible. What you should look to for answers then is Domain-driven Design (DDD) where <a href="http://lostechies.com/jimmybogard/2008/08/21/services-in-domain-driven-design/" rel="nofollow">Application Services</a> can solve such low-level needs.</p> <p>Application Services will have access to the database (either directly, or better yet; indirectly through <a href="http://lostechies.com/jimmybogard/2009/09/03/ddd-repository-implementation-patterns/" rel="nofollow">repositories</a>) and can perform low-level validation and throw <code>ValidationException</code> or something similar (with detailed information the <code>Controller</code> can act upon and respond to the user) when a prerequisite or business rule isn't met.</p> <p><a href="http://www.sharparchitecture.net/" rel="nofollow">S#arp Architecture</a> implementes all of this in a best-practice framework that you can use as a basis for your ASP.NET MVC applications. It is highly opinionated towards DDD principles and NHibernate, and it will sometimes force your hand on how you do stuff, which is kind of the point. The most important part about it is that it learns you how to deal with these kinds of problems.</p> <p>To answer your question more concretely and in the spirit of DDD, this is how I would solve it:</p> <pre><code>public class UserController { private readonly IUserService userService; public UserController(IUserService userService) { // The IUserService will be injected into the controller with // an "Inversion of Control" container like NInject, Castle Windsor // or StructureMap: this.userService = userService; } public ActionResult Save(UserFormModel userFormModel) { if (userFormModel.IsValid) { try { // Mapping can be performed by AutoMapper or some similar library UserDto userDto = Mapper.Map&lt;UserDto&gt;(userFormModel); this.userService.Save(userDto); } catch (ValidationException ve) { ViewBag.Error = ve.Detail; } } // Show validation errors or redirect to a "user saved" page. } } public class UserService : IUserService { private readonly IUserRepository userRepository; public UserService(IUserRepository userRepository) { // The IUserRepository will be injected into the service with // an "Inversion of Control" container like NInject, Castle Windsor // or StructureMap: this.userRepository = userReposityr; } public UserDto Save(UserDto userDto) { using (this.userRepository.BeginTransaction()) { if (!this.userRepository.IsUnique(userDto.UserName)) { // The UserNameNotUniqueValidationException will inherit from ValidationException // and build a Detail object that contains information that can be presented to // a user. throw new UserNameNotUniqueValidationException(userDto.UserName); } userDto = this.userRepository.Save(userDto); this.userRepository.CommitTransaction(); return userDto; } } } </code></pre>
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    1. POHow to ensure unique column value inside code?
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    1. USAsbjørn Ulsberg
    1. USAsbjørn Ulsberg
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    1. COThank you for your answer. However, since I am learning asp.net mvc, I am not too sure as to whether this answer is really basics or it includes (some) advanced feature. Can you please comment on this
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      1. USmliya
    2. COMy answer is an attempt to teach a "best practice" implementation of an n-tier ASP.NET MVC application. It's mostly relevant if you want to build very large-scale applications, but once you have all the plumbing for IoC and such in place, it's not much more difficult than a more basic implementation. If you want your application to grow, this is the way to do it.
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      1. USAsbjørn Ulsberg
    3. COUnless I'm mistaken this solution is performing a test against the database before the save, and it would still be theoretically possible for 2 users to be entering the data at exactly the same time, both pass the IsUnique test but then one will fail the database unique constraint. I guess a try/catch around the save which translates the DB error into a user error would handle that?
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      1. USDale Burrell
 

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