Server side validation in asp.net MVC 3 with entity framework

I agree with Ladislav Mrnka but if you are not able to change the properties of the entity class, you have to source it out:

[MetadataType(typeof(YourOwnClassForValidation))]
public partial class ClassOfYourDbDeveloper
{
    // db developer doesn't allow you to change this
    public string Title { get; set; }
}

public class YourOwnClassForValidation
{
    // here you can use your data annotations
    // important to use object
    [Required]
    public object Title { get; set; }
}

It's just for setting the properties, read more about that here at the bottom: http://www.asp.net/mvc/tutorials/validation-with-the-data-annotation-validators-cs

You can register a class to be used as metadata/validation provider for your models.

Having your entity, that cannot be changed:

public class MyModel 
{
    public int IntProperty { get; set; }

    public DateTime DateProperty { get; set; }
}

You can have it's metadata on MVC side, that you can validate and provide metadata as if it is the original class:

[ValidationAttribute(...)]
[ValidationAttribute(...)]
public class MyModelValidator
{
    [Required]
    [Display("My Integer")]
    public int IntProperty { get; set; }

    [DataType(DataType.DateTime)]
    public DateTime DateProperty { get; set; }
}

Finally you just have to attach the Metadata/Validator class to the correspondent base class, for example on Global.asax:

protected void Application_Start()
{
    AssociatedMetadataTypeTypeDescriptionProvider typeDescriptionProvider;

    typeDescriptionProvider = new AssociatedMetadataTypeTypeDescriptionProvider(
            typeof(MyModel),
            typeof(MyModelValidator));

    TypeDescriptor.AddProviderTransparent(typeDescriptionProvider, typeof(MyModel));

    // register other metadata classes
}

You can do this for every Entity you want.

Use view models. One of their intended purposes is to abstract away model (as M in MVC) objects from the views.

Put validation attributes in your view models and use a mapper (like automapper) to copy information to/from the db entities.

Use ModelState.IsValid in your controller actions to check if the view models are valid.

My POST action methods usually look someting liek this:

[HttpPost]
public virtual ActionResult Edit(EditModel model)
{
    if (!ModelState.IsValid)
        return View(model);

    try
    {
        // fetch db entity
        var template = _templateService.Get(model.Id);

        // copy info from view model to db entity
        Mapper.Map(model, template);

        // save db entity
        templateService.Save(template);

        return RedirectToAction("Details", new { id = template.Id });
    }
    catch (Exception err)
    {
        Logger.Error("Failed to save template", err);
        ModelState.AddModelError("", err);
        return View(model);
    }
}