Pure POCO entity update problem in repository pattern

问题

I have a problem in my UserRepository in which I want to update a user. I dont want certain fields updated, such as password, unless specified. For example, When I pass the User from the view, to the service to the repository, it sends up the user with a null or empty password string. This null gets written to the database (which I dont want).

How do I handle a situation like this?

Domain

public class User
{
    public int UserId { get; set; }

    public string Email { get; set; }
    public string Password { get; set; }
}

Repository

    public User Save(User user)
    {
        if (user.UserId > 0)
        {
            User dbUser = context.Users.FirstOrDefault(u => u.UserId == user.UserId);
            //What do I do here?
        }
        context.Users.AddObject(user);
        context.SaveChanges();
        return user;
    }

Lets say in this case, my view allows me to change only Email, so the only thing that gets sent back to the Save() method are: user.UserId and user.Email while user.Password is null. In my case, the database throws error because Password should be nullable.

回答1:

Detached POCO scenario (you will not load user from DB before update):

You can selectively say which properties must be updated:

public User Save(User user)     
{         
    if (user.UserId == 0)         
    {             
        context.Users.AddObject(user);         
    }
    else
    {
        context.Users.Attach(user);
        ObjectStateEntry entry = context.ObjectStateManager.GetObjectStateEntry(user);
        entry.SetModifiedProperty("Email");
    }

    context.SaveChanges();         
    return user;     
}

You can also create two overloads of you Save method. First will update whole object, second will update only explicitly selected properties:

public User Save(User user)     
{         
    if (user.UserId == 0)         
    {             
        context.Users.AddObject(user);         
    }
    else
    {
        context.Users.Attach(user);
        context.ObjectStateManager.ChangeObjectState(user, EntityState.Modified);        
    }

    context.SaveChanges();         
    return user;     
}

public User Save(User user, IEnumerable<Expression<Func<User, object>>> properties)     
{         
    if (user.UserId == 0)         
    {             
        context.Users.AddObject(user);         
    }
    else
    {
        context.Users.Attach(user);
        ObjectStateEntry entry = context.ObjectStateManager.GetObjectStateEntry(user);
        foreach(var selector in properties)
        {
            string propertyName = PropertyToString(selector.Body);
            entry.SetModifiedProperty(propertyName);
        }
    }

    context.SaveChanges();         
    return user;     
}

// Doesn't work for navigation properties!
private static string PropertyToString(Expression selector)
{
    if (selector.NodeType == ExpressionType.MemberAccess)
    {
        return ((selector as MemberExpression).Member as PropertyInfo).Name;
    }

    throw new InvalidOperationException();
}

You will call the second overload this way:

userRepository.Save(user, new List<Expression<Func<User, object>>> 
    { 
        u => u.Email 
    });

Attached scenario (you will load user from DB before update):

You can modify your Save method to accept delegate so that you can control how update will be performed:

public User Save(User user, Action<User, User> updateStrategy)                                
{                                  
    if (user.UserId > 0)                                  
    {
        User dbUser = context.Users.FirstOrDefault(u => u.UserId == user.UserId);
        updateStrategy(dbUser, user);                                                                        
    }        
    else
    {                          
        // New object - all properties should be saved
        context.Users.AddObject(user);
    }

    context.SaveChanges();                                  
    return user;                              
}  

You will call the method this way:

var user = GetUpdatedUserFromSomewhere();
repository.Save(user, (dbUser, mergedUser) => 
    {
        dbUser.Email = mergedUser.Email;
    });

Anyway, despite of my examples you should definitely think about Darin's post and special ModelViews for updating.

回答2:

You should use view models. View models are classes which are specifically tailored to the needs of a view and contain only the properties needed by this given view. So your controller action should look like this:

[HttpPost]
public ActionResult Update(UserViewModel model) { ... }

instead of:

[HttpPost]
public ActionResult Update(User model) { ... }

Inside the controller action you could map between the view model and the model. AutoMapper is a great tool that could simplify this task.

You should really be very careful and never expose your models like this. Always use view models to and from a view. Just imagine if there was an IsAdministrator boolean property on your model.

回答3:

Can you do this?

public User Save(User user)
    {
        if (user.UserId > 0)
        {
            User dbUser = context.Users.FirstOrDefault(u => u.UserId == user.UserId);
            //What do I do here?
            dbUser.Email = user.Email
            user = dbUser;
        }
        else
        {
            context.Users.AddObject(user);
        }
        context.SaveChanges();
        return user;
    }

来源：https://stackoverflow.com/questions/4697203/pure-poco-entity-update-problem-in-repository-pattern