Entity Framework 6.1 Updating a Subset of a Record

Question

I have a view model that encapsulates only some of the database model properties. These properties contained by the view model are the only properties I want to update. I want the other properties to preserve their value.

During my research I found this answer which appears to be perfect for my needs however, despite my best efforts, I cannot get the code to work as expected.

Here is an isolated example of what I came up with:

static void Main() {
    // Person with ID 1 already exists in database.

    // 1. Update the Age and Name.
    Person person = new Person();
    person.Id = 1;
    person.Age = 18;
    person.Name = "Alex";

    // 2. Do not update the NI. I want to preserve that value.
    // person.NINumber = "123456";

    Update(person);
}

static void Update(Person updatedPerson) {
    var context = new PersonContext();

    context.Persons.Attach(updatedPerson);
    var entry = context.Entry(updatedPerson);

    entry.Property(e => e.Name).IsModified = true;
    entry.Property(e => e.Age).IsModified = true;

    // Boom! Throws a validation exception saying that the 
    // NI field is required.
    context.SaveChanges();
}

public class PersonContext : DbContext {
    public DbSet<Person> Persons { get; set; }
}

public class Person {
    public int Id { get; set; }
    [Required]
    public string Name { get; set; }
    [Required] 
    public int Age { get; set; } // this is contrived so, yeah.
    [Required]
    public string NINumber { get; set; }
}

What am I doing wrong?

Answer 1

You based your work on the post https://stackoverflow.com/a/15339512/2015959, but in the other thread the fields that weren't changed (and as such weren't in the attached model) weren't mandatory, and that's why it worked. Since your fields are required, you'll get this validation error.

Your problem can be solved by the solution provided in question Entity Framework validation with partial updates

Answer 2

It is the validation that is causing it not to be saved. You can disable validation with context.Configuration.ValidateOnSaveEnabled = false; and it will work. To validate specific fields you can call var error = entry.Property(e => e.Name).GetValidationErrors();. So you certainly can make an 'UpdateNameAndAge' method that only only enforces business rules and flags those properties as modified. No double query required.

  private static bool UpdateNameAndAge(int id, string name, int age)
  {
     bool success = false;

     var context = new PersonContext();
     context.Configuration.ValidateOnSaveEnabled = false;

     var person = new Person() {Id = id, Name = name, Age = age};
     context.Persons.Attach(person);
     var entry = context.Entry(person);

     // validate the two fields
     var errorsName = entry.Property(e => e.Name).GetValidationErrors();
     var errorsAge = entry.Property(e => e.Age).GetValidationErrors();

     // save if validation was good
     if (!errorsName.Any() && !errorsAge.Any())
     {
        entry.Property(e => e.Name).IsModified = true;
        entry.Property(e => e.Age).IsModified = true;

        if (context.SaveChanges() > 0)
        {
           success = true;
        }
     }

     return success;
  }

Answer 3

(Edited for clarity)

The context must have a complete copy of the object to enforce business rules. This can only happen if the attached object has all the necessary properties populated or the partial view is merged with a complete copy before updating.

I believe that what you want to do is conceptually impossible: doing updates like this will require either a preserved pre-change copy, or two queries to the database because the business layer needs a full copy of the object for validation.