How to update record using Entity Framework 6?

Question

You're trying to update the record (which to me means "change a value on an existing record and save it back"). So you need to retrieve the object, make a change, and save it.

using (var db = new MyContextDB())
{
    var result = db.Books.SingleOrDefault(b => b.BookNumber == bookNumber);
    if (result != null)
    {
        result.SomeValue = "Some new value";
        db.SaveChanges();
    }
}

I have been reviewing the source code of Entity Framework and found a way to actually update an entity if you know the Key property:

public void Update<T>(T item) where T: Entity
{
    // assume Entity base class have an Id property for all items
    var entity = _collection.Find(item.Id);
    if (entity == null)
    {
        return;
    }

    _context.Entry(entity).CurrentValues.SetValues(item);
}

Otherwise, check the AddOrUpdate implementation for ideas.

Hope this help!

You can use the AddOrUpdate method:

db.Books.AddOrUpdate(book); //requires using System.Data.Entity.Migrations;
db.SaveChanges();

So you have an entity that is updated, and you want to update it in the database with the least amount of code...

Concurrency is always tricky, but I am assuming that you just want your updates to win. Here is how I did it for my same case and modified the names to mimic your classes. In other words, just change attach to add, and it works for me:

public static void SaveBook(Model.Book myBook)
{
    using (var ctx = new BookDBContext())
    {
        ctx.Books.Add(myBook);
        ctx.Entry(myBook).State = System.Data.Entity.EntityState.Modified;
        ctx.SaveChanges();
    }
}

You should use the Entry() method in case you want to update all the fields in your object. Also keep in mind you cannot change the field id (key) therefore first set the Id to the same as you edit.

using(var context = new ...())
{
    var EditedObj = context
        .Obj
        .Where(x => x. ....)
        .First();

    NewObj.Id = EditedObj.Id; //This is important when we first create an object (NewObj), in which the default Id = 0. We can not change an existing key.

    context.Entry(EditedObj).CurrentValues.SetValues(NewObj);

    context.SaveChanges();
}

Attaching an entity will set its tracking state to Unchanged. To update an existing entity, all you need to do is set the tracking state to Modified. According to the EF6 docs:

If you have an entity that you know already exists in the database but to which changes may have been made then you can tell the context to attach the entity and set its state to Modified. For example:

var existingBlog = new Blog { BlogId = 1, Name = "ADO.NET Blog" };

using (var context = new BloggingContext())
{
    context.Entry(existingBlog).State = EntityState.Modified;

    // Do some more work...  

    context.SaveChanges();
}