EF dbcontext.Entry in ASP.NET app

Question

I know it can be silly question, but although ...

Im learning ASP.NET MVC 4 and got one question about the dbcontext.Entry method, lets assume we got such method :

public ActionResult Edit(Movie movie)
{
    if (ModelState.IsValid)
    {
        db.Entry(movie).State = EntityState.Modified;
        db.SaveChanges();
        return RedirectToAction("Index");
    }
    return View(movie);
}

How the db.Entry(movie) determine which exactly row from the database is this "movie" instance? If all properties of this movie parameter object will be changed, how will it be possible to determine which row this instance should be attached to? I got smth on my head like it will determine by the Id which we cant change but not sure, so better if I ask :)

Answer 1

This method has a parameter Movie , but it has nothing to do with database. You must update object that you will get from db like:

public ActionResult Edit(Movie updatedMovie)
{
  if (ModelState.IsValid)
  {
   //db=your entities context
   //here you will get instance of record from db that you want to update
   Movie movie = db.Movie.Find(updatedMovie.Id);
   TryUpdateModel(movie);
   ctx.SaveChanges();
   return RedirectToAction("Index");
  }
  return View(movie);
}

Maybe there is more efficient way , but I'm using it like this and it works fine.

If code in your question is a version that is supposed to be a working version, then it wasn't working for me when I have been trying to solve a similar problem so I had to solve it by using code above finally.

Answer 2

EF使用的每个表都必须有一个主键，除非它是一个collection，所以EF知道由主键保持不变的行。

Answer 3

When EF generates the models it includes information on what column is the primary key in the database. db.Entry(model) binds the model to the context using that primary key and whatever primary key it shares in the database will be overwriten by the data in the model when SaveChanges() is called. If the primary key is 0, or empty you must set the State to EntityState.Added instead of EntityState.Modified .

As another answer said this can fail. The reason that binding can fail is that a context can only track a single distinct entity once (entity being a distinct class and id). If you attempt to do the following you'll get an exception:

var dbModel = db.Movie.Find(movie.ID);
db.Entry(movie.ID) // Doesn't matter what you do, this will always fail 

Because the db context is already tracking a Movie with the same ID.

This is one of the larger reasons that you don't want to share db contexts between users, because another user attempting to update the same entry will cause the second one to throw an exception. While the transaction will be ACID, you can end up with edge case race conditions (for lack of a better term) between users.