Prevent adding duplicated values - entity-framework

Question

In my application I want to assign movie with director. Of course there is no point to store duplicated directors, so somehow I need to prevent adding duplicated rows.
I remember old days having fun with sql server and that did the trick:
1. Get Director Id
1a. If director exist return existing director Id
1b. If director not exist add new and return added director Id
2. Add new Movie with directorId assigned.
I am just learning Entity Framework and I cant do this in that way. I have to change the way I look at my database as collections. So I am adding Director to the Movie. I am not sure if I explained that correctly, so I think it would be better to show code:
class Program {
static void Main(string[] args) {
Movie firstMovie = new Movie() { Name = "Titanic" };
Movie secondMovie = new Movie() { Name = "Pulp Fiction" };
Movie thirdMovie = new Movie() { Name = "Matrix" };
Director director = new Director() { Name = "Warner Bros" };
firstMovie.Director = director;
secondMovie.Director = director;
thirdMovie.Director = director;
Db.AddMovie(firstMovie);
Db.AddMovie(secondMovie);
Db.AddMovie(thirdMovie);
}
}
public class Movie {
public int Id { get; set; }
public string Name { get; set; }
public int DirectorId { get; set; }
[ForeignKey("DirectorId")]
public virtual Director Director { get; set; }
}
public class Director {
public int Id { get; set; }
public string Name { get; set; }
public virtual List<Movie> Movies { get; set; }
public Director() {
Movies = new List<Movie>();
}
}
public class Db {
public static int AddMovie(Movie movie) {
using (var context = new MovieContext()) {
context.Movies.Add(movie);
context.SaveChanges();
return movie.Id;
}
}
}
public class MovieContext : DbContext {
public DbSet<Movie> Movies { get; set; }
public DbSet<Director> Directories { get; set; }
}
And the thing is after that I have 3x the same Director in my database. I expected to have only 1. But how can I archieve that? How can I prevent adding the same Directories? I mean I cant check if there is already directory with the same name, because I am not adding them manually. Entity Framework is adding new Directory, not me. So, what can I do with it?
What I've manged is to add directorId and let entity framework assign with proper object. This is how I've changed main:
static void Main(string[] args) {
Movie firstMovie = new Movie() { Name = "Titanic" };
Movie secondMovie = new Movie() { Name = "Pulp Fiction" };
Movie thirdMovie = new Movie() { Name = "Matrix" };
Director director = Db.GetDirector("Warner Bros");
firstMovie.DirectorId = director.Id;
secondMovie.DirectorId = director.Id;
thirdMovie.DirectorId = director.Id;
Db.AddMovie(firstMovie);
Db.AddMovie(secondMovie);
Db.AddMovie(thirdMovie);
}
And that's my new method which will return Director (and add if it is necessary)
public static Director GetDirector(string directorName) {
using (var context = new MovieContext()) {
if (context.Directories.Any(x => x.Name == directorName))
return context.Directories.FirstOrDefault(x => x.Name == directorName);
else {
Director newDirector = new Director() { Name = directorName };
context.Directories.Add(newDirector);
context.SaveChanges();
return newDirector;
}
}
}
So it is the best way to do it? I am not sure if adding new Director is proper coded. Do you have any tips?

Answer 1

You could go the way from the other side.
Just add the Movie objects to your one Director object's collection. After that save the Director object to database and it should work like you want.
Only as sample code:
Movie firstMovie = new Movie { Name = "Titanic" };
Movie secondMovie = new Movie { Name = "Pulp Fiction" };
Movie thirdMovie = new Movie { Name = "Matrix" };
// here you should take one from data context or create it, if it does not exist
Director director = new Director { Name = "Warner Bros" };
director.Movies.Add(firstMovie);
director.Movies.Add(secondMovie);
director.Movies.Add(thirdMovie);
// add new or update director with new movies
Db.AddOrUpdateDirector(director);
And of course, instead of just create a new one you should check if the directory already exist in datacontext and use that one.

Answer 2

Unfortunately, the way you're doing it will always result in a new director row for each reference to the same director object. Why? Because they are being added in three different contexts.
EF only tracks objects through DbContext. It will (sadly, maybe) not attempt to glean anything about the state of an object within a graph from properties of that object. So, the second time you save a Movie with the same director as the first, EF simply sees that it doesn't know anything about that Director and proceeds to add it to the graph as a new object as well. When SaveChanges() is called, it is duly added to the database.
You can watch this happen by looking at the DbEntry for the Director reference before SaveChanges(), and you'll also see Director.Id change each time as well.
There are a few strategies you can use to mitigate this behavior:
Add everything in a single context.
Add the director, then for each movie, only set DirectorId from Director.Id.
Add the director, the for each movie manually set the entry-state for the director object to "unchanged".
In the context, first do a context.Directors.Find(movie.Director.Id); to make sure the director is already in the context before adding the movie.

Answer 3

I have 3x the same Director in my database.
I think you have three different directors, they will have a different ID.
If you want to have just one director you need to let know to the DataContext about him. Maybe you can add a method AddDirector and add the director to the database before to use it.
Then when you do:
firstMovie.Director = director;
secondMovie.Director = director;
thirdMovie.Director = director;
The DataContext will know the director, and you will have the same for your three movies.
I mean I cant check if there is already directory with the same name,
because I am not adding them manually. Entity Framework is adding new
Directory, not me. So, what can I do with it?
For Entity Framework two directors with no ID value are two different directors. The only way to let it know that you want to use the same director is using a director with an ID value.
So you can:
- Create your director with a name and an id new Director { Id = 1, Name="Warner Bros" }
- Use a director that exist in the database (Entity Framework know about him, he has an ID)