ASP.NET MVC 3 Database Context Querying

Question

I have my Database Context:

public class ProductContext : DbContext
{
    public ProductContext() : base ("DefaultConnection") {}

    public DbSet<Product> Products {get;set;}
}

and my Repository:

public class ProductRepository : IProductRepository
{
    private ProductContext _dbContext = new ProductContext();

    public IQueryable<Product> Products { get { return _dbContext.Products; } }
}

when I query my database in the Edit Action:

public ActionResult Edit(Guid id)
{
    var item = _repository.Products.FirstOrDefault(x => x.Id.Equals(id));

    return View(item);
}

I would usually use a ViewModel but this is purely to show the scenario.

When I query the database using the var item line, does EntityFramework change the state of that item.

Can I pass around that item through a multitude of Services in the Service Layer and then finally save it using my method:

public void SaveEntity<TEntity>(TEntity entityToSave) where TEntity : DbEntity
{
    if (entityToSave.Id.Equals(Guid.Empty))
        _dbContext.Set<TEntity>().Add(entityToSave);
    else
        _dbContext.Entry<TEntity>(entityToSave).State = EntityState.Modified;

    _dbContext.SaveChanges();
}

It won't throw an exception saying that there is already a Entity with the same Id as the one you're trying to Save?

Callum Linington · Accepted Answer · 2013-07-09 14:55:54Z

1

So after trial and error, it seems that this works perfectly fine, and it doesn't bring back any errors. There is one thing to look out for:

This navigation property:

public virtual Category Category { get;set; }
public Guid CategoryId { get;set; }

That could reside in the Product model has a little gotcha, that is:

When editing or saving a new Product, you should only set the CategoryId and not just the Category exclusively because you will get duplicate Category entries every time you edit or save if you use the a Category that already exist within the database...

I think you should the navigation property solely for your ease, not for use when modifying entities...

answered Jul 9, 2013 at 14:55

Callum Linington

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4 Comments

Henk Mollema Over a year ago

EF will try to insert the category again because it's most likely attached to another instance of your DbContext. EF cannot know the category already exsists, so it tries to insert it which causes a duplicate key error. This problem can be solved by using the same instance of the DbContext the whole request. Filling in the foreign key field as you mentioned will work just fine though.

Callum Linington Over a year ago

Yeah I think that is the issue, although I am injecting the repository which will use one instance of the DbContext, I moving the entities around the place, so it is just easier to use the Id fields.

Henk Mollema Over a year ago

Make sure you create an instance for the scope of a request.

Callum Linington Over a year ago

Ninject takes care of creating them, I just pull them through the controllers.

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