Cannot save many-to-many Entity Framework

Question

I have a simple many-to-many relational database with these entities:

public class Product
{
    public Product()
    {
        this.Category = new HashSet<Category>();
    }

    // just other fields...
    public virtual ICollection<Category> Category { get; set; }
}

public class Category
{
    public Category()
    {
        this.Product = new HashSet<Product>();
    }

    // other fields
    public virtual ICollection<Product> Product { get; set; }
}

I'm trying to save a product, but it throws an exception:

System.InvalidOperationException: 'The relationship between the two objects cannot be defined because they are attached to different ObjectContext objects.'

My code:

public void SaveProduct(Product product, IEnumerable<Category> categories)
{
    if (product.ProductID == null)
    {
        product.ProductID = RandomKeyGeneration();
        context.Products.Add(product);
        context.Products.Attach(product);

        foreach (var cat in categories)
            product.Category.Add(cat);
    }

    context.SaveChanges();
}

I googled and tried to solve this issue, but it still didn't work.

Answer 1

An entity object category was referenced by multiple instances of my context class. The problem was in controller, where I used two repositories: one for Products, another - Category. And in the both classes I declare instance of context. Because of this, there were declared two instances of context and was thrown an exception.

It's my fault and inattentiveness. Question closed.