Entity Framework Generic insert method is inserting already existing entity again along with the new entity

Question

I have following Insert method:

  public static bool Insert<T>(T item) where T : class 
    {
        using (ApplicationDbContext ctx = new ApplicationDbContext())
        {
            try
            {
                ctx.Set<T>().Add(item);
                ctx.SaveChanges();
                return true;
            }
            catch (Exception ex)
            {
               // ...
            }
        }
    }

This works as expected but when i want to insert a new entity, which has a relation to a existing entity, EF is re-inserting this relational (already existing) entity again along with the new entity.

Details: I have a entity Supplier which already exists in my database. I want to insert a new entity Product which has this existing Supplier entity as relation so i retrieve this Supplier from the database and add it to this Product Entity. When i insert it using the generic method, it re-inserts this Supplier and obviously i don't want this behavior.

Am i doing something wrong here or is this by design and should i not use a generic insert function when having relational entities attached to my new entities?

Thank you for any suggestion or information! Kind regards

EDIT:

Product Entity:

// ... non relational properties

public ICollection<Price> Prices { get; set; }
public ICollection<Supplier> Suppliers { get; set; }
public ICollection<Productnumber> ProductNumbers { get; set; }

Price Entity:

public Product Product { get; set; }
public Supplier Supplier { get; set; }

Supplier Entity:

public ICollection<Productnumber> ProductNumbers { get; set; }
public ICollection<Product> Products { get; set; }
public ICollection<Price> Prices { get; set; }

ProductNumber Entity:

 public Supplier Supplier { get; set; }
 public Product Product { get; set; }

How should i proceed to insert a new product? Is this possible having this structure and using a Generic insert?

Answer 1

If you want to add a new product to an existing Supplier you need to do as following:

1- Retrieve the supplier entity, let's call it sup

2- Add the new Product to it,

sup.Product = new Product{properties
    ...}

3- Update the supplier entity,

ctx.Entry(sup).State = EntityState.Modified;
ctx.SaveChanges();

You were adding a new supplier entity each time you were using the bool Insert<T> method and this why you got unexpected behaviour, so just update the existing entry instead.

Hope this helps,