如何在 Entity Framework Core 中更新多对多关系中的表？

Question

I have two entities in a many-to-many relationship and 3 tables:

• Customer
• Shop
• CustomerAndShopAssignemnt (join table)

I have list of shops for example:

ShopA, ShopB, ShopC, ShopD 

and I want to add a new Customer, for which I choose 2 shops (for example ShopA and ShopC).

What I do is:

var listOfChoosenChops = // get the shops entities from db by ID's

var newCustomer = new Customer()
{
   Name = "Martin",
   Age = 44,
   Shops = listOfChoosenChops 
}

_context.Customers.Add(newCustomer)
_context.SaveChanges();

I get error from EF that it can't put the entities with the same primary keys. That means that EF tries to store new Shops into the database.

My question is: how to solve it and update only join table?

PS: I saw another posts but I'm not able to find solution which will help me there.

Answer 1

You should have provide more information. by the way.

Here is the updated code:-

var existingShops = _context.Shops.Where(s => s.ShopId == shopId).ToList();

var newCustomer = new Customer()
{
   Name = "Martin",
   Age = 44
};

newCustomer.Shops = existingShops;

_context.Customers.Add(newCustomer);
_context.SaveChanges();

Hope it will resolve your issue.

Answer 2

I want to add a new Customer, for which I choose 2 shops (for example ShopA and ShopC).

I have a suggestion like below:

var ShopA = // get the ShopA from db by ID
var ShopC = // get the ShopC from db by ID
var newCustomer = new Customer()
{
   Name = "Martin",
   Age = 44,
};
newCustomer.Shops.Add(ShopA);
newCustomer.Shops.Add(ShopC);
_context.Customers.Add(newCustomer)
_context.SaveChanges();

My demo code like:

            var s1 = _context.Supplier.FirstOrDefault(x => x.SupplierID == 1);
            var s2 = _context.Supplier.FirstOrDefault(x => x.SupplierID == 2);
            var newCustomer = new Product()
            {
                ProductName = "p3",
             
            };
            newCustomer.Supplier.Add(s1);
            newCustomer.Supplier.Add(s2);
            _context.Product.Add(newCustomer);
            _context.SaveChanges();

result:

Answer 3

Typically when I see this error, it's due to using the AsNoTracking method.

If you're using the AsNoTracking method when getting your list of shops, removing that method should solve your error.