EF Core Duplicate entry Exception - asp.net

Question

EF core throws an exception in the project I am working on
MySqlException: Duplicate entry '2' for key 'customers.IX_Customers_UserID'
UserID gives such an error when it is present I do not understand exactly what the error is, please help
public class Context : DbContext
{
public DbSet<Product> Products { get; set; }
public DbSet<Category> Categories { get; set; }
public DbSet<User> Users { get; set; }
public DbSet<Customer> Customers { get; set; }
public DbSet<Address> Addresses { get; set; }
protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
{
//optionsBuilder.UseSqlite("Data Source = Shop.db");
//optionsBuilder.UseSqlServer(#"Data Source = (localdb)\v11.0; Initial Catalog=ShopDb; Integrated Security = SSPI;");
optionsBuilder.UseMySql(#"server=localhost;port=3306;database=ShopDb1;user=root;password=admin123;");
}
public Context()
{
Database.EnsureCreated();
}
}
public class User
{
public int ID { get; set; }
public string UserName { get; set; }
public string Email { get; set; }
public Customer Customer { get; set; }
public List<Address> Addresses { get; set; }
}
public class Customer
{
public int ID { get; set; }
public int IdentifyNumber { get; set; }
public string FirstName { get; set; }
public string LastName { get; set; }
public User User { get; set; }
[Required]
public int UserID { get; set; }
}
public class Suplier
{
public int ID { get; set; }
public string Name { get; set; }
public string TaxNumber { get; set; }
}
public class Address
{
public int ID { get; set; }
public string FullName { get; set; }
public string Title { get; set; }
public string Body { get; set; }
public User User { get; set; }
public int UserID { get; set; }
}
public class Product
{
public int ProductID { get; set; }
[MaxLength(100)]
[Required]
public string Name { get; set; }
public decimal Price { get; set; }
public int CategoryID { get; set; }
}
public class Category
{
public int CategoryID { get; set; }
[MaxLength(100)]
[Required]
public string Name { get; set; }
}
class Program
{
static void Main(string[] args)
{
using (var db = new Context())
{
var customer = new Customer()
{
IdentifyNumber = 123,
FirstName = "Remzi",
LastName = "Balakishiyev",
UserID = 2
};
db.Customers.Add(customer);
db.SaveChanges();
}
Console.ReadLine();
}
static void InsertUsers()
{
var users = new List<User> {
new User() { UserName ="Remzi",Email = "remzi.balakisiyev#gmail.com"},
new User() { UserName ="Xezri",Email = "xezri.balakisiyev#gmail.com"},
new User() { UserName ="Nurane",Email = "nurane.tarverdiyeva#gmail.com"}
};
using (var db = new Context())
{
db.Users.AddRange(users);
db.SaveChanges();
}
}
static void InsertAddresses()
{
var addresses = new List<Address> {
new Address(){FullName = "Remzi Balakisiyev", Title = "Ev addressi", Body = "Masalli",UserID=1},
new Address(){FullName = "Remzi Balakisiyev", Title = "Ish addressi", Body = "Baki",UserID=1},
new Address(){FullName = "Xezri Balakisiyev", Title = "Ev addressi", Body = "Masalli",UserID=2},
new Address(){FullName = "Nurane Tarverdiyeva", Title = "Ev addressi", Body = "Naxcivvan",UserID=3},
new Address(){FullName = "Rena Heyderova", Title = "Ev addressi", Body = "Xachmaz",UserID=2},
new Address(){FullName = "Memmed Bedelov", Title = "Ev addressi", Body = "Sumqayit",UserID=1}
};
using (var db = new Context())
{
db.Addresses.AddRange(addresses);
db.SaveChanges();
Console.WriteLine("Ishledi");
}
}

Answer 1

For each customer entity framework adds a new user with the primary key 2. To add a relationship to the user with id 2 you need to insert a user object to the user property.
var customer = new Customer()
{
IdentifyNumber = 123,
FirstName = "Remzi",
LastName = "Balakishiyev",
User = db.Users.FirstOrDefault(x => x.ID == 2)
};
db.Customers.Add(customer);
db.SaveChanges();