Loading particular peoperties for entity and assigning it as reference to another entity - entity-framework

Question

Example:
I am entering a new invoice. For this invoice I need to enter a customer. Lets assume that we retrieved a list of customers:
var list = Context.Set<Customer>().ToList();
Here I see two issues:
1) I do not need to bring all information for customer, I only need Id, Code and Name
2) Customer in current DbContext is read-only, so it would be nice if it is possible to tell DbContext not to monitor their states, to improve performance.
Questions:
1) Can we load only partial data for customer, but still be able to assign it to Invoice (see code bellow)?
2) Can we tell DbContext not to monitor Customers for changes, and still be able to do this:
Invoice.Customer = CustomerList[10];

Answer 1

There's not a direct way to do exactly what you want, but you might be able to achieve your goals with some compromise.
I do not need to bring all information for customer, I only need Id,
Code and Name
There isn't a way for EF to create a partially loaded entity, but you could create an anonymous type:
Context.Customers.Select(c => new {Id = c.CustomerId, Code = c.Code, Name = c.Name}).Tolist()
If you could live with the new anonymous type then use that, or you could then iterate through that list, creating actual customer objects.
Customer in current DbContext is read-only, so it would be nice if it
is possible to tell DbContext not to monitor their states, to improve
performance.
EF provides an Extension of AsNoTracking() which will do exactly what you're looking for:
var list = Context.Set<Customer>().AsNoTracking().ToList();
Depending on what you choose from above, the following code may change, but this code does achieve what you're looking for. Partially loads the customer, but still allows you to attach the customer to the invoice.
Note: You'll need to attach the customer to your context before you can use it, and then setting it to a state of Unchanged will prevent it from overwriting exiting data.
m = new Model();
var list = m.Customers.Select(c => new {Id = c.CustomerId, Code = c.Code, Name = c.Name});
List<Customer> customerList = new List<Customer>();
foreach (var item in list)
{
customerList.Add(new Customer()
{
CustomerId = item.Id,
Code = item.Code,
Name = item.Name
});
}
Invoice i = new Invoice();
var customer = customerList.First();
m.Customers.Attach(customer);
m.Entry(customer).State = EntityState.Unchanged;
i.Customer = customer;
m.Invoices.Add(i);
m.SaveChanges();