实体框架：如何选择嵌套实体

Question

my entity hierarchy is Customer > Address > Contact. so i use this code to iterate in contact details.

Contacts CurrentContacts = null;

foreach (var existingContacts in existingCustomer.Addresses.Select(a => a.Contacts.Where(cc=> cc.ContactID==5)))
{
    CurrentContacts = existingContacts;
}

but this line CurrentContacts = existingContacts; throwing error

Cannot implicitly convert type 'System.Collections.Generic.IEnumerable' to 'EFTest.Contacts'. An explicit conversion exists (are you missing a cast?)

what is my mistake. existingContacts will have contacts related data and CurrentContacts is a variable of contact type.

so please tell me how could i do it.

Answer 1

Use SelectMany instead of Select .

Select projects each individual element and does nothing to the rest of the enumerable . So Addresses.Select(a=>a.Contacts.Where(...)) is an enumerable of whatever Address.Contacts.Where(...) is. Ie. you get an enumerable of enumerables.

To get a single enumerable you need to flatten this list. Which is what SelectMany does.

That said even

foreach (var existingContacts in <some enumerable>)
{
    CurrentContacts = existingContacts;
}

Looks strange. Unless there are other logic in the loop you could replace the loop with;

CurrentContacts= <some enumerable>.LastOrDefault();

And if you don't actually care which Contact you use, as long as it has ContactID=5;

CurrentContacts= <some enumerable>.FirstOrDefault();

Answer 2

I think you have two problems, but without seeing how the Contacts type is defined it's just a guess.

As @Taemyr points out, you need to use SelectMany instead of Select to flatten the collection of collections.

But unless Contacts is inherited from List<EFTest.Contacts> you still aren't going to be able to do the assignment, you need to change your variable declaration to:

IEnumerable<Contacts> CurrentContacts = null;