LINQ Join with where condition - asp.net-mvc

Question

I can use LINQ's Join with Lambda notations no problem, but I can't work out how one would then add a where condition.
var q = query.Join(context.CustomerIds,
x => x.CustomerId,
y => y.CustomerId,
(x, y) => new CustomerLookupResult()
{
dob = x.DateOfBirth.ToString(),
forenames = x.Forenames,
surname = x.Surname,
loyaltyNo = y.Identifier,
customerId = x.CustomerId
});
The table I'm joining the first to contains the loyaltyNo in its Identifier column, but also contains other information in the same column and so uses a second column IdentifierTypeCode to allow filtering.
So how do I now add .Where(x => x.IdentifierTypeCode == "LOYALTY") like I would in SQL. Appending this to end refers to the new object.

Answer 1

You could apply your Where before doing the join.
var q = customerLoyalties
.Where(x => x.IdentifierTypeCode == "LOYALTY")
.Join(customers,
x => x.CustomerId,
y => y.CustomerId,
(x, y) => new CustomerLookupResult()
{
CustomerId = y.CustomerId,
Name = y.Name,
IdentifierTypeCode = x.IdentifierTypeCode
});

Answer 2

You can also use this way to achieve that using Linq.
var match = from t1 in context.orders
join t2 in context.orderdetails on
new { t1.OrderID } equals
new { t2.OrderID }
join t3 in context.products on
new { t2.ProductID } equals
new { t3.ProductID }
where t3.ProductID == id
select t3;
return match.ToList();

Answer 3

The first parameter to the Join takes any IEnumerable, so you can apply the Where at that point, or earlier
var q = query.Join(context.CustomerIds.Where(x=>x.IdentifierTypeCode=="LOYALTY"),
x => x.CustomerId,
y => y.CustomerId,
(x, y) => new CustomerLookupResult()
{
dob = x.DateOfBirth.ToString(),
forenames = x.Forenames,
surname = x.Surname,
loyaltyNo = y.Identifier,
customerId = x.CustomerId
});
alternatively, if you don't like to put too much on one line:
var filteredLoyalties = context.CustomerIds.Where(x=>x.IdentifierTypeCode=="LOYALTY");
var q = query.Join(filteredLoyalties,
...