Question

I have two tables, Orders and Customers, with Orders referencing Customers, and their mapped classes Order and Customer.

I want to write a method which receives a Linq Where() condition for both Orders and Customers:

void ProcessOrders(Expression<Func<Order, bool>> orderCondition, 
        Expression<Func<Customer, bool>> customerCondition)
{
    var q = Database.Query<Order>().Where(orderCondition);
    q = q.Where(o => o.Customer APPLY customerCondition); // how ?

    ... process records in q ...
}

Even after reading lots of SO answers on this topic and related articles on MSDN, it seems I cannot figure out how to apply the customer condition on the Order.Customer property.

(DB/ORM in use is Oracle/DevExpress, but I’m fine with a generic solution, so I did not include them in the tags. Please mention if your answer is restricted to a specific DB or ORM)

Answer 1

This is one way to achieve it.

void ProcessOrders(Expression<Func<Order, bool>> orderCondition, 
        Expression<Func<Customer, bool>> customerCondition>>)
{
    var orders = Database.Query<Order>().Where(orderCondition);
    var customers = Database.Query<Customer>().Where(customerCondition);

    var q = from o in orders join c in customers on o.CustomerId == c.Id
            select new {o, c}
}

Again, depending on the ORM, something like this may also work (I’m assuming a lazily loaded Customer.Orders collection property here):

void ProcessOrders(Expression<Func<Order, bool>> orderCondition, 
        Expression<Func<Customer, bool>> customerCondition>>)
{
    var customers = Database.Query<Customer>().Where(customerCondition);

    var q = from c in customers
            let orders = c.Orders.Where(orderCondition)
            select new {c, orders}

}

Any solution has to be tested against the specific linq provider, since different linq providers may or may not support certain queries…