Sorting array with different order

[Grzeogorz Łuszczek" <grzegorz at piklus.pl>]

I'm writing small web server to serve data as JSON. This data 
could be sorted based on various parameters (order and field to 
sort).

In this example I will use only 2 classes (but I need serve 10-20 
classes and each class have 5-10 attributes):

class Port {
   int number;
   Host[] host;
}

class Host {
   string macAddress;
   string ipAddress;
}

I have idea to use this struct:

struct Sort {
   string order = "desc"; // Sort order - DESC/ASC
   string sort_by;        // Filed to sort by
}

To create an associative array with sort function:

static this(){
   portSortMethods[ Sort( "desc", "number" ) ] = &sort!( "a.number 
 > b.number", SwapStrategy.unstable, Port[] );
   portSortMethods[ Sort( "asc", "number" ) ] = &sort!( "a.number 
< b.number", SwapStrategy.unstable, Port[] );
   hostSortMethods[ Sort( "desc", "macAddress" ) ] = &sort!( 
"a.macAddress > b.macAddress", SwapStrategy.unstable, Host[] );
   hostSortMethods[ Sort( "asc", "macAddress" ) ] = &sort!( 
"a.macAddress < b.macAddress", SwapStrategy.unstable, Host[] );
   hostSortMethods[ Sort( "desc", "ipAddress" ) ] = &sort!( 
"a.ipAddress > b.ipAddress", SwapStrategy.unstable, Host[] );
   hostSortMethods[ Sort( "asc", "ipAddress" ) ] = &sort!( 
"a.ipAddress < b.ipAddress", SwapStrategy.unstable, Host[] );
}

And write function like this for Hosts:

Hosts[] sortHosts( Host[] collection, string order, string by ) {
   auto sortMethod = hostsSortMethods[ Sort( "desc", "macAddress ) 
];
   return sortMethod( collection ).release();
}

But I have problem with type of portSortMethod and hostSortMethod.

I will be glad if someone could help me with my problem.