Currently, custom polymorphic types can be specified by either an array of morph classes or by specifying a map of type strings to classes (https://laravel.com/docs/5.2/eloquent-relationships#polymorphic-relations). However, it is not possible to use numeric keys as type "strings".

Given the following array
Relation::morphMap([ "1" => \App\Models\ModelOne::class, "2" => \App\Models\ModelTwo::class, ]);
The resulting morphMap is:
array:2 [ 0 => "App\Models\ModelOne" 1 => "App\Models\ModelTwo" ]

It should be possible to specify numeric ids which is common in database environments and also less storage hungry in terms of INT vs. VARCHARS.