fix(model): allow virtual ref function to return arrays

Author: brunohcastro

lib/model.js

@@ -3580,7 +3580,15 @@ function getModelsMapForPopulate(model, docs, options) {
           if (typeof ref === 'function') {
             ref = ref.call(doc, doc);
           }
-          modelNames = [ref];
+          
+          // When referencing nested arrays, the ref should be an Array 
+          // of modelNames.
+          if (Array.isArray(ref)) {
+            modelNames = ref;
+          } else {
+            modelNames = [ref];
+          }
+
           isVirtual = true;
         } else {
           // We may have a discriminator, in which case we don't want to