fix(populate): handle doc.populate() with virtuals

Fix #5240

Author: vkarpov15

lib/document.js

@@ -311,6 +311,7 @@ Document.prototype.init = function(doc, opts, fn) {
   }
 
   this.isNew = false;
+  this.$init = true;
 
   // handle docs with populated paths
   // If doc._id is not null or undefined

lib/model.js

@@ -19,6 +19,7 @@ var cast = require('./cast');
 var castUpdate = require('./services/query/castUpdate');
 var discriminator = require('./services/model/discriminator');
 var isPathSelectedInclusive = require('./services/projection/isPathSelectedInclusive');
+var mpath = require('mpath');
 var parallel = require('async/parallel');
 var util = require('util');
 var utils = require('./utils');
@@ -3190,7 +3191,14 @@ function assignVals(o) {
     if (o.isVirtual && !o.justOne && !Array.isArray(rawIds[i])) {
       rawIds[i] = [rawIds[i]];
     }
-    utils.setValue(o.path, rawIds[i], docs[i], setValue);
+
+    if (o.isVirtual && docs[i].constructor.name === 'model' && docs[i].$init) {
+      // If virtual populate and doc is already init-ed, need to walk through
+      // the actual doc to set rather than setting `_doc` directly
+      mpath.set(o.path, rawIds[i], docs[i]);
+    } else {
+      utils.setValue(o.path, rawIds[i], docs[i], setValue);
+    }
   }
 }