fix(populate): allow accessing populate virtual prop underneath array…

… when virtual defined on top level

Fix #8198
Re: #8210

lib/schema.js

@@ -10,6 +10,7 @@ const SchemaType = require('./schematype');
 const VirtualType = require('./virtualtype');
 const applyTimestampsToChildren = require('./helpers/update/applyTimestampsToChildren');
 const applyTimestampsToUpdate = require('./helpers/update/applyTimestampsToUpdate');
+const arrayParentSymbol = require('./helpers/symbols').arrayParentSymbol;
 const get = require('./helpers/get');
 const getIndexes = require('./helpers/schema/getIndexes');
 const handleTimestampOption = require('./helpers/schema/handleTimestampOption');
@@ -1724,6 +1725,22 @@ Schema.prototype.virtual = function(name, options) {
     return mem[part];
   }, this.tree);
 
+  // Workaround for gh-8198: if virtual is under document array, make a fake
+  // virtual. See gh-8210
+  let cur = parts[0];
+  for (let i = 0; i < parts.length - 1; ++i) {
+    if (this.paths[cur] != null && this.paths[cur].$isMongooseDocumentArray) {
+      const remnant = parts.slice(i + 1).join('.');
+      const v = this.paths[cur].schema.virtual(remnant);
+      v.get((v, virtual, doc) => {
+        const parent = doc.__parentArray[arrayParentSymbol];
+        const path = cur + '.' + doc.__index + '.' + remnant;
+        return parent.get(path);
+      });
+      break;
+    }
+  }
+
   return virtuals[name];
 };