fix(populate): make ArraySubdocument#populated() return a value whe…

…n the path is populated

Re: #8247

lib/document.js

@@ -486,8 +486,7 @@ Document.prototype.$__init = function(doc, opts) {
 
   // handle docs with populated paths
   // If doc._id is not null or undefined
-  if (doc._id !== null && doc._id !== undefined &&
-    opts.populated && opts.populated.length) {
+  if (doc._id != null && opts.populated && opts.populated.length) {
     const id = String(doc._id);
     for (let i = 0; i < opts.populated.length; ++i) {
       const item = opts.populated[i];
@@ -501,6 +500,8 @@ Document.prototype.$__init = function(doc, opts) {
 
   init(this, doc, this._doc, opts);
 
+  markArraySubdocsPopulated(this, opts.populated);
+
   this.emit('init', this);
   this.constructor.emit('init', this);
 
@@ -509,6 +510,44 @@ Document.prototype.$__init = function(doc, opts) {
   return this;
 };
 
+/*!
+ * If populating a path within a document array, make sure each
+ * subdoc within the array knows its subpaths are populated.
+ *
+ * ####Example:
+ *     const doc = await Article.findOne().populate('comments.author');
+ *     doc.comments[0].populated('author'); // Should be set
+ */
+
+function markArraySubdocsPopulated(doc, populated) {
+  if (doc._id == null || populated == null || populated.length === 0) {
+    return;
+  }
+
+  const id = String(doc._id);
+  for (const item of populated) {
+    if (item.isVirtual) {
+      continue;
+    }
+    const path = item.path;
+    const pieces = path.split('.');
+    for (let i = 0; i < pieces.length - 1; ++i) {
+      const subpath = pieces.slice(0, i + 1).join('.');
+      const rest = pieces.slice(i + 1).join('.');
+      const val = doc.get(subpath);
+      if (val == null) {
+        continue;
+      }
+      if (val.isMongooseDocumentArray) {
+        for (let j = 0; j < val.length; ++j) {
+          val[j].populated(rest, item._docs[id][j], item);
+        }
+        break;
+      }
+    }
+  }
+}
+
 /*!
  * Init helper.
  *