Changes

node_modules/mongoose/lib/helpers/populate/assignVals.js

@@ -18,6 +18,7 @@ module.exports = function assignVals(o) {
     justOne: o.justOne
   });
   populateOptions.$nullIfNotFound = o.isVirtual;
+  const populatedModel = o.populatedModel;
 
   const originalIds = [].concat(o.rawIds);
 
@@ -39,12 +40,31 @@ module.exports = function assignVals(o) {
     if (val instanceof SkipPopulateValue) {
       return val.val;
     }
+
     if (o.justOne === true && Array.isArray(val)) {
-      return valueFilter(val[0], options, populateOptions);
+      // Might be an embedded discriminator (re: gh-9244) with multiple models, so make sure to pick the right
+      // model before assigning.
+      const ret = [];
+      for (const doc of val) {
+        const _docPopulatedModel = leanPopulateMap.get(doc);
+        if (_docPopulatedModel == null || _docPopulatedModel === populatedModel) {
+          ret.push(doc);
+        }
+      }
+      // Since we don't want to have to create a new mongoosearray, make sure to
+      // modify the array in place
+      while (val.length > ret.length) {
+        Array.prototype.pop.apply(val, []);
+      }
+      for (let i = 0; i < ret.length; ++i) {
+        val[i] = ret[i];
+      }
+
+      return valueFilter(val[0], options, populateOptions, populatedModel);
     } else if (o.justOne === false && !Array.isArray(val)) {
-      return valueFilter([val], options, populateOptions);
+      return valueFilter([val], options, populateOptions, populatedModel);
     }
-    return valueFilter(val, options, populateOptions);
+    return valueFilter(val, options, populateOptions, populatedModel);
   }
 
   for (let i = 0; i < docs.length; ++i) {
@@ -84,6 +104,16 @@ module.exports = function assignVals(o) {
       }, new Map());
     }
 
+    if (isDoc && Array.isArray(valueToSet)) {
+      for (const val of valueToSet) {
+        if (val != null && val.$__ != null) {
+          val.$__.parent = docs[i];
+        }
+      }
+    } else if (isDoc && valueToSet != null && valueToSet.$__ != null) {
+      valueToSet.$__.parent = docs[i];
+    }
+
     if (o.isVirtual && isDoc) {
       docs[i].populated(o.path, o.justOne ? originalIds[0] : originalIds, o.allOptions);
       // If virtual populate and doc is already init-ed, need to walk through