fix(query): propagate top-level session down to populate()

Fix #6754

Author: vkarpov15

lib/query.js

@@ -1601,7 +1601,6 @@ Query.prototype._find = function(callback) {
     }
 
     const pop = helpers.preparePopulationOptionsMQ(_this, mongooseOptions);
-    pop.__noPromise = true;
     completeManyOptions.populated = pop;
     _this.model.populate(docs, pop, function(err, docs) {
       if (err) return callback(err);

lib/queryhelpers.js

@@ -48,6 +48,19 @@ exports.preparePopulationOptionsMQ = function preparePopulationOptionsMQ(query,
       forEach(makeLean(options.lean));
   }
 
+  const session = get(query, 'options.session', null);
+  if (session != null) {
+    pop.forEach(path => {
+      if (path.options == null) {
+        path.options = { session: session };
+        return;
+      }
+      if (!('session' in path.options)) {
+        path.options.session = session;
+      }
+    });
+  }
+
   return pop;
 };