fix(document): if setting unselected nested path, don't overwrite nes…

…ted path

Fix #5800

lib/document.js

@@ -670,6 +670,14 @@ Document.prototype.$set = function(path, val, type, options) {
     curPath += (curPath.length > 0 ? '.' : '') + parts[i];
     if (!cur) {
       this.$set(curPath, {});
+      // Hack re: gh-5800. If nested field is not selected, it probably exists
+      // so `MongoError: cannot use the part (nested of nested.num) to
+      // traverse the element ({nested: null})` is not likely. If user gets
+      // that error, its their fault for now. We should reconsider disallowing
+      // modifying not selected paths for v5.
+      if (!this.isSelected(curPath)) {
+        this.unmarkModified(curPath);
+      }
       cur = this.getValue(curPath);
     }
   }