In this wiki I will show you how to save/update a model with many related model For example: How to set a product to multiple categories only in one post form ?
The only differences in Yii 2 is that its much simpler due to available functions in the Model class for loading and validating models.
This tutorial shows how to use a model to save and validate related (MANY_MANY) models.
If you tried this extension with success, maybe you thought how to do it without page submission or ajax calls.