Hi Yang,

When an object is committed through a default Save button, a commit activity, or web services, it always triggers the commit events. The platform also evaluates all associated objects. To guarantee data consistency, the platform may also autocommit associated objects.

An autocommit is an automatic commit from the platform, which is done to keep the domain model in sync. If your application ends up having autocommitted objects, then you will have a modelling error. Since an association is also a member of an object, the association is stored in the database as well. This means that if you create an order line inside an order and the order line is the parent of the association, when you commit the order line, the order is autocommitted.

https://docs.mendix.com/refguide/committing-objects/#autocommit-and-associated-objects

The documentation shows how autocommits can occur and how you should correct the Domain Model (cardinality) and the object where the commit should be. (Parrent side) 

If you need more guidance, let us know. 

Go Make It.