Hi Harry,
The association is saved in the ‘parent’ entity (the place where the association starts from). To ensure data integrity, Mendix will auto-commit the ‘child’ object if it is not commited to the database already. Therefore you should model your app in such a way that auto-commits are not necessary. In practice this means making sure your ‘child' object is commited before you commit an association reference to it.
You can read more about this here: