The normal flow should be from C to A, I mean that C1 should be already defined when the view A is opened, and that's why it's a related field. But there's a rare scenario where an user could want start defining from A to C, and that's why I need to implement it this way.

Pau, even if it is a rare chance, you still need to implement the synchronization, i.e. change A1 if C1 is updated and C1 if A1 is updated. You still can default A1 to display whatever value C1 is holding. Another approach that you may want to consider is to display B in A's form and C in B's form, hence user can change C1 from C's form instead of changing it from A's form. More longwinded for user, I understand.

John, what do you mean by implement the synchronization? Do you have a link to any document explaining how to do it? The request is to display C1 in A, and then filter B by this value.

Pau, your first problem description is that user need to be able to change A1 and then C1 need to be change to be equal to A1. Does this still a requirement? If so, my answer stands. Synchronization is basically changing C1 when A1 is changed by user and A1 if C1 is selected by user. You need to inherit method write of both class A and C. For, say class A, you need to detect if there is any change in A1, and then change the related C1 value to be the same as A1, vv. If your first problem description is not correct, then I would suggest that you restate what you really need so the answer can be crafted more accurately.