Why is the "constraints" feature available only in 1:N relationships and not for the N:1 relationships ?

You are right, constraints are super useful.

When you are on the child selecting a parent, that parent can already have been linked to a different child and it's still available to add to child you are on, so constraining the number of parents is a useful thing but when you are on the parent either the child composiiton is Yes and you will only be able to add new child records, or No and only children not already linked to a parent will be available. This constraint has been set for you, so to speak. There should be no or very few children free as they are orphans without a parent.

When it comes to a many to many relationship it may look like you need a workaround, for example I'm creatig an invoice and I only want to add lines where the stock item is allowed to be sold to the customer but in fact assigning a stock item to a line is a case of selecting a parent from a child record and so you can set a constrain on stock to only show items allowed.

Hope that makes sense.

Regards John

I dont really understand why I some times get right answer when I use the "b.Contact = a", and some times not?
I suppose the b is the child and the a parent?