[IMP] models: do not use compute methods to determine default values anymore

[IMP] models: do not use compute methods to determine default values anymore

Compute methods could give results that should not be considered as default
values.  For instance, a related field usually defaults to a null value, which
is then set to the field with its inverse method by create().  This may violate
a non-null constraint if the original field is required.  Therefore, compute
methods are no longer used to determine default values.