[FIX] CONTRIBUTING: dead link
[FIX] models: do not prefetch fields to recompute, and recompute once only

The method _prefetch_field() was accidentally prefetching fields to recompute;
which was skipping the actual recomputation, since a value was put in cache.
But sometimes the field's value was fixed by an extra recomputation of the
field.  Here we remove the extra recomputation and fix the cache corruption.