You can't. The objects in memory represent a subset of the real entities in the database. This means that when you fetch for entity E a subset of the related entities R, this may lead to 0 entities being fetched. Though there might be entities R in the database, though they didn't match the filter being set for the particular fetch for R.
Also, after your fetch, some other user might have added an entity which thus is present, though not in memory of your process.