populate functionality to be consistent with push/set

[zbenmo]

I find the populate functionality very useful. You store in the database object ids, yet you need the actual documents so you just ask for them and instead of ids you have the objects.

But, the same is needed for push/set. You set an object and a moment later only the id is there. You don't have any more the object (at least associated with the main document).

I recommend that the duality be exposed explicitly, and be consistent all the way at all times. This will make the push/set/populate functionality truly useful and consistent.

With kind regards, Oren.

[aheckmann]

duplicate of #570 and fixed in 3.6x branch.