Support Model.update

[Astrid Ivy]

I think this involves writing a MongoDB update query processor, so not an easy fix. But worth doing, certainly.

[Astrid Ivy]

I've given this some thought, and we could implement it in layers of varying efficiency. $set operations are easy to type check; that wouldn't require pulling down the model before calling update. The mathematical update operators would only need to be pulled down if there were relevant validators. ($mul might result in a value too small or too large etc.) $push, etc. are easy to type check, but we would have to pull down the model if there was a notEmpty validator.

Basically, we can perform some update operations flying blind, but others would require a find() before we were sure the update would not result in invalid values

[Astrid Ivy]

In the spirit of having the user be free from writing update queries by hand, update could also take a function that looks like

Model.update(instance => {
   instance.field = 'new value'
   instance.numField.$mul(42)
   instance.dateField.$currentDate()
})

where instance is a special instance on which you can perform whatever updates you like. I'd then write the query from the instance's _updates object.