Adding current user available to pre/post save hooks

[estilles]

This PR makes the current user available to pre and post save hooks.

I do not alter the model per se. I simply add a _user property to the item object just before it is saved in updateHandler, but since mongoose is unaware of it's existence, it does not save it to the database. The property is temporary in nature and does not survive the current transaction.

Here's a usage example.

List.schema.pre('save', function(next, done) {
    console.log('current user =>', this._user);
    next();
});

The idea of having the current user available to pre and post save hooks was originally raised in #458 in the context of using it for audit purposes (which I addressed in #490). However, having the current user available in pre and post save hooks is also quite useful in use cases other than this, which is why I decided to submit this as a separate pull request.

[JedWatson]

This is pretty much exactly how I was planning to do this, nice work.

I can't remember why I hadn't added it yet, I think I wanted to (either) write tests first, or do some testing to make it really clear in which situations this will and won't work...

For example this._user won't be available in the following context:

new Thing.model({ name: "My New Thing" }).save();

... which is a bit of a trap, it means if you don't consistently use the UpdateHandler, or explicitly set _user when you're not, middleware won't necessarily work as expected.

I do think it's better to have this even with that limitation, I just wanted to make sure it was well tested and understood :)

Also, what do you think of calling it __user? or possibly even something like _req_user which is more explicit and probably never going to potentially conflict with actual schema paths.

[SharadKumar]

This is nice - I can use this on my project. It further sits well with #490
Would love to see a merge. ;-)

[estilles]

@JedWatson, you're absolutely correct. I should have included the caveat that it will not work in circumstances where UpdateHandler is not used.

I see your point about a potential schema conflict. I'll rename it to _req_user, as per your suggestion.

Let me work on this a bit. I was to include some tests as well.

[estilles]

@SharadKumar, thanks for the comments. The audit meta is something I've been using with Keystone since I started using it last year. I think it was one of the first mods I made. #490 is a slightly modifed/more testable version of my original code. Sorry I didn't share it with everyone until now.

[JedWatson]

Merged as is to unblock some work, will rename to _req_user in a moment. If you could do some tests that would be great, as a new PR though :)

[estilles]

No problem. Will work on some tests tonight and submit a new PR.

[alisajidcs]

This feature should be in pre/post remove too