support the $changeStream aggregation in 3.6+

[domodwyer]

The $changeStream aggregation isn't very well documented, but looks very interesting. An overview can be found here:

https://mongodb.github.io/mongo-java-driver/3.6/driver/tutorials/change-streams/

This commit is a squash of 4 commits from the 10gen fork of mgo:
* go-mgo@e78a135
* go-mgo@31a6d57
* go-mgo@7940d20
* go-mgo@0d8351c

[szank]

We are missing the values that can be set in the FullDocument field.
This is:

• DEFAULT
• UPDATE_LOOKUP
  And possibly some documentation re ResumeAfter field.
  I assume this is a query that would return a single document, no?

[domodwyer]

Keeping this on hold for the time being - ideally we want to wait for $changeStream to become a little better documented.

[amineck]

Any update on this thread ?
Thank you

[domodwyer]

Hi @surfeurX

I think it's definitely mature enough to consider implementing now the 3.6 release is imminent. Some helpful integration docs have appeared too:

• MongoDB $changeStream documentation
• Driver specification

Our team is rather busy at the moment so we're open to outside PR's for this I guess - it's our intention to get this feature in one way or another though, I just can't put a timeline on it yet.

Dom

[amineck]

Thank you @domodwyer, I may have some time to implement it next week 👍

[mcmurdont]

We are looking at implementing this too. We will review the docs tomorrow, 2017-12-07, and come up with a testing strategy.

[peterdeka]

Any news on this? Are just tests needed or is there something else missing?
I can do my best to write a couple of tests in case.

[peterdeka]

If i understand correctly, Next() is currently a blocking function and the MaxAwaitTimeMS parameter is a mock, it is not used. The code is pretty useless right now as a full blocking function is rarely useful in real case scenarios. We should make use of the MaxAwaitTimeMS, if i understand correctly this is a deeper option of the iterator cursor. Any help/idea on how to proceed down this way? It seems the driver is not ready for this type of options.

[peterdeka]

Opened PR #97 hoping we can move on.

[domodwyer]

Hi @peterdeka - really sorry for the lack of a reply, I've been away at a conference.

Thanks for opening a PR - it's really appreciated! We'll take a look 👍

Dom