Implement limit method for MongoDB driver

[nazriel]

My first naive implementation.
Seems to work good but I am still new to vibe.d internals so I am not sure if it's the best approach.

[s-ludwig]

If you have the time, I'd like to suggest some improvements. In particular, the amount of returned documents per chunk should also be limited to the given limit and the cursor should be discarded when the limit is reached. I'll make some inline comments for this.

[s-ludwig]

An additional if (m_nret <= 0 || count < m_nret) m_nret = min(count, 1024); should limit the amount of results per chunk here.

[nazriel]

I might be wrong but I think it should be:

if (count != m_nret) 
    m_nret = min(count, 1024);

    m_limit = count;

Because according to MongoDB docs:
"A limit() value of 0 (e.g. “.limit(0)”) is equivalent to setting no limit."

So such code:

db["users"].find().sort(Bson(["title": Bson(-1)])).limit(3).limit(0);

Should reset limit and also rising limit should be possible, for example:

db["paste"].find().sort(Bson(["title": Bson(-1)])).limit(3).limit(10);

@s-ludwig what you think?

[s-ludwig]

I don't know about the last two use cases, I think it's ok and expected if limit... limits (i.e. it can only lower the limit and not raise it). And it should still be possible to specify a chunk size that is lower than the limit somehow, which cannot work if limit always adjusts m_nret.

But you are definitely right about the zero case (note that the if statement in the empty method also needs to be adjusted to cope with m_limit == 0.

[nazriel]

@s-ludwig updated, hope I understood your correctly.

[s-ludwig]

Looks good, except the zero case, which I would rather handle like this:

if (count > 0) {
    if (m_nret == 0 || m_nret > count)
         m_nret = count;
}

This would be more in line with the tought that each limit call successively limits the output further.

[nazriel]

@s-ludwig updated. Hope I got it correctly this time 👻

[s-ludwig]

Thanks a lot! Merging now.

[s-ludwig]

Just discovered that m_limit was still handled differently. But now everything should be good.

[nazriel]

Yeah I left limit(0) case as "limit-reset" option.
But I think your approach is more consistant so yeah looks great now!