If you read my posts, you know that I'm a big fan of MongoDB Aggregation Framework.  It was brand new in 2.2 over a year ago and last year in 2.4 release it got faster and picked up some cool new features.   The next production release is 2.6 and aggregation framework continues growing up as a major feature of MongoDB.  Since MongoDB 2.6.0 is now available, I thought I would point out some of the coolest improvements that will make aggregation even simpler and more powerful.

If you want to see the complete list of SERVER tickets fixed for 2.6, it's HERE.  A more readable summary is in the release notes for 2.6.

The biggest and baddest is SERVER-5932, support for aggregations returning a cursor.   Some think that SERVER-3253 is the biggie, that's the one that allows you to redirect the output of of aggregation to another collection (and its cousin SERVER-10097 that allows you the same capability on a sharded cluster).  But if you ask me, getting back a cursor pointing to the results, so I can iterate over it the same way we already do over results from find(), that's the more important one and if I want to, I can then send the results to another target collection (or some other place), so I've been most excited about that.  Needless to say, this makes the old limitation of aggregation returning only 16MB history.

Of course there are many other new features, from major ones or minor ones.  I'm sure you'll be seeing them show up in my future posts as I think of new and novel ways of torturing documents into new shapes, forms and uses!

One of the most disappointing things about Aggregation Framework in MongoDB is that it doesn't give you many ways to figure out dynamically what schema you are working with.  There's the expected '$ifNull' operator, since many fields might show up only in some documents, but I wish there was a simple way to tell if something is or isn't an array.

It turns out there is a "stupid trick" for that.  You can use "$project" stage to output a new field which tells you whether or not another field is an array like this:

This isn't strictly "kosher" but it works in 2.4 and in upcoming 2.6 so I'm going to keep using it - hopefully by the time it stops working, a real operator to project type may be available.

What's the point of doing this, other than just "for fun"?  Well, it turns out there are some operators which only work on arrays and if you try to use them on non-arrays you will get an error.  One of them in "$unwind" stage, another is the new in 2.6 "$size" operator which returns the size of an array field, but gives an error if the field is not an array. 

We took an assortment of documents which had tags field as an array sometimes, as non-array of different types sometimes, and absent some of the times.  The projection we created outputs the size of the field tags in every case, where if the field is null, absent or a non-array we output size 'null' and in all other cases we output the actual size of the array.

Of course the '$size' operator is quite handy in much simpler scenarios where you might be creating an array of unique values during a '$group' stage using'$addToSet' and then you can use '$size' to find out how many unique values ended up being accumulated in that array.

For those looking forward to other new features of aggregation in 2.6 I figured I'd show an alternate way to do this with only a single "$project" stage, instead of three in sequence above:

Here, I'm using the new in 2.6 "$let" expression, which allows me to create a named variable "isArray" which I later reference using "$$isArray" syntax.  Note that I'm also using the new in 2.6 "$cond" syntax which lets me use a document with "if", "then", and "else" field names, hopefully making it more readable than just using an array of three expressions (which is still supported so that you don't have to rewrite your existing aggregations).

I hope you enjoy some of the new 2.6 aggregation enhancements - I will probably continue writing about them, as they make some of our old tricks unnecessary, but open the door to a whole new set of tricks to come!