I recently had a requirement for conditional emission from a map function. Essentially I only wanted to emit where the date was within a given range.
In SQL, grouping a count for a given time granularity within a date range would look something like this:
I wasn't 100% sure about the 'right way' to achieve this in NOSql/MongoDB. So this is a solution.
It requires you to know about scope variables. Problem is, I found that scope variables are not very well documented. You can find more about scope variables in the MongoDB documentation MapReduce-Overview. The relevant parts are:
scope - can pass in variables that can be access from map/reduce/finalize.
Back to this example. First, let's define some data:
Before we implement this, lets get it working at the command-line, in our mongo shell:
What is happening here?
Well, we're selecting the sub-document of this collection where the value of "meh" is "meh". Then we've defined two dates; from and to to represent the boundaries of the date range, we're including these within the MapReduce function call. Basically what this means is that we can use what ever is defined here in the Map function (btw, we can also use them in the Reduce and Finalize functions).
Once we have this working from the shell, it is straight forward to implement it. This is the very same implemented in Java.
Caveat: This is an example of how to use scope variables, I'm sure if you go to any of the Events or gmane.comp.db.mongodb.user group you'll get some advice straight from the 10Gen guys.