Any way to see map results in a mapreduce function? - ruby-on-rails

Question

For debugging purposes it would be great to be able to see just what you are getting back from the map method. Is this possible in ruby?

Answer 1

To do this in the Mongo shell, you can define you own debug version of the emit() function to print trace information.
function emit(k, v) {
print("emit");
print(" k:" + k + " v:" + tojson(v));
}
Check out Troubleshooting MapReduce in the MongoDB docs for more info.

Answer 2

I know the Mongo documentation recommends defining your own emit function, but I find it easier to instead use print() directly in my map- and reduce-functions while I watch Mongo's log.
Just put any print()'s in your code, run tail -f /var/log/mongodb/mongodb.log, then run your code. You should see the print()'s output to the console.
Here's a few of the benefits:
Ability to debug the reduce() function – defining your own emit() doesn't help here
No need to define the emit() function every time you fire up the mongo console
Write your code in your editor instead of going back and forth between console and IDE
Ability to do code generation and variable interpolation in your native language