MapReduce Functions

Working with Data Connectors & Integrations
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1

Hello! I am very new to MongoDB.
I am trying to write a map reduce function (not an aggregation):

So far I have:
var mapHorse1 = function(){
var key = this.shows; if(this.shows <50) emit(key,1);};

I need to show the number of horses who have gender = female, and less than 50 shows - but I don’t know how to add in the gender filter to the above function. Can someone please assist?
Many thanks

2

Hey Kristen, welcome to the MongoDB community

Try this:

var mapHorse1 = function() {
    var key = this.shows;
    if (this.shows < 50 && this.gender === 'female') {
        emit(key, 1);
    }
};

var reduceFunction1 = function(key, values) {
    return Array.sum(values);
};

db.coll.mapReduce(
    mapHorse1,
    reduceFunction1,
    {
        out: "map_reduce_result"  // Name collection out
    }
)
3

Thank you so much! I’ll give that a go!

4

You’re welcome, I’m at your disposal and if your problem has been resolved, leave this topic as resolved so that other people can benefit from it!

5

Hi Samuel - sorry that did not work. Whilst there were no errors - when I called the functions, no results / data displayed.

6

Hi Samuel - I have worked it out - I just had to make one very small change to the map function! Thank you so much for your help!

7

It would be interesting and fair to all to see the final working map function.

8

Hiya! Apologies - yes of course!
I just had to change the var key assignment to “this.name”, in the map function.
Thank you so much for your help!

var mapHorse1 = function() {
var key = this.name;
if (this.shows < 50 && this.gender === ‘female’) {
emit(key, 1);
}
};

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