How to group by uniform intervals of data between a maximum and minimum using the MongoDB aggregator? - mongodb

Question

Let's say I have a whole mess of data that yields a range of integer values for a particular field... I'd like to see those ranked by a grouping of intervals of occurrence, perhaps because I am clustering...like so:
[{
_id: {
response_time: "3-4"
},
count: 234,
countries: ['US', 'Canada', 'UK']
}, {
_id: {
response_time: "4-5"
},
count: 452,
countries: ['US', 'Canada', 'UK', 'Poland']
}, ...
}]
How can I write a quick and dirty way to A) group the collection data by equally spaced intervals over B) a minimum and maximum range using a MongoDB aggregator?

Answer 1

Well, in order to quickly formulate a conditional grouping syntax for MongoDB aggregators, we first adopt the pattern, per MongoDB syntax:
$cond: [
{ <conditional> }, // test the conditional
<truthy_value>, // assign if true
$cond: [ // evaluate if false
{ <conditional> },
<truthy_value>,
... // and so forth
]
]
In order to do that muy rapidamente, without having to write every last interval out in a deeply nested conditional, we can use this handy recursive algorithm (that you import in your shell script or node.js script of course):
$condIntervalBuilder = function (field, interval, min, max) {
if (min < max - 1) {
var cond = [
{ '$and': [{ $gt:[field, min] }, { $lte: [field, min + interval] }] },
[min, '-', (min + interval)].join('')
];
if ((min + interval) > max) {
cond.push(ag.$condIntervalBuilder(field, (max - min), min, max));
} else {
min += interval;
cond.push(ag.$condIntervalBuilder(field, interval, min, max));
}
} else if (min >= max - 1 ) {
var cond = [
{ $gt: [field, max] },
[ max, '<' ].join(''), // Accounts for all outside the range
[ min, '<' ].join('') // Lesser upper bound
];
}
return { $cond: cond };
};
Then, we can invoke it in-line or assign it to a variable that we use elsewhere in our analysis.