Aggregation

MongoDB has a fun feature called aggregation. There's two ways of doing it, aggregation pipelines and map-reduce. Map-reduce is exactly what you'd expect if you're from a functional programming background: you provide MongoDB a map function that it will run every item in the array and then a reduce function to aggregate your collection into a smaller set of data.

MongoDB also released a newer feature of aggregation pipelines that tend to perform better and can also be easier to maintain. With these you provide a configuration object to the aggregation pipeline

What if we wanted to know how many puppies, adult, and senior dogs we have in our pets collection? Let's try just that

db.pets.aggregate([
  {
    $bucket: {
      groupBy: "$age",
      boundaries: [0, 3, 9, 15],
      default: "very senior",
      output: {
        count: { $sum: 1 },
      },
    },
  },
]);

• With the aggregation pipelines, you provide a step of things to do. In this case we only have one step, bucket pets into 0-2 years old, 3-8 years old, 9-15 years, and "very senior" (which is the default bucket.)
• With the output you're defining what you want to pass to the next step. In this case we just want to sum them up by adding 1 to the count each time we see a pet that matches a bucket.

This is all pets. We want just dogs. Let's add another stage.

db.pets.aggregate([
  {
    $match: {
      type: "dog",
    },
  },
  {
    $bucket: {
      groupBy: "$age",
      boundaries: [0, 3, 9, 15],
      default: "very senior",
      output: {
        count: { $sum: 1 },
      },
    },
  },
]);

Using the $match stage of the aggregation, we can exclude every pet that isn't a dog.

Last one, what if we wanted to sort the results by which group had the most pets?

db.pets.aggregate([
  {
    $match: {
      type: "dog",
    },
  },
  {
    $bucket: {
      groupBy: "$age",
      boundaries: [0, 3, 9, 15],
      default: "very senior",
      output: {
        count: { $sum: 1 },
      },
    },
  },
  {
    $sort: {
      count: -1,
    },
  },
]);

As you can see, you just add more stages to the aggregation until you gather the insights you're looking for. There are many more things you can do, so here's a link to all the existing aggregation stages.

This is definitely one of the most fun parts about MongoDB. We used to use MongoDB's aggregation features to catch fraudsters in our classifieds app!