In last week's article, you learned about bitwise query operators in MongoDB and how storing bitmaps in MongoDB can help you avoid using JOINs. In this article, you'll learn about 2 new aggregation framework features in MongoDB 3.2: the $lookup and $sample operators. You've probably heard a great deal about the $lookup operator, but, as you'll see in this article, $sample is also an important addition.

$lookup

The $lookup operator is a way for your MongoDB aggregation pipeline to pull in documents from another collection. Note that the $lookup operator is only in the aggregation framework, you can't use $lookup with CRUD operations like .find(), .findOne(), etc.

Let's take a look at how the $lookup operator works in the MongoDB shell. Suppose you have a collection of users like you see below.

> db.users.find().pretty()
{
    "_id" : ObjectId("56743c5af418925185babf08"),
    "name" : "Val",
    "likes" : [
        "bacon"
    ]
}

Suppose you also have a collection of related events. Note that each event has a user field, which contains the ObjectId of the associated user.

> db.events.find().pretty()
{
    "_id" : ObjectId("56743c87f418925185babf09"),
    "user" : ObjectId("56743c5af418925185babf08"),
    "action" : "registered",
    "time" : ISODate("2015-12-18T17:04:07.487Z")
}
{
    "_id" : ObjectId("56743c8ef418925185babf0a"),
    "user" : ObjectId("56743c5af418925185babf08"),
    "action" : "logged in",
    "time" : ISODate("2015-12-18T17:04:14.799Z")
}

You can use the $lookup operator to get the user associated with each event. The $lookup operator takes an object with 4 properties:

  • from: the collection to get documents from
  • localField: the field in the local document
  • foreignField: find documents in the from collection where this field equals the value of localField in the local document.
  • as: the field in the local document to store the results in

For instance, suppose you wanted to run an aggregation on the 'events' collection that found documents in the 'users' collection whose _id field was equal to the event's user field. Here's how that works in the shell.

> db.events.aggregate([{ $lookup: { from: 'users', localField: 'user', foreignField: '_id', as: 'user' } }]).pretty();
{
    "_id" : ObjectId("56743c87f418925185babf09"),
    "user" : [
        {
            "_id" : ObjectId("56743c5af418925185babf08"),
            "name" : "Val",
            "likes" : [
                "bacon"
            ]
        }
    ],
    "action" : "registered",
    "time" : ISODate("2015-12-18T17:04:07.487Z")
}
{
    "_id" : ObjectId("56743c8ef418925185babf0a"),
    "user" : [
        {
            "_id" : ObjectId("56743c5af418925185babf08"),
            "name" : "Val",
            "likes" : [
                "bacon"
            ]
        }
    ],
    "action" : "logged in",
    "time" : ISODate("2015-12-18T17:04:14.799Z")
}

One case where $lookup is particularly useful is for data migrations. Suppose you realized that only storing the user's id is not terribly helpful. You can use the $out operator to create a new collection where each event embeds the user document. To do this, you need a 3-stage aggregation pipeline:

  • $lookup the user documents
  • $unwind so the user field is not an array
  • $out to the new collection.

Below is the corresponding aggregation pipeline in the shell.

db.events.aggregate([
  {
    $lookup: {
      from: 'users',
      localField: 'user',
      foreignField: '_id',
      as: 'user'
    }
  },
  { $unwind: '$user' },
  { $out: 'new_events' }
]);

After you run this aggregation pipeline, the 'new_events' collection will contain all the events with embedded user data.

> db.new_events.find().pretty()
{
    "_id" : ObjectId("56743c87f418925185babf09"),
    "user" : {
        "_id" : ObjectId("56743c5af418925185babf08"),
        "name" : "Val",
        "likes" : [
            "bacon"
        ]
    },
    "action" : "registered",
    "time" : ISODate("2015-12-18T17:04:07.487Z")
}
{
    "_id" : ObjectId("56743c8ef418925185babf0a"),
    "user" : {
        "_id" : ObjectId("56743c5af418925185babf08"),
        "name" : "Val",
        "likes" : [
            "bacon"
        ]
    },
    "action" : "logged in",
    "time" : ISODate("2015-12-18T17:04:14.799Z")
}

Using the $lookup operator in Node.js is easy. If you're going to use the $lookup operator, you should use version >= 2.1.0 of the MongoDB driver or version >= 4.3.0 of mongoose. Mongoose 4.3.x also has a handy .lookup() helper as part of its chainable aggregation pipeline builder. In mongoose, you could run the 'new_events' migration aggregation as shown below.

Events.aggregate().
  lookup({
    from: 'users',
    localField: 'user',
    foreignField: '_id',
    as: 'user'
  }).
  unwind('user').
  out('new_events').
  exec();

$sample

While $lookup is extremely popular, I'm more excited about the $sample operator. The $sample operator is the answer to the age-old question of how to get a random document or documents from a MongoDB collection. There are numerous mechanisms for getting a single random document from MongoDB:

  • Setting a random skip value: MyModel.find().skip(Math.random() * count);
  • Associating a random (x, y) coordinate pair with each document and execute a query with a 2d index to find the document closest to a random point.

The first approach doesn't have great performance: you need to execute a count query followed by a find with a skip. The second approach is faster, but has poor randomness and uses a legacy feature. Neither approach works well with getting multiple random documents.

The $sample operator makes it easy to get a random document or documents without any additional overhead. To use $sample, you pass in an object with a single property, size, that defines the number of documents you want. For instance, here's how you would get a random event from the 'new_events' collection using the $sample operator in the shell.

> db.new_events.aggregate([{ $sample: { size: 1 } }]).pretty();
{
    "_id" : ObjectId("56743c8ef418925185babf0a"),
    "user" : {
        "_id" : ObjectId("56743c5af418925185babf08"),
        "name" : "Val",
        "likes" : [
            "bacon"
        ]
    },
    "action" : "logged in",
    "time" : ISODate("2015-12-18T17:04:14.799Z")
}
> db.new_events.aggregate([{ $sample: { size: 1 } }]).pretty();
{
    "_id" : ObjectId("56743c87f418925185babf09"),
    "user" : {
        "_id" : ObjectId("56743c5af418925185babf08"),
        "name" : "Val",
        "likes" : [
            "bacon"
        ]
    },
    "action" : "registered",
    "time" : ISODate("2015-12-18T17:04:07.487Z")
}

If the size you specified is larger than the number of documents in the collection, you get all the documents back.

Like $lookup, you should use version >= 2.1.0 of the MongoDB driver or version >= 4.3.0 of mongoose for $sample. Mongoose >= 4.3.1 has a .sample() aggregation helper. Below is how you'd run the 'new_events' sample aggregation pipeline in Mongoose.

NewEvents.aggregate().
  sample(1).
  exec();

Moving On

The $lookup and $sample aggregation operators are two of MongoDB 3.2's killer features. They're easy to use in Node.js, so go ahead and try them out!

Found a typo or error? Open up a pull request! This post is available as markdown on Github
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