MongoDB: The Complete Guide/

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Aggregation Pipeline: Part 2

Aggregation Pipeline: Part 2

Learn and practice the $project, $limit, and $sort aggregation pipelines stages.

We'll cover the following...

$lookup stage

We use the $lookup stage to perform the join operations with other collections. This stage performs a left outer join for each input document, and adds a new array field where the results for the join are added. Next, the transformed documents are sent to the next stage.

We use the below syntax to define the $lookup stage.

{
    $lookup: {
        from: <target collection to join to>,
        localField: <field of the source document>,
				foreignField: <field of the target collection>,
        let: <variables to use in pipeline>,
        pipeline: <pipeline result is returned on out field>,
        as: <output field name>
    }
 }

To perform the $lookup stage, we need another collection that will have some referenced data in it. We insert some data into the users collection.

Markdown
db.users.insertMany([{
  username: "onkar",
  name: 'Onkar',
  age: 25,
}, 
{
  username: "shankar",
  name: 'Shankar',
  age: 20,
}]);

Next, we reference the tasks.user field to the users.username field.

Let’s perform a join operation and return the user profile with tasks.

Markdown
db.tasks.aggregate([
{
  $match: {
      status: "completed",
  }
},
{
  $lookup: {
      from: "users",
      localField: "user",
      foreignField: "username",
      as: "user_profile",
  }
},
]);

This returns the below output.

[
  {
    _id: ObjectId("6136f1dae5020a122bb6b17c"),
    name: 'Learn MongoDB Topic 2',
    date: ISODate("2021-09-07T05:00:10.680Z"),
...