📘 Structure of an Aggregation Pipeline

To interact with a relational database, we typically use SQL, a 4th-generation language, to access our data. With MongoDB, we instead get data and transform it in incremental steps.

An aggregation pipeline is composed of stages.

We pass an array of stages to the aggregate method like so:

db.mycollection.aggregate([
    stage1,
    stage2,
    stage3
])

Example

// Inside the current database, in the collection named reviews
db.reviews.aggregate([
  { 
    // group all reviews for the same book
    $group: {
        _id: "$bookId",
        averageRating: {
          $avg: "$rating",
        },
      },
  },
  // filter out all reviews that have an average other than 5
  { $match: { averageRating: 5 } },
  // JOIN with author collection to get all the author info
  { $lookup: {
        from: "authors",
        localField: "_id",
        foreignField: "books",
        as: "author",
      },
  },
  // add a field called bio
  {$addFields: {
    bio: "$author.bio"
  }},
])

Example​

Example