Aggregation Pipeline Stage - $bucket

Build the $bucket stage through the aggregation pipeline stage builder aggregation.NewStageBuilder using the method Bucket.

Suppose we want to group users into different age groups:

• 18 and under
• 19 to 30
• 31 to 45
• 46 and over

type User struct {
	mongox.Model `bson:"inline"`
	Name         string `bson:"name"`
	Age          int    `bson:"age"`
}

// mongo.Pipeline{bson.D{bson.E{Key:"$bucket", Value:bson.D{bson.E{Key:"groupBy", Value:"$age"}, bson.E{Key:"boundaries", Value:[]interface {}{0, 19, 31, 46, +Inf}}, bson.E{Key:"default", Value:"Other"}, bson.E{Key:"output", Value:bson.D{bson.E{Key:"count", Value:bson.D{bson.E{Key:"$sum", Value:1}}}, bson.E{Key:"names", Value:bson.D{bson.E{Key:"$push", Value:"$name"}}}}}}}}}
// [
//  {
//    $bucket: {
//      groupBy: "$age",  // Specifies the field to group by
//      boundaries: [0, 19, 31, 46, Infinity],  // Defines the boundaries for age groups
//      default: "Other",  // Documents that do not match any boundary condition are assigned to a default bucket
//      output: {
//        "count": { $sum: 1 },  // Calculates the number of documents in each bucket
//        "names": { $push: "$name" }  // Collects the names of all users in each bucket
//      }
//    }
//  }
//]
aggregation.NewStageBuilder().Bucket(
    "$age",
    []any{0, 19, 31, 46, math.Inf(1)},
    &types.BucketOptions{
        DefaultKey: "Other",
        Output:     aggregation.NewBuilder().Sum("count", 1).Push("names", "$name").Build(),
    },
).Build()

The Bucket method takes three parameters: groupBy any, boundaries []any, and opt *types.BucketOptions. To retain documents that do not meet any boundary condition, you can pass the opt parameter, setting the default and output parameters to include these documents in the specified default bucket. The output parameter defines the format of the output documents.