Mongo median operation in Golang

Mongo median operation in Golang is an important technology that is of great significance to developers. It allows us to be more flexible and efficient when dealing with Mongo databases. In Golang, we can use Mongo median operations to implement various data operations, such as insertion, update, and deletion. This computing method allows us to better utilize the powerful functions of Mongo and improve our development efficiency. This article will introduce the usage and precautions of Mongo median operation in Golang through practical cases to help developers better master this technology.

Question content

I have a collection in mongo (go) whose type is:

type CreateFeedbackRequest struct {
    UserID     string    `json:"user_id" validate:"required"`
    WaybillID  uint64    `json:"waybill_id" validate:"required"`
    Rating     int       `json:"rating" validate:"required"`
    Comment    string    `json:"comment"`
    ReceivedAt time.Time `json:"received_at" validate:"required"`
}

I need to evaluate the median rating of the last 5 records (via the receivedAt time field) of a certain user (via his user_id). I've got this:

matchStage := bson.D{{"$match", bson.D{{"_id", userID}}}}
sortStage := bson.D{{"$sort", bson.D{{"created_at", 1}}}}
limitStage := bson.D{{"$limit", tripsCount}}

cursor, err := r.c.Aggregate(ctx, mongo.Pipeline{matchStage, sortStage, limitStage})

But I don't know how to get the median rating of these 5 rows. I'm not sure the correct way for me to do this. Help, thanks

Solution

After the $limit stage, an option since mongodb version 7.0 is $group with $median Accumulator

groupgStage := bson.D{{"$group", bson.D{
  {"_id", 0}, 
  {"median", bson.D{{"$median", 
    bson.D{{"$input", "$rating"}, {"method", "approximate"}}
  }}}
}}}

For older versions, you can

1. $sort by rating
2. $group and $push all ratings into an array (all 5 after limiting)
3. $project The project in the middle of the array

It looks like this:

sortRatingStage := bson.D{{"$sort", bson.D{{"rating", 1}}}}
groupStage := bson.D{{"$group", bson.D{{"_id", 0}, {"ratings", bson.D{{"$push", "ratings"}}}}}}
projectStage := bson.D{{"$project", bson.D{
  {"_id", 0}, 
  {median, bson.D{{"$arrayElemAt", bson.D{
    {"$ratings", bson.D{{"$floor", bson.D{
      {"$divide", bson.A{{bson.D{{"$size", "$ratings"}}, 2}}}
    }}}}
  }}}}
}}}}

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