Only return the looked up document with Mongo and Golang

Question

I have these two models :

// EventBoost describes the model of a EventBoost
type EventBoost struct {
    ID          string    `bson:"_id" json:"_id" valid:"alphanum,printableascii"`
    Name        string    `bson:"name" json:"name"`
    Description string    `bson:"description" json:"description"`
    Level       string    `bson:"level" json:"level"`
    EventID     string    `bson:"_event_id" json:"_event_id" valid:"alphanum,printableascii"`
    StartDate   time.Time `bson:"start_date" json:"start_date"`
    EndDate     time.Time `bson:"end_date" json:"end_date"`
    IsPublished bool      `bson:"is_published" json:"is_published"`
    CreatedBy   string    `bson:"created_by" json:"created_by"`
    CreatedAt   time.Time `bson:"created_at" json:"created_at"`
    ModifiedAt  time.Time `bson:"modified_at" json:"modified_at"`
}

// Event describes the model of an Event
type Event struct {
    ID            string      `bson:"_id" json:"_id" valid:"alphanum,printableascii"`
    OldID         string      `bson:"old_id" json:"old_id" valid:"alphanum,printableascii"`
    ParentID      string      `bson:"_parent_id" json:"_parent_id" valid:"alphanum,printableascii"`
    Name          string      `bson:"name" json:"name"`
    Content       string      `bson:"content" json:"content"`
    Slug          string      `bson:"slug" json:"slug"`
    LocationID    string      `bson:"_location_id" json:"_location_id"`
    Price         string      `bson:"price" json:"price"`
    Categories    []string    `bson:"categories" json:"categories"`
    Tags          []string    `bson:"tags" json:"tags"`
    Organisers    []string    `bson:"organisers" json:"organisers"`
    Artists       []string    `bson:"artists" json:"artists"`
    Image         string      `bson:"image" json:"image"`
    IsPublished   bool        `bson:"is_published" json:"is_published"`
    IsProposed    bool        `bson:"is_proposed" json:"is_proposed"`
    CreatedBy     string      `bson:"created_by" json:"created_by"`
    CreatedAt     time.Time   `bson:"created_at" json:"created_at"`
    ModifiedAt    time.Time   `bson:"modified_at" json:"modified_at"`
}

I want, when I lookup an EventBoost, to return only an Event without having to perform the cleaning with Golang logic. Because actually, the documents that are return have a property named event. I directly want have an Event document.

Here is my method that needs to return []*models.Event :

// Boosted returns the boosted events
func (dao *eventBoostDAO) Boosted() ([]*models.Event, error) {
    // Clone the session
    session := dao.session.Clone()
    defer session.Close()

    // Get the time
    now := time.Now()

    // Create the pipe
    pipe := session.DB(shared.DatabaseNamespace).C(dao.collection).Pipe([]bson.M{
        {
            "$match": bson.M{
                "is_published": true,               // Boost is active
                "start_date":   bson.M{"$lt": now}, // now is between start and end
                "end_date":     bson.M{"$gt": now}, // now is between start and end
            },
        },
        {
            "$lookup": bson.M{
                "from":         "events",
                "localField":   "_event_id",
                "foreignField": "_id",
                "as":           "event",
            },
        },
    })

    var result []*models.Event
    err := pipe.All(&result)
    if err != nil {
        return nil, err
    }

    return result, nil
}

Looking at the Mongo documentation, I found that $project should help me doing what I want, but I did not find how to transform a nested document to the final document.

Answer 1

You may use $unwind to "transform" the event array field to a single embedded document, then $replaceRoot to "promote" this event field to be the new "root":

pipe := session.DB(shared.DatabaseNamespace).C(dao.collection).Pipe([]bson.M{
    {
        "$match": bson.M{
            "is_published": true,               // Boost is active
            "start_date":   bson.M{"$lt": now}, // now is between start and end
            "end_date":     bson.M{"$gt": now}, // now is between start and end
        },
    },
    {
        "$lookup": bson.M{
            "from":         "events",
            "localField":   "_event_id",
            "foreignField": "_id",
            "as":           "event",
        },
    },
    {"$unwind": "$event"},
    {"$replaceRoot": bson.M{ "newRoot": "$event" }},
})

This solution handles properly if there are multiple events for a given EventBoost.