带有$ lookup查询的MGO Pipe将不会附加“已加入”集合中的匹配文档

My Models

type (
    //Category implements item category in database
    Category struct {
        ID          bson.ObjectId `bson:"_id,omitempty" json:"id"`
        Name        string        `bson:"name" json:"name" form:"name" valid:"Required"`
        IsActive    bool          `bson:"is_active" json:"is_active" form:"is_active" valid:"Required"`
        Slug        string        `bson:"slug" json:"slug"`
        Icon        string        `bson:"icon" json:"icon" form:"icon"`
        SidebarIcon string        `bson:"sidebar_icon" json:"sidebar_icon" form:"sidebar_icon"`
        Parent      bson.ObjectId `bson:"parent,omitempty" json:"parent,omitempty" form:"parent"`
        CreatedAt   time.Time     `bson:"created_at" json:"-"`
        UpdatedAt   time.Time     `bson:"updated_at" json:"-"`
        IsDeleted   bool          `bson:"is_deleted" json:"-"`
    }
)

My get collection query:

categories := []models.Category{}

f := func(collection *mgo.Collection) error {
        query := []bson.M{
            {
                "$match": bson.M{
                    "is_deleted": bson.M{
                        "$ne": true,
                    },
                },
            },
            {
                "$sort": bson.M{
                    orderBy: pipeOrder,
                },
            },
            {
                "$limit": limit,
            },
            {
                "$skip": skip,
            },
            {
                "$lookup": bson.M{
                    "from":         "categories",
                    "localField":   "_id",
                    "foreignField": "parent",
                    "as":           "parentlist",
                },
            },
        }

        return collection.Pipe(query).All(&categories)

Goal : retrieve all the category along with their parent if their parent id matches with one of the document inside the collection.

Issue : retrieved all the category but missing 'parentlist' joined attribute

Stack : mgo to interact with DB and golang version 1.8

In your aggregation you lookup the parents and they will be stored in a field called parentlist. And you try to unmarshal the results into a slice of Category, but the type Category has no field that would match parentlist. So that field will be "lost" during the unmarshaling process.

There are many ways to get the additional parentlist, some of the possibilities are detailed in this answer: Mgo aggregation: how to reuse model types to query and unmarshal "mixed" results?

One option is to use a wrapper struct like this:

type CategoryWithParents struct {
    Category models.Category    `bson:",inline"`
    Parents  []*models.Category `bson:"parentlist"`
}

And unmarshal into a slice of this:

var results []CategoryWithParents

err := collection.Pipe(query).All(&results)

This will get all the parents.

If there may be one parent at most for all categories, you can modify the aggregation to $unwind the parentlist, and Parents may be a single *model.Category instead of a slice:

type CategoryWithParents struct {
    Category       models.Category  `bson:",inline"`
    OptionalParent *models.Category `bson:"parentlist"`
}

var results []CategoryWithParents

f := func(collection *mgo.Collection) error {
    query := []bson.M{
        {
            "$match": bson.M{
                "is_deleted": bson.M{
                    "$ne": true,
                },
            },
        },
        {
            "$sort": bson.M{
                orderBy: pipeOrder,
            },
        },
        {"$limit": limit},
        {"$skip": skip},
        {
            "$lookup": bson.M{
                "from":         "categories",
                "localField":   "_id",
                "foreignField": "parent",
                "as":           "parentlist",
            },
        },
        {
            "$unwind": bson.M{
                "path":                       "parentlist",
                "preserveNullAndEmptyArrays": true,
            },
        },
    }

    return collection.Pipe(query).All(&results)
}