TIL: MongoDB array update with $push and $pull

In mdbssg my small static site generator project I started to continue sharpening Go skills and begin learning MongoDB basics for my current job, I keep user sessions in an array within the user document, like so:

{
	"_id": ObjectID("..."),
	"username"": "...",
	"sessions": [
		{"token": "...", "expires_at": "..."}, ...
	],
}

Without looking into the db, I was writing update logic in Go like this snippet, to add a new session:

// error handling elided for space
sessions, err := u.getSessionsByUsername(ctx, username)

session := Session{
	Token:     uuid.NewString(),
	ExpiresAt: time.Now().Add(5 * time.Minute),
}
sessions = append(sessions, session)

users := u.client.Database(u.dbName).Collection("users")
_, err = users.UpdateOne(ctx, bson.M{"username": username}, bson.M{"$set": bson.M{"sessions": sessions}})

I would build a new slice in Go and $set the sessions array for the given user document in Mongo.

I learned later that I could instead rewrite similar logic with $push to add new array entries and $pull to delete entries, like this logic to remove all expired sessions for a given user:

// given a username, remove expired sessions with an updateMany(...{ '$pull': ... })
func (u *UserModel) removeExpiredSessions(ctx context.Context, username string) (int, error) {
	sessions, err := u.getSessionsByUsername(ctx, username)
	if err != nil {
		return 0, err
	}

	sessTokens := make([]string, 0)
	for i := range sessions {
		if sessions[i].ExpiresAt.Before(time.Now()) {
			sessTokens = append(sessTokens, sessions[i].Token)
		}
	}

	users := u.client.Database(u.dbName).Collection("users")

	// note: db.collection.updateMany() is not atomic. I think this could cause issues if a user is trying
	// to sign in at one location (so this method is called) while that account is in use at another location
	mr, err := users.UpdateMany(ctx, bson.M{},
		bson.M{"$pull": bson.M{"sessions": bson.M{"token": bson.M{"$in": sessTokens}}}})
	return int(mr.ModifiedCount), err
}

which produces a query like:

db.users.updateMany(
	{
		username: "..."
	},
	{
		$pull: { sessions: { token: { $in: [...] } } }
	}
)

I’m not sure which is more efficient to use–but I think $pull and $push produce clearer, more idiomatic Mongo queries.

Tags: til mongo golang