MongoDB字符串切片存储问题：无法向likes数组添加userID

问题详情 #

尝试向MongoDB的likes字段添加userID，该字段预期为字符串数组[]string，但当前值为null。先后使用$set操作、手动将字段设为数组后改用$setOnInsert、$addToSet，均未成功触发写入。

创建文档时使用的结构体：

comment := entity.Comment{
    PostID:    postID,
    Content:   content,
    AuthorID:  authorID,
    Likes:     []string{},
    Dislikes:  []string{},
    Votes:     []string{},
    CreatedAt: time.Now(),
}

数据库中目标文档结构：

{"_id":{"$oid":"67de0487653132119ba14548"},"postid":"67de00a0eea2fe8f5a55e1da","content":"awesome Pizza day","authorid":"3204aa18-2006-4aa1-ad88-1b71256d8a36","createdat":{"$date":{"$numberLong":"1742603399198"}},"updatedat":{"$date":{"$numberLong":"1742603992797"}},"likes":null,"dislikes":null,"votes":null}

向"likes":null字段添加userID时，在UpdateOne步骤触发写入异常。已确认userID和comment.ObjectId均不为空，且comment.ObjectId对应的文档在数据库中存在。

原尝试代码 #

func (r *CommentsRepository) ToggleLikeComment(ctx context.Context, comment *entity.Comment, userID string) error {
    filter := bson.D{{Key: "_id", Value: comment.ObjectId}}
    update := bson.D{}

    // 检查用户是否已点踩
    dislikeFilter := bson.D{{Key: "_id", Value: comment.ObjectId}, {Key: "dislikes", Value: userID}}
    dislikeCount, err := r.collection.CountDocuments(ctx, dislikeFilter)
    if err != nil {
        return err
    }
    if dislikeCount > 0 {
        // 移除点踩
        update = append(update, bson.E{Key: "$pull", Value: bson.D{{Key: "dislikes", Value: userID}}})
    }

    // 检查用户是否已点赞
    likeFilter := bson.D{{Key: "_id", Value: comment.ObjectId}, {Key: "likes", Value: userID}}
    likeCount, err := r.collection.CountDocuments(ctx, likeFilter)
    if err != nil {
        return err
    }
    if likeCount > 0 {
        // 移除点赞
        update = append(update, bson.E{Key: "$pull", Value: bson.D{{Key: "likes", Value: userID}}})
    } else {
        // 添加点赞
        update = append(update, bson.E{Key: "$set", Value: bson.D{{Key: "likes", Value: userID}}})
    }
    update = append(update, bson.E{Key: "$set", Value: bson.D{{Key: "updatedat", Value: time.Now()}}})

    // 写入失败位置
    _, err = r.collection.UpdateOne(ctx, filter, bson.D{{Key: "$set", Value: update}})
    return err
}

修正后的可行代码 #

在@aneroid的帮助下，修改后的查询可正常完成写入：

likeFilter := bson.D{
        {Key: "_id", Value: objectID},
        {Key: "likes", Value: bson.D{{Key: "$ne", Value: userID}}},
    }
    likeUpdate := bson.A{
        bson.D{{Key: "$set", Value: bson.D{
            {Key: "likes", Value: bson.D{
                {Key: "$cond", Value: bson.D{
                    {Key: "if", Value: bson.D{{Key: "$eq", Value: bson.A{"$likes", nil}}}},
                    {Key: "then", Value: bson.A{userID}},
                    {Key: "else", Value: bson.D{{Key: "$concatArrays", Value: bson.A{"$likes", bson.A{userID}}}},
                }},
            }},
            {Key: "updatedat", Value: time.Now()},
        }}},
    }
    result, err := r.collection.UpdateOne(ctx, likeFilter, likeUpdate)

内容的提问来源于stack exchange，提问作者Mike3355