Question

我有一个包含集合的数据库，这些集合包含表单上的文档：

{ "_id" : 4, "value" : 2 }

我希望以有效的方式在所有集合中找到最大"_id"。目前，我有一个工作方法：

public long getLastTimestamp()
{
    var tempList = getChannelNames();
    var channelList = new List<IMongoCollection<BsonDocument>>();
    var docuList = new List<BsonDocument>();

    foreach (var channel in tempList)
    {
        channelList.Add(_database.GetCollection<BsonDocument>(channel.name));
    }

    foreach (var channel in channelList)
    {
        var filter = Builders<BsonDocument>.Filter.Exists("_id");
        var result = channel.FindAsync(filter).Result.ToList();

        foreach (var docu in result)
        {
            docuList.Add(docu);
        }
    }

    var timeList = new List<long>();

    foreach (var docu in docuList)
    {
        Console.WriteLine(docu);
        if (!docu["_id"].IsObjectId)
            timeList.Add(docu["_id"].ToInt64());
    }

    return timeList.Max();
}

它有效，但我不认为它非常有效。有没有人有一些意见或建议？

编辑：我最终这样做了：

    public long getLastTimestamp()
    {
        var filter = Builders<BsonDocument>.Filter.Exists("value");
        return getChannelNames()
            .Select(channel => _database.GetCollection<BsonDocument>(channel.name))
            .SelectMany(channel => channel.FindAsync(filter).Result.ToList())
            .Max(docu => docu["_id"].ToInt64());
    }

Answer 1

检查出来：

public long getLastTimestamp()
{
    //var tempList = getChannelNames();
    var channelList = new List<IMongoCollection<BsonDocument>>();
    var docuList = new List<BsonDocument>();

    foreach (var channel in getChannelNames())
    {
        var filter = Builders<BsonDocument>.Filter.Exists("_id");
        var result = _database.GetCollection<BsonDocument>(channel.name)
                     .FindAsync(filter).Result.ToList();

        return result.Where(x => !x["_id"].IsObjectId)
               .Max(entry => entry["_id"].ToInt64);
    }

    return 0;
}

无法测试它，因为我可以使用这些对象。在linq-part上你可能需要转换为list或array才能获得Where()和Max()。

再次存储和迭代总是比较慢。

public long getLastTimestamp()
{
    var filter = Builders<BsonDocument>.Filter.Exists("_id");

    return getChannelNames()
    .Select(channel => _database.GetCollection<BsonDocument>(channel.name).FindAsync(filter).Result.ToList())
    .Where(doc => !doc["_id"].IsObjectId)
    .Max(doc => doc["_id"].ToInt64);
}

Answer 2

这样的东西？

var filter = Builders<BsonDocument>.Filter.Exists("_id");

getChannelNames()
    .SelectMany(channel => _database.GetCollection<BsonDocument>(channel.name))
    .SelectMany(channel => channel.FindAsync(filter).Result.ToList()) // Even better w/o ToList
    .Where(docu => !docu["_id"].IsObjectId)
    .Max(docu => docu["_id"].ToInt64());

在所有集合中查找字段的最大值

2 个答案: