Mongodb查询，用于从集合中拾取随机文档，直到达到数量

我收集了如下文件：

歌曲

{
  "title": "Highway To Hell",
  "duration": 4
  "tags": [{"label":"rock"},{"label":"heavy"}]
}
{
  "title": "Never Be The Same",
  "duration": 3
  "tags": [{"label":"pop"}]
}
{
  "title": "Wake Up",
  "duration": 4
  "tags": [{"label":"metal"},{"label":"heavy"}]
}
{
  "title": "Catharsis",
  "duration": 3
  "tags": [{"label":"metal"},{"label":"heavy"}]
}

我想查询最长持续时间为8分钟、标签为“重”的随机歌曲

查询结果1

{
  "title": "Highway To Hell",
  "duration": 4
  "tags": [{"label":"rock"},{"label":"heavy"}]
}
{
  "title": "Wake Up",
  "duration": 4
  "tags": [{"label":"metal"},{"label":"heavy"}]
}

查询结果2

{
  "title": "Highway To Hell",
  "duration": 4
  "tags": [{"label":"rock"},{"label":"heavy"}]
}
{
  "title": "Catharsis",
  "duration": 3
  "tags": [{"label":"metal"},{"label":"heavy"}]
}

---

我已经检查了$sample操作符，但您需要随机文档的数量，但在本例中，数量不是固定的。需要一个变量来保存“持续时间”，但我不确定如何实现这一点。

您可以尝试这种聚合吗

$match

-仅匹配标记包含重

$sample

-获取n个样本（未保证的元素顺序）

$group

-按id分组，以便在下一阶段减少

$project

-使用$reduce计算每首歌曲的持续时间和最大持续时间，如果未达到最大持续时间，则将歌曲添加到数组中，否则忽略它

$unwind

-解除过滤后的歌曲

$replaceRoot

-获取原始文档结构中的歌曲

这里假设歌曲的持续时间至少为1分钟，如果不增加样本数

管道

    db.songs.aggregate(
      [
        {$match : {"tags.label" : "heavy"}},
        {$sample : {size : 8}},
        {$group : {_id : null, songs : {$push : "$$ROOT"}}},
        {$project : {
          songs : {
            $reduce : {
                input : "$songs",
                initialValue : { filtered : [], total : 0 },
                in : {              
                  total : {$add : ["$$value.total",{$cond :[{$lte : [{$add : ["$$value.total", "$$this.duration"]}, 8]},"$$this.duration", 0]}]},
                  filtered : {$concatArrays : [ "$$value.filtered", {$cond :[{$lte : [{$add : ["$$value.total", "$$this.duration"]}, 8]},["$$this"], []]}]}
                }
              }
            }
          }
        },
        {$unwind : "$songs.filtered"},
        {$replaceRoot : {newRoot : "$songs.filtered"}}
      ]
    ).pretty()

为问题中的测试数据生成的可能输出

> db.songs.aggregate(   [     {$match : {"tags.label" : "heavy"}},     {$sample : {size : 8}},     {$group : {_id : null, songs : {$push : "$$ROOT"}}},     {$project : {       songs : {         $reduce : {             input : "$songs",             initialValue : { filtered : [], total : 0 },             in : {                             total : {$add : ["$$value.total",{$cond :[{$lte : [{$add : ["$$value.total", "$$this.duration"]}, 8]},"$$this.duration", 0]}]},               filtered : {$concatArrays : [ "$$value.filtered", {$cond :[{$lte : [{$add : ["$$value.total", "$$this.duration"]}, 8]},["$$this"], []]}]}             }           }         }       }     },     {$unwind : "$songs.filtered"},     {$replaceRoot : {newRoot : "$songs.filtered"}}   ] ).pretty()
{
    "_id" : ObjectId("5a7739693eafa689da47ba81"),
    "title" : "Wake Up",
    "duration" : 4,
    "tags" : [
        {
            "label" : "metal"
        },
        {
            "label" : "heavy"
        }
    ]
}
{
    "_id" : ObjectId("5a7739693eafa689da47ba7f"),
    "title" : "Highway To Hell",
    "duration" : 4,
    "tags" : [
        {
            "label" : "rock"
        },
        {
            "label" : "heavy"
        }
    ]
}
> 
> db.songs.aggregate(   [     {$match : {"tags.label" : "heavy"}},     {$sample : {size : 8}},     {$group : {_id : null, songs : {$push : "$$ROOT"}}},     {$project : {       songs : {         $reduce : {             input : "$songs",             initialValue : { filtered : [], total : 0 },             in : {                             total : {$add : ["$$value.total",{$cond :[{$lte : [{$add : ["$$value.total", "$$this.duration"]}, 8]},"$$this.duration", 0]}]},               filtered : {$concatArrays : [ "$$value.filtered", {$cond :[{$lte : [{$add : ["$$value.total", "$$this.duration"]}, 8]},["$$this"], []]}]}             }           }         }       }     },     {$unwind : "$songs.filtered"},     {$replaceRoot : {newRoot : "$songs.filtered"}}   ] ).pretty()
{
    "_id" : ObjectId("5a7739693eafa689da47ba7f"),
    "title" : "Highway To Hell",
    "duration" : 4,
    "tags" : [
        {
            "label" : "rock"
        },
        {
            "label" : "heavy"
        }
    ]
}
{
    "_id" : ObjectId("5a7739693eafa689da47ba82"),
    "title" : "Catharsis",
    "duration" : 3,
    "tags" : [
        {
            "label" : "metal"
        },
        {
            "label" : "heavy"
        }
    ]
}
> db.songs.aggregate(   [     {$match : {"tags.label" : "heavy"}},     {$sample : {size : 8}},     {$group : {_id : null, songs : {$push : "$$ROOT"}}},     {$project : {       songs : {         $reduce : {             input : "$songs",             initialValue : { filtered : [], total : 0 },             in : {                             total : {$add : ["$$value.total",{$cond :[{$lte : [{$add : ["$$value.total", "$$this.duration"]}, 8]},"$$this.duration", 0]}]},               filtered : {$concatArrays : [ "$$value.filtered", {$cond :[{$lte : [{$add : ["$$value.total", "$$this.duration"]}, 8]},["$$this"], []]}]}             }           }         }       }     },     {$unwind : "$songs.filtered"},     {$replaceRoot : {newRoot : "$songs.filtered"}}   ] ).pretty()
{
    "_id" : ObjectId("5a7739693eafa689da47ba82"),
    "title" : "Catharsis",
    "duration" : 3,
    "tags" : [
        {
            "label" : "metal"
        },
        {
            "label" : "heavy"
        }
    ]
}
{
    "_id" : ObjectId("5a7739693eafa689da47ba81"),
    "title" : "Wake Up",
    "duration" : 4,
    "tags" : [
        {
            "label" : "metal"
        },
        {
            "label" : "heavy"
        }
    ]
}
> 

---

你能提出你的问题吗？