这是来自 mongo 文档的 View 。我只想保留文档中键值最长的那些条目。这里的键值是一个字符串，所以应该只保留字符串长度最长的键

{ 
    "_id" : ObjectId("585a431415c7a981b47ac4ee"), 
    "key" : "http://www.adnansami.com", 
    "value" : "A"
}
{ 
    "_id" : ObjectId("585a431415c7a981b47ac4ef"), 
    "key" : "http://www.leap-networks.com", 
    "value" : "BB"
}
{ 
    "_id" : ObjectId("585a431d15c7a981b47ac4f0"), 
    "key" : "http://www.leap-networks.com", 
    "value" : "B"
}
{ 
    "_id" : ObjectId("585a431d15c7a981b47ac4f1"), 
    "key" : "http://www.machinelearningmastery.com", 
    "value" : "C"
}
{ 
    "_id" : ObjectId("585a432515c7a981b47ac4f2"), 
    "key" : "http://www.leap-networks.com", 
    "value" : "BBB"
}
{ 
    "_id" : ObjectId("585a432815c7a981b47ac4f3"), 
    "key" : "http://www.machinelearningmastery.com", 
    "value" : "CC"
}
{ 
    "_id" : ObjectId("585a432d15c7a981b47ac4f4"), 
    "key" : "http://www.leap-networks.com", 
    "value" : "BBBB"
}
{ 
    "_id" : ObjectId("585a433115c7a981b47ac4f5"), 
    "key" : "http://www.machinelearningmastery.com", 
    "value" : "CCC"
}
{ 
    "_id" : ObjectId("585a433615c7a981b47ac4f6"), 
    "key" : "http://www.leap-networks.com", 
    "value" : "BBBBB"
}
{ 
    "_id" : ObjectId("585a433d15c7a981b47ac4f7"), 
    "key" : "http://www.machinelearningmastery.com", 
    "value" : "CCCC"
}
{ 
    "_id" : ObjectId("585a434915c7a981b47ac4f8"), 
    "key" : "http://www.machinelearningmastery.com", 
    "value" : "CCCCC"
}

所以输出应该是

  { 
   "_id" : ObjectId("58539dc715c7a964817686f9"), 
   "http://www.adnansami.com" : "A "
  }
  { 
    "_id" : ObjectId("585a433615c7a981b47ac4f6"), 
    "key" : "http://www.leap-networks.com", 
    "value" : "BBBBB"
  }
  { 
    "_id" : ObjectId("585a434915c7a981b47ac4f8"), 
    "key" : "http://www.machinelearningmastery.com", 
    "value" : "CCCCC"
  }

我怎样才能做到这一点？

最佳答案

由于散列键的缘故，如果不诉诸于某些 map-reduce 操作，在 mongo 中进行查询将非常复杂。 Mongo 与嵌入式结构配合得很好，在这种结构中，您有一个像

{ 
    "_id" : ObjectId("58539dfa15c7a96481768700"),        
    "key": "http://www.leap-networks.com", 
    "value": "AAAAAAAA" 
} 

相反，您应该考虑重组您的文档，使其可索引并更容易在 MongoDB 中搜索。

对于上述建议的模式，您可以应用聚合框架，您可以在其中使用 $strLenCP Mongodb 3.4中计算值字段长度的运算符:

db.collection.aggregate([
    {
        "$addFields": {
            "strLength": { 
                "$strLenCP": "$value"
            }
        }
    },
    { "$sort": { "strLength": -1 } },
    {
        "$group": {
            "_id": "$key",
            "value": { "$first": "$value" },
            "doc_id": { "$first": "$_id" }          
        }
    }    
])

示例输出

{ 
    "doc_id": ObjectId("58539dc715c7a964817686f9"),
    "_id" : "http://www.adnansami.com", 
    "value":  "A "      
},
{ 
    "doc_id": ObjectId("58539dd515c7a964817686fc"),
    "_id" : "http://www.movies.yahoo.com",
    "value": "AAAA"     
},
{ 
    "doc_id": ObjectId("58539dfa15c7a96481768700"),
    "_id" : "http://www.leap-networks.com", 
    "value": "AAAAAAAA"     
}

关于python - 过滤mongo文档 - python，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/41179944/