如何编写查询以基于MongoDB中的两个相关输入变量获取值？

Question

I have a mongoDB collection that contains JSON documents in the following format. This is just a sample not the full document.

{
    "_id": ObjectId("555ba8a6ae96b63b98969192"),
    "toptags": {
        "@attr": {
            "artist": "Rihanna"
        },
        "tag": [
            {
                "count": "100",
                "name": "pop",
                "url": "http://www.last.fm/tag/pop"
            },
            {
                "count": "89",
                "name": "rnb",
                "url": "http://www.last.fm/tag/rnb"
            },
            {
                "count": "60",
                "name": "female vocalists",
                "url": "http://www.last.fm/tag/female%20vocalists"
            },
            {
                "count": "55",
                "name": "dance",
                "url": "http://www.last.fm/tag/dance"
            },
            {
                "count": "40",
                "name": "Hip-Hop",
                "url": "http://www.last.fm/tag/hip-hop"
            },
            {
                "count": "21",
                "name": "Rihanna",
                "url": "http://www.last.fm/tag/rihanna"
            },
      ]
      }
}

I have hundreds of similar documents in the collection. I want to write a query that will return the "artist" names that have a given set of tags and "count" values of those tags is greater than a given value.

These are the two queries I have tried so far

1.  collection_name.find({'$and': [{"toptags.tag.name":tag_array}, {"toptags.tag.count":{'$gte':count_value}}]}, {"_id":"1","toptags.@attr.artist":"1"}) 

2.  collection_name.find({"toptags.artist": {$all : [{"$elemMatch" : {"name":tag_array, "count": {'$gt': count_value}}},]}) 

None of the above queries work. I realize that the first one is fundamentally wrong because it does not take "count" value for the tags passed as a parameter. But the second one I think should work. But I think my syntax is wrong. Where am I going wrong ?

Answer 1

From what I understand:

• You have an array of tags to match;
• You only take into account tags above a certain threshold.

As suggested by @yogesh in a comment, you should first ensure that your tag count is a number. Not as string. Once done, you have to build your query based on your tag list. Something like that maybe:

> THRESHOLD=50
> TAGS=['dance', 'rnb']
> for (idx in TAGS) {
    QTAGS[idx]={"$elemMatch": {"name":TAGS[idx], "count":{"$gt": THRESHOLD}}}
  }
> QTAGS
[
    {
        "$elemMatch" : {
            "name" : "dance",
            "count" : {
                "$gt" : 50
            }
        }
    },
    {
        "$elemMatch" : {
            "name" : "rnb",
            "count" : {
                "$gt" : 50
            }
        }
    }
]

Now, you can query your DB:

> db.w.find({"toptags.tag": { "$all": QTAGS}})
{ "_id" : ObjectId("555ba8a6ae96b63b98969192"), "toptags" : { "@attr" : { "artist" : "Rihanna" }, "tag" : [ { "count" : 100, "name" : "pop", "url" : "http://www.last.fm/tag/pop" }, { "count" : 89, "name" : "rnb", "url" : "http://www.last.fm/tag/rnb" }, { "count" : 60, "name" : "female vocalists", "url" : "http://www.last.fm/tag/female%20vocalists" }, { "count" : 55, "name" : "dance", "url" : "http://www.last.fm/tag/dance" }, { "count" : 40, "name" : "Hip-Hop", "url" : "http://www.last.fm/tag/hip-hop" }, { "count" : 21, "name" : "Rihanna", "url" : "http://www.last.fm/tag/rihanna" } ] } }

Raise the threshold an do it all again, and you end-up selecting nothing:

> THRESHOLD=100
> for (idx in TAGS) {   QTAGS[idx]={"$elemMatch": {"name":TAGS[idx], "count":{"$gt": THRESHOLD}}} }
> db.w.find({"toptags.tag": { "$all": QTAGS}})
> // nothing