CODEWITHSUNDEEP
javascriptmongodbpymongo
S.D.
S.D.

Reputation: 1201

find query to find retrieve the data of specific element in mongodb

I have a document in my mongo instance in below format, 

{
"_id"   : "08d4a242-08fb-07f7-46e5-8717a81d5b70",
"fname" :   "john",
"created_date" : ISODate("2017-05-24T01:13:06.829Z"),
"customProp"    : [
    [
        "customX","{\"some data related to X \"}"
    ],
    [
        "customY","{\"some data related to Y \"}"
    ],
    [
        "customZ","{\"some data related to Z \"}"
    ]
]

}

the elements/values like "customX","customY" & "customZ" are not necessarily be in all documents. How to retrieve all the values in second element of "customProp" array, in this document it contains "customZ"?

I'm able to use following query to filter & find all the documents which are having "customZ" element,

db.getCollection('col1').find({$and : [{"customProp":{$elemMatch:{0:"customZ"}}}, {"created": { $gte: ISODate("2017-05-22T00:00:00.000Z") }}] },{"created":1}).limit(1) .pretty()

output:

    {
        "_id" : "08d4a242-08fb-07f7-46e5-8717a81d5b45",
        "created" : ISODate("2017-05-24T01:13:06.829Z")
}

but finding a way to retrieve all the values in second element of array where the first value is "customZ".

expected result:

   {
    "_id" : "08d4a242-08fb-07f7-46e5-8717a81d5b45",
    "created" : ISODate("2017-05-24T01:13:06.829Z"),
    "customPro": ["customZ","{\"some data related to Z \"}"]
    }

I'm fine if my query just returns 

{
"{\"some data related to Z \"}"
}

Upvotes: 1

Views: 74

Answers (1)

Neil Lunn
Neil Lunn

Reputation: 151220

Well it is a nested array, which is not a great idea but you are in fact matching the element with the $elemMatch expression, so you do get the position in the "outer" array of customProp, which allows you to project with the positional $ operator:

db.getCollection('coll1').find(
  { 
    "customProp":{ "$elemMatch": { "0": "customZ" } }, 
    "created_date": { "$gte": ISODate("2017-05-22T00:00:00.000Z") }
  },
  { "created_date": 1, "customProp.$": 1 }
)

That yields the result:

{
        "_id" : "08d4a242-08fb-07f7-46e5-8717a81d5b70",
        "created_date" : ISODate("2017-05-24T01:13:06.829Z"),
        "customProp" : [
                [
                        "customZ",
                        "{\"some data related to Z \"}"
                ]
        ]
}

Where customProp is of course still in a nested array, but when processing the individual documents in python you can just access the property at the array index:

doc['customProp'][0][1]

Which of course returns the value:

'{"some data related to Z "}'

Same goes for JavaScript really, which is basically identical in syntax. As a shell example:

db.getCollection('coll1').find(
  { 
    "customProp":{ "$elemMatch": { "0": "customZ" } }, 
    "created_date": { "$gte": ISODate("2017-05-22T00:00:00.000Z") }
  },
  { "created_date": 1, "customProp.$": 1 }
).map(function(doc) {
  doc['customProp'] = doc['customProp'][0][1];
  return doc;
})

And the output:

{
        "_id" : "08d4a242-08fb-07f7-46e5-8717a81d5b70",
        "created_date" : ISODate("2017-05-24T01:13:06.829Z"),
        "customProp" : "{\"some data related to Z \"}"
}

And the positional $ projection here ensures there is only one element in the returned array, so the notation is always the same to extract from all document results. So you get the matched element from the database, and you extract the property through the code.

Also note that you do not need $and here since all the query arguments are already AND conditions. This is the MongoDB default, so you do not need to explicitly express it. See how much nicer this looks without it.

Upvotes: 1

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