How to count a specific element in a collection in mongodb - mongodb

Question

I know you can count how many elements are in a collection with
collection.find().count().
However, I was wondering how can I count a certain element inside the item inside the collection. For example I have four images inside a Documents like this:
"Documents":{
"1":"image.png",
"2":"Test.jpg",
"3":"Next.png"
}
I was wondering how I can count all the items in Documents? I have tried a few things but none of them are working. Can anyone help?
Example data:
{
"name": "Oran",
"username": "Oran.Hammes",
"avatar": "https://s3.amazonaws.com/uifaces/faces/twitter/brandonflatsoda/128.jpg",
"email": "xxxxxx#hotmail.com",
"dob": "1953-03-21T17:40:17.020Z",
"phone": "364-846-1607",
"address": {
"street": "Schultz Stream",
"suite": "Suite 618",
"city": "North Muriel mouth",
"zipcode": "06447-1081",
"geo": {
"lat": "57.1844",
"lng": "-56.8890"
}
},
"website": "misty.net",
"company": {
"name": "Hettinger, Reilly and Stracke",
"catchPhrase": "Multi-tiered system-worthy database",
"bs": "best-of-breed evolve e-markets"
},
"Documents": [
{
"id": "1",
"name": "image.png",
},
{
"id": "2",
"name": "Test.jpg",
},
{
"id": "3",
"name": "Next.png"
}
]
}
After this comment I changed my document's structure and it looks like this:
{
"_id" : ObjectId("568703d08981f193cf343698"),
"name" : "Oran",
"username" : "Oran.Hammes",
"email" : "xxxxxxx#hotmail.com",
"dob" : "1953-03-21T17:40:17.020Z",
"phone" : "364-846-1607",
"company" : {
"name" : "Hettinger, Reilly and Stracke",
"catchPhrase" : "Multi-tiered system-worthy database",
"bs" : "best-of-breed evolve e-markets"
},
"Documents" : [
{
"id" : "1",
"name" : "image.png"
},
{
"id" : "2",
"name" : "Test.jpg"
},
{
"id" : "3",
"name" : "Next.png"
}
]
}

Answer 1

Generally speaking it's not a good idea to have dynamic keys in your documents. Your best bet in situation like this is mapReduce
var map = function() {
emit(this._id, Object.keys(this.Documents).length);
};
var reduce = function(key, values) {};
db.collection.mapReduce(map, reduce, { "out": { "inline": 1 } } )
Which yields:
{
"results" : [
{
"_id" : ObjectId("5686e2a98981f193cf343697"),
"value" : 3
}
],
"timeMillis" : 697,
"counts" : {
"input" : 1,
"emit" : 1,
"reduce" : 0,
"output" : 1
},
"ok" : 1
}
Definitely the best thing to do is to change your documents structure and make Documents an array of sub-documents so that the Documents field value looks like this:
"Documents": [
{
"id": "1",
"name": "image.png",
},
{
"id": "2",
"name": "Test.jpg",
},
{
"id": "3",
"name": "Next.png"
}
]
and use the .aggregate() method to $project your documents and return the number of elements in the "Documents" array using the $size operator
db.collections.aggregate([
{ "$project": { "count": { "$size": "$Documents" } } }
] )