Counting elements inside an array in mongodb

Question

I want to count the number of elements in an array using python and pymongo. Here is the data.

{
"_id": 5,
"type": "Student",
"Applicates": [
    {
        "appId": 100,
        "School": "dfgdfgd",
        "Name": "tony",
        "URL": "www.url.com",
        "Time": "5/5/5-6/6/6",
        "Research": "dfgdfg",
        "Budge": 5000,
        "citizenship": "us",
        "Major": "csc",
        "preAwards": "None",
        "Advisor": "dfgdfg",
        "Evaluators": [
            {
                "abstractScore": 10,
                "goalsObjectivesScore": 20,
                "evalNum": 1
            },
            {
                "abstractScore": 30,
                "goalsObjectivesScore": 40,
                "evalNum": 2
            },
            {
                "abstractScore": 50,
                "goalsObjectivesScore": 60,
                "evalNum": 3
            }
        ]
    },
    {
        "appId": 101,
        "School": "dvdu",
        "Name": "jessy",
        "URL": "www.url.com",
        "Time": "4/4/4-6/6/6",
        "Research": "dfgdfg",
        "Budge": 7500,
        "citizenship": "us",
        "Major": "dfgdfg",
        "preAwards": "dfgfd",
        "Advisor": "dfgdfg",
        "Evaluators": [
            {
                "abstractScore": 70,
                "goalsObjectivesScore": 80,
                "evalNum": 1
            },
            {
                "abstractScore": 90,
                "goalsObjectivesScore": 100,
                "evalNum": 2
            }
        ]
    }
]}

So I want to get the size of the Evaluators array. {"appId" : 100} would give 3 and {"appId" : 101} would give 2. I have been playing around with $size but cant seem to get it.

Answer 1

Queries return documents. No query will return the size of the Evaluators array in the array element with "appId" : 100`. But the following awkwardly formatted expression will so what you want:

len(coll.find_one(
    { "Applicates.appId" : 100 }, 
    { "Applicates.$.Evaluators" : 1 }
)["Applicates"][0]["Evaluators"])

where coll is the Collection object.

Answer 2

One approach would be to loop through your array, and then on each iteration use len() on that dict's Evaluators property.

for obj in Applicates:
    count = len(obj['Evaluators'])

Answer 3

With this syntax { $size: <expression> } you can count number of items in an array. See here for more > $size