how to get embedded array's size in mongo? [duplicate]

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Possible Duplicate:

Querying internal array size in MongoDB

I have a document which contains an array category. I just want to know how many items there are in category. All I could find in the documentation is how to count the number of top level docume开发者_如何学运维nts.

{
 "_id": {
     "$oid": "4e73a30466ca1a1f56000001"
    },
 "category": [
     "Food",
     "Entertainment"
    ]
}

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Add new field to hande category size. It's a usual practice in mongo world.

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To find a single example element in a collection callend mycollection:

db.mycollection.find().limit(1)[0];

To get the number of elements in the arry category:

db.mycollection.find().limit(1)[0].category.length;

Or:

var elem = db.mycollection.find().limit(1)[0];
elem.category.length;