get size of array?

[Earthson]

for document {'a':(1,2,3), '_id':Object(xxxx...)}, how could I get the size of 'a'. 

I do not find a way to do this, someone help~ 

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Perfection is achieved 

not when there is nothing more to add

 but when there is nothing left to take away

[Scott Hernandez]

You need to do this at the client/app.

In the mongo javascript shell you can do this: doc.a.length

You may also want to look at the query operators:
http://mongodb.org/display/DOCS/Advanced+Queries

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[Asoka Sampath Edussooriya]

I am not sure about the exact way of getting the size or length of the array. but this is similar what you asking. 

db.collection.find({a:{$size:3}});

this will match document which has the array size of 3

Thanks!

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Asoka Sampath Edussooriya

[Earthson]

May be I should add an extra field to count the length(with limit operations, not addToSet).

thx~

[Scott Hernandez]

If you need to do queries on it with an index, then yes.