MongoDB - Check if value exists for a field in a document

You need the $size operator. To find something with no elements do the following

db.collection.find({ channels: {$size: 0} })

If you know you have a fixed size then do that

db.collection.find({ channels: {$size: 2} })

Otherwise reverse that with $not

db.collection.find({ channels: {$not:{$size: 0}} })

And you can combine with $and:

db.collection.find({ $and: [ 
    { channels: {$not:{$size: 0}} },
    { channels: {$exists: true } }
]})

I did it using this :

db.event_copy.find({'channels.0' : {$exists: true}}).count()

Check out the size operator here

db.event_copy.find( { channels: { $size: 0 } } );

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