Question

I have a problem try to use $elemMatch in dual nested array:

Suppose I have this a document:

a = {'cart': [[{'id': 1, 'count': 1}, {'id': 2, 'count': 3}], [{'id': 1, 'count': 5}]]}

And I want to select a document out when id is 1 and count greater than 2:

db.cart.find_one({'cart.0.id': 1, 'cart.0.count': {'$gt': 2}})

But this query will select a out.

Then I have tried these queries:

db.cart.find_one({'cart': {'$elemMatch': {'id': 1, 'count': {'$gt': 2}}}})
db.cart.find_one({'cart': {'$elemMatch': {'id': 2, 'count': {'$gt': 2}}}})
db.cart.find_one({'cart.0': {'$elemMatch': {'id': 1, 'count': {'$gt': 2}}}})
db.cart.find_one({'cart.0': {'$elemMatch': {'id': 2, 'count': {'$gt': 2}}}})

But all return None.

So do $elemMatch support the nested array match? If so, how shall I tune my query?

Answer 1

Given the fact that you have an array within an array, I think you could try something like

db.cart.find_one({'cart': {'$elemMatch': { '$elemMatch' : {'id': 1, 'count': {'$gt': 2}}}}})