Given a collection with the following documents:

{ arr: [ {a: 1, b: 1}, {a: 2, b: 3} ] }
{ arr: [ {a: 1, b: 8}, {a: 2, b: 7} ] }
{ arr: [ {a: 1, b: 4}, {a: 2, b: 5} ] }

This new feature would allow the positional operator ($) to be used to sort by a field of the matched documents inside the array. For example, the following query would sort by the "b" field of the inner documents with "a" equal to 2:

db.test.find({"arr.a": 2}).sort({"arr.$.b": -1});

The documents matching documents would then be returned by the cursor in the following order:

{ arr: [ {a: 1, b: 8}, {a: 2, b: 7} ] }
{ arr: [ {a: 1, b: 4}, {a: 2, b: 5} ] }
{ arr: [ {a: 1, b: 1}, {a: 2, b: 3} ] }

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Related posts:

https://groups.google.com/forum/#!topic/mongodb-user/w5ZVI4yraik

https://gist.github.com/e9182ceee4dc9de26f15