MongoDB projections and fields subset - mongodb

Question

I would like to use mongo projections in order to return less data to my application. I would like to know if it's possible.
Example:
user: {
id: 123,
some_list: [{x:1, y:2}, {x:3, y:4}],
other_list: [{x:5, y:2}, {x:3, y:4}]
}
Given a query for user_id = 123 and some 'projection filter' like user.some_list.x = 1 and user.other_list.x = 1 is it possible to achieve the given result?
user: {
id: 123,
some_list: [{x:1, y:2}],
other_list: []
}
The ideia is to make mongo work a little more and retrieve less data to the application. In some cases, we are discarding 80% of the elements of the collections at the application's side. So, it would be better not returning then at all.
Questions:
Is it possible?
How can I achieve this. $elemMatch doesn't seem to help me. I'm trying something with unwind, but not getting there
If it's possible, can this projection filtering benefit from a index on user.some_list.x for example? Or not at all once the user was already found by its id?
Thank you.

Answer 1

What you can do in MongoDB v3.0 is this:
db.collection.aggregate({
$match: {
"user.id": 123
}
}, {
$redact: {
$cond: {
if: {
$or: [ // those are the conditions for when to include a (sub-)document
"$user", // if it contains a "user" field (as is the case when we're on the top level
"$some_list", // if it contains a "some_list" field (would be the case for the "user" sub-document)
"$other_list", // the same here for the "other_list" field
{ $eq: [ "$x", 1 ] } // and lastly, when we're looking at the innermost sub-documents, we only want to include items where "x" is equal to 1
]
},
then: "$$DESCEND", // descend into sub-document
else: "$$PRUNE" // drop sub-document
}
}
})
Depending on your data setup what you could also do to simplify this query a little is to say: Include everything that does not have a "x" field or if it is present that it needs to be equal to 1 like so:
$redact: {
$cond: {
if: {
$eq: [ { "$ifNull": [ "$x", 1 ] }, 1 ] // we only want to include items where "x" is equal to 1 or where "x" does not exist
},
then: "$$DESCEND", // descend into sub-document
else: "$$PRUNE" // drop sub-document
}
}
The index you suggested won't do anything for the $redact stage. You can benefit from it, however, if you change the $match stage at the start to get rid of all documents which don't match anyway like so:
$match: {
"user.id": 123,
"user.some_list.x": 1 // this will use your index
}

Answer 2

Very possible.
With findOne, the query is the first argument and the projection is the second. In Node/Javascript (similar to bash):
db.collections('users').findOne( {
id = 123
}, {
other_list: 0
} )
Will return the who'll object without the other_list field. OR you could specify { some_list: 1 } as the projection and returned will be ONLY the _id and some_list

Answer 3

$filter is your friend here. Below produces the output you seek. Experiment with changing the $eq fields and target values to see more or less items in the array get picked up. Note how we $project the new fields (some_list and other_list) "on top of" the old ones, essentially replacing them with the filtered versions.
db.foo.aggregate([
{$match: {"user.id": 123}}
,{$project: { "user.some_list": { $filter: {
input: "$user.some_list",
as: "z",
cond: {$eq: [ "$$z.x", 1 ]}
}},
"user.other_list": { $filter: {
input: "$user.other_list",
as: "z",
cond: {$eq: [ "$$z.x", 1 ]}
}}
}}
]);