how to sort before querying in the embedded document - mongodb

Question

I know how to sort the embedded document after the find results but how do I sort before the find so that the query itself is run on the sorted array ? I know this must be possible if I use aggregate but i really like to know if this is possible without that so that I understand it better how it works.
This is my embedded document
"shipping_charges" : [
{
"region" : "region1",
"weight" : 500,
"rate" : 10
},
{
"region" : "Bangalore HQ",
"weight" : 200,
"rate" : 40
},
{
"region" : "region2",
"weight" : 1500,
"rate" : 110
},
{
"region" : "region3",
"weight" : 100,
"rate" : 50
},
{
"region" : "Bangalore HQ",
"weight" : 100,
"rate" : 150
}
]
This is the query i use to match the 'region' and the 'weight' to get the pricing for that match ..
db.clients.find( { "shipping_charges.region" : "Bangalore HQ" , "shipping_charges.weight" : { $gte : 99 } }, { "shipping_charges.$" : 1 } ).pretty()
This query currently returns me the
{
"shipping_charges" : [
{
"region" : "Bangalore HQ",
"weight" : 200,
"rate" : 40
}
]
}
The reason it possibly returns this set is because of the order in which it appears(& matches) in the embedded document.
But, I want this to return me the last set that best matches to closest slab of the weight(100grams)
What changes required in my existing query so that I can sort the embedded document before the find runs on them to get the results as I want it ?
If for any reasons you are sure this cant be done without a MPR, let me know so that i can stay away from this method and focus only on MPR to get the desired results as I want it .

Answer 1

You can use an aggregation pipeline instead of map-reduce:
db.clients.aggregate([
// Filter the docs to what we're looking for.
{$match: {
'shipping_charges.region': 'Bangalore HQ',
'shipping_charges.weight': {$gte: 99}
}},
// Duplicate the docs, once per shipping_charges element
{$unwind: '$shipping_charges'},
// Filter again to get the candidate shipping_charges.
{$match: {
'shipping_charges.region': 'Bangalore HQ',
'shipping_charges.weight': {$gte: 99}
}},
// Sort those by weight, ascending.
{$sort: {'shipping_charges.weight': 1}},
// Regroup and take the first shipping_charge which will be the one closest to 99
// because of the sort.
{$group: {_id: '$_id', shipping_charges: {$first: '$shipping_charges'}}}
])
You could also use find, but you'd need to pre-sort the shipping_charges array by weight in the documents themselves. You can do that by using a $push update with the $sort modifier:
db.clients.update({}, {
$push: {shipping_charges: {$each: [], $sort: {weight: 1}}}
}, {multi: true})
After doing that, your existing query will return the right element:
db.clients.find({
"shipping_charges.region" : "Bangalore HQ",
"shipping_charges.weight" : { $gte : 99 }
}, { "shipping_charges.$" : 1 } )
You would, of course, need to consistently include the $sort modifier on any further updates to your docs' shipping_charges array to ensure it stays sorted.