MongoDB query group with 'sub' group -

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from product stocks log have created mongodb collection. relevant fields are: sku, stock , date. every time products stock updated there new entry total stock.

the skus made of 2 parts. parent part, 'a' , variant or child part, '1', '2', '3', etc.. sku might this: 'a2'.

i can query single products stock, grouped day, query:

    [{         $match: {             sku: 'a2'         }     },     {         $group: {             _id: {                 year: {$year: '$date'},                  day: {$dayofyear: '$date'}             },             stock: {                 $min: '$stock'             },             date: {                 $first: '$date'             }         }     },     {         $sort: {             date: 1         }     }]

note: want minimum stock each day.

but need query variations (minimum) stocks added up. can change $match object to:

    [{         $match: {             sku: /^a/         }     }

how create 'sub' group in $group stage?

edit:

the data looks this:

{     sku: 'a1',     date: '2015-01-01',     stock: 15 }  {     sku: 'a1',     date: '2015-01-01',     stock: 14 }  {     sku: 'a2',     date: '2015-01-01',     stock: 20 }

two stocks 'a1' , 1 'a2' on single day. query (all skus grouped day) give me stock 14 result ($min of 3 values). want result 34. 20 (min a2) plus 14 (min a1)

if add sku _id field in group phase aggregate on well, i.e. group per sku, year & day.

db.stocks.aggregate( [   {     $group: {         _id: {             sku: '$sku',             year: {$year: '$date'},              day: {$dayofyear: '$date'}         },         stock: {             $min: '$stock'         },         date: {             $first: '$date'         }     }   },   {     $sort: {         date: 1     }   }] )

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