mongodb - Changing from .group() to Aggregation -

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i'm doing query in mongodb. want query iusing aggregation , not group. query using:

var query_part = {     "shipdate" : { "$gte" : 19940101, "$lt" : 19950101 },     "partsupp.supplier.nation.name" : { $regex : '^canada'},      "partsupp.part.name" : { $regex : '^forest', $options : 'i' } };  var red = function(doc, out) {     out.sum += doc.quantity; };  var half_total_quantity = db.lineitems.group( {     key : "sum_quantity",     cond : query_part,     initial : { sum : 0 },     reduce : red })[0].sum / 2;

i'm trying change half_total_quantity calculation this:

db.lineitems.aggregate( { $match : {$and : [{"shipdate" : { "$gte" : 19940101, "$lt" : 19950101 }},                     {"partsupp.supplier.nation.name" : { $regex : '^canada'}},                     {"partsupp.part.name" : { $regex : '^forest', $options : 'i' }}                     ]}}, { $group : {      sum_quantity : { $sum: "$quantity" } }} );

but not works, can me? thank you.

you never need $and since all mongodb query operations "and" conditions unless explicitly stated otherwise.

also $group requires , _id, null here indicate grouping "everything":

db.lineitems.aggregate([     { "$match": {          "shipdate" : { "$gte" : 19940101, "$lt" : 19950101 },         "partsupp.supplier.nation.name" : { "$regex": '^canada' },         "partsupp.part.name" : { "$regex": '^forest', "$options": 'i' }     }},     { "$group": {          "_id": null,         "sum_quantity": { "$sum": "$quantity" }     }} ]);

not forget brackets [] since "list" of pipeline expressions, afterall.

and add "halftotalquantity" thing, $divide in $project stage, if must:

db.lineitems.aggregate([     { "$match": {          "shipdate" : { "$gte" : 19940101, "$lt" : 19950101 },         "partsupp.supplier.nation.name" : { "$regex": '^canada' },         "partsupp.part.name" : { "$regex": '^forest', "$options": 'i' }     }},     { "$group": {          "_id": null,         "sum_quantity": { "$sum": "$quantity" }     }},     { "$project": {         "half_quantity": { "$divide": [ "$sum_quantity", 2 ] }      }} ]);

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