javascript - MongoDB Match and Slice multiple child arrays -

i have schema

var dataschema = new schema({ hid: { type: string }, sensors: [{     nid: { type: string },     sid: { type: string },     data: {         param1: { type: string },         param2: { type: string },         data: { type: string }     },     date: { type: date, default: date.now } }], actuators: [{     nid: { type: string },     aid: { type: string },     control_id: { type: string },     data: {         param1: { type: string },         param2: { type: string },         data: { type: string }     },     date: { type: date, default: date.now } }], status: [{     nid: {type: string},     status_code: {type: string},     date: { type: date, default: date.now } }], updated: { type: date, default: date.now }, created: { type: date } }); 

the query i'm trying build should search schema "hid", pick "sensors", "actuators" , "status" arrays objects match nid provide , limit result 10 element each array.

actually, don't use multiple pipeline phases when "one" do. every pipeline stage include adding "time" processing since it's pass through data.

so logically works in "single" stage, should stay in "single" stage:

data.aggregate([     { "$match": { "hid": hid } },     { "$project": {         "sensors": {             "$slice": [                 { "$filter": {                      "input": "$sensors",                      "as": "sensor",                      "cond": { "$eq": [ "$$sensor.nid", nid ] }                  }},                 -10             ]         },         "actuators": {             "$slice": [                 { "$filter": {                      "input": "$actuators",                      "as": "actuator",                      "cond": { "$eq": [ "$$actuator.nid", nid ] }                  }},                 -10             ]         },         "status": {             "$slice": [                 { "$filter": {                      "input": "$status",                      "as": "status",                      "cond": { "$eq": [ "$$status.nid", nid ] }                  }},                 -10             ]         },         "updated": 1,         "created": 1     }} ]) 

also, it's not necessary use "_id": 1 in inclusions since "_id" always included unless explicitly "excluded".

the main case try not create unnecessary stages, since it's bad performance. indicators of are:

• $project followed $project, means can in 1 stage.
• $project followed $group going compacted "optimizer" anyway, "should" in habit of of combining two.
• $project followed $match, should indicate should using $redact in single stage instead. since did $project produce fields based on calculations, considered in $match.

and finally:

• $unwind before $match, should have been $filter in $project "before" $unwind. since in fact work lot faster "filter" within document, , saves cost on processing $unwind, due less output filtered content.

but 1 more:

• all of "array" operations such $map, $filter, $slice , "set operators" should used "in-line" each other, in cases manipulating same array content. same applies wrapping $sum or $max these can work directly on arrays themselves, , strange looking effective:

  { "$sum": { "$sum": "$array" } } 

  structure within $group stage.