Count and apply condition to slice the mongodb array document

Question

My document structure looks like this:

{
   "_id" : ObjectId("5aeeda07f3a664c55e830a08"),
   "profileId" : ObjectId("5ad84c8c0e71892058b6a543"),
   "list" : [ 
      {           
        "content" : "answered your post",
        "createdBy" : ObjectId("5ad84c8c0e71892058b6a540")
      }, 
      {           
        "content" : "answered your post",
        "createdBy" : ObjectId("5ad84c8c0e71892058b6a540")
      }, 
      {
        "content" : "answered your post",
        "createdBy" : ObjectId("5ad84c8c0e71892058b6a540")
      }, 
    ],   
}

I want to count array of list field. And apply condition before slicing that

if the list<=10 then slice all the elements of list

else 10 elements.

P.S I used this query but is returning null.

db.getCollection('post').aggregate([
 {
  $match:{
       profileId:ObjectId("5ada84c8c0e718s9258b6a543")}
 },
 {$project:{notifs:{$size:"$list"}}},    
 {$project:{notifications:
            {$cond:[
                {$gte:["$notifs",10]},
                {$slice:["$list",10]},
                {$slice:["$list","$notifs"]}
            ]}
          }}
])

Try using $addFields instead of the first $project.

dnickless
– dnickless

2018-05-09 10:57:41 +00:00
Commented May 9, 2018 at 10:57 — dnickless
– dnickless, Commented May 9, 2018 at 10:57
thanks @dnickless it worked.

Raju Adhikari
– Raju Adhikari

2018-05-09 11:04:40 +00:00
Commented May 9, 2018 at 11:04 — Raju Adhikari
– Raju Adhikari, Commented May 9, 2018 at 11:04

dnickless · Accepted Answer · 2018-05-09 15:59:31Z

Your first $project stage effectively wipes out all result fields but the one(s) that it explicitly projects (only notifs in your case). That's why the second $project stage cannot $slice the list field anymore (it has been removed by the first $project stage).

Also, I think your $cond/$slice combination can be more elegantly expressed using the $min operator. So there's at least the following two fixes for your problem:

Using $addFields:

db.getCollection('post').aggregate([
  { $match: { profileId: ObjectId("5ad84c8c0e71892058b6a543") } },
  { $addFields: { notifs: { $size: "$list" } } },    
  { $project: {
    notifications: {
      $slice: [ "$list", { $min: [ "$notifs", 10 ] } ]
    }
  }}
])

Using a calculation inside the $project - this avoids a stage so should be preferable.

db.getCollection('post').aggregate([
  { $match: { profileId: ObjectId("5ad84c8c0e71892058b6a543") } },
  { $project: {
    notifications: {
      $slice: [ "$list", { $min: [ { $size: "$list" }, 10 ] } ]
    }
  }}
])

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