min value of field in array and 1st object in array mongodb aggregation query

Question

Let me use following pseudo example to better explain my need.

I have a group of people and some images which belong to group schema

{
  images:['ARRAY OF IMAGES'],
  members:[{
    age:NUMBER,
    name:STRING, 
    email:EMAIL_OF_MEMBER
  }], 
  groupName:String
  ....etc
}

Now i have a aggregation query

group.aggregate([
  {
    $project:{
       //some code happening to support query in the below $match
       'original':$$ROOT
    }
  },
  {
    $match:{//some query code happening and all params from $project above will be available here.  the $original field contains all fields of document}
  },
  {
    //Now i want to extract 1st image from Images array, Min and Max age of members in group.
  }
],function(err,results){//dosomething with results})

If I don't use 3rd pipeline I am getting the whole document which is not required for me. I just need 1 image and min-max age of people in group to display a web page. I don't need other details.

Yathish Manjunath · Accepted Answer · 2015-10-25 13:31:58Z

Below part of the query will gives you the first image as well as min and max age of the members.

db.[collection].aggregate([
                            { 
                              $unwind : "$members" 
                            },
                            { 
                              $group : 
                                     { _id: "$_id" , 
                                       images : { $first: "$images"},
                                       minAge : {$min : "$members.age"},
                                       maxAge : {$max: "$members.age"}
                                     }
                            }
                          ]).pretty();

For the mongoDb Version 3.1.6 and above, you can use $Slice in aggregation pipeline to limit the contents of array.

db.[collection].aggregate([
                            { 
                              $unwind : "$members" 
                            },
                            { 
                              $group : 
                                     { _id: "$_id" , 
                                       images : {$push : "$images"},
                                       minAge : {$min : "$members.age"},
                                       maxAge : {$max: "$members.age"}
                                     }
                            },
                            { 
                              $project : 
                                       { images : 
                                           { images :{$slice:1} }
                                       },
                                       minAge : 1 ,
                                       maxAge  : 1
                           }
                         ]).pretty();

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