aggregate.js

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//example object of document
{
  "_id": "10280",
  "city": "NEW YORK",
  "state": "NY",
  "pop": 5574,
  "loc": [
    -74.016323,
    40.710537
  ]
}

//above query is returns the smallest and largest cities by population for each state
aggregate([
  {
    {
      $group:{
        _id: {state: '$state', city: '$city'},
        pop: {$sum: '$pop'}
      },
    },
    {$sort: {pop: 1}},
    {
      $group: {
        _id: '$_id.state',
        biggestCity: {$last: '$_id.city'},
        biggestPop: {$last: '$pop'},
        smallestCity: {$first: '$_id.city'},
        smallestPop: {$first: '$pop'}
      }
    },
    {
      $project: {
        _id: 0,
        state: '$_id',
        biggestCity: {name: '$biggestCity', pop: '$biggestPop'},
        smallestCity: {name: '$smallestCity', pop: '$smallestPop'}
      }
    }
  }  
]);



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//example object of document
//in accessPassHistory Array, objects has readerGroup and type (0 is out, 1 is in)
{ 
    "_id" : ObjectId("570cb14a95aa4a70229fed81"), 
    "accessPassHistory" : [
        {
            "_id" : ObjectId("570cd5ce2134d7e41be0249a"), 
            "type" : "0", 
            "date" : ISODate("2016-04-12T11:02:38.689+0000"), 
            "readerGroup" : "570a639c1ffb50b8086f4bc1", 
            "reader" : "06011615"
        }, 
        {
            "_id" : ObjectId("570cd5d72134d7e41be0249c"), 
            "type" : "1", 
            "date" : ISODate("2016-04-12T11:02:47.300+0000"), 
            "readerGroup" : "570a639c1ffb50b8086f4bc1", 
            "reader" : "06011614"
        }
    ]
}

//above query is returns, that is if last type in matched readerGroup is current type access is denied else entered
/**
 * @param {string} readerGroup
 * @param {string} cardId 
 */
Card.aggregate([
    {"$match": {cardId: cardId}},
    {"$project": {access: "$accessPassHistory"}},
    {$unwind: "$access"},
    {$group: {_id: "$access.readerGroup", lastAccess: {"$last": "$access.type"}}},
    {$match: {'_id': readerGroup}}
  ]);

MONGODB AGGREGATE