Aggregation Framework: $match, $unwind, $redact, and $filter

Aggregation Framework

Given a array of document as below, we want to display all sub documents whose value of favoritesList.book is "Journey to the West".

{
...
	"favoritesList" : [
		{
			"sport" : "Ski",
			"music" : "Easy Listening",
			"city" : "Istanbul",
			"book" : "Moby Dick",
			"movie" : "The Wizard of Oz"
		},
		{
			"book" : "Don Quixote",
			"movie" : "Casablanca",
			"sport" : "Mountaineering",
			"music" : "Soul",
			"city" : "Bangkok"
		},
		{
			"movie" : "Gone with the Wind",
			"sport" : "Baseball",
			"music" : "Blues",
			"city" : "Taipei",
			"book" : "Journey to the West"
		}
	],
...
}

We can use the find() command, but first let's use keyhole to seed randomized documents.

keyhole --uri mongodb://localhost/examples --seed --drop --total 100000

and apply index.

mongo mongodb://localhost/examples
> db.favorites.createIndex({ "favoritesList.book": 1 })

Find

We can use $elemMatch to query.

db.favorites.explain("executionStats").find(
    {
        favoritesList: {$elemMatch: {book: "Journey to the West"} }
    }, {
        _id: 0, 
        "favoritesList.$": 1
    }
)

"executionTimeMillis" : 271

However, because we specify a single query predicate in the $elemMatch expression, $elemMatch is not necessary. The commands below are as good as the above command.

db.favorites.explain("executionStats").find(
    {
        "favoritesList.book": "Journey to the West"
    }, {
        _id: 0, 
        "favoritesList.$": 1
    }
)

"executionTimeMillis" : 265

db.favorites.explain("executionStats").find(
    {
        "favoritesList.book": "Journey to the West"
    }, {
        _id: 0, 
        favoritesList: {$elemMatch: {book: "Journey to the West"} }
    } 
)

"executionTimeMillis" : 289

But, what if there are multiple matched document in the array and we want to list them as single document, we have to use the aggregation framework.

Aggregation

See golang example.

$match

The commands below, $match again after $unwind, solve the problems find() cannot.

db.favorites.explain("executionStats").aggregate([
  {
    $match: {
      "favoritesList.book": "Journey to the West"
    }
  }, {
    $project: {
      _id: 0, 
      favoritesList: 1
    }
  }, {
    $unwind: {
      path: '$favoritesList'
    }
  }, {
    $match: {
      'favoritesList.book': 'Journey to the West'
    }
  }
])

"executionTimeMillis" : 530

$redact

Operator $redact, introduced in 2.6, restricts the contents of the documents based on information stored in the documents themselves.

db.favorites.explain("executionStats").aggregate([
  {
    $match: {
      "favoritesList.book": "Journey to the West"
    }
  }, {
    $project: {
      _id: 0, 
      favoritesList: 1
    }
  }, {
    $redact: {
      $cond: {
        'if': {
          $or: [
            {
              $eq: ['$book', 'Journey to the West']
            }, {
              $not: '$book'
            }
          ]
        },
        then: '$$DESCEND', 
        'else': '$$PRUNE'
      }
    }
  }, {
    $unwind: {
      path: '$favoritesList'
    }
  }
])

"executionTimeMillis" : 551

$filter

Operator $filter, introduced in 3.2, selects a subset of an array to return based on the specified condition. Returns an array with only those elements that match the condition. The returned elements are in the original order.

db.favorites.explain("executionStats").aggregate( [
  {
    $match: {
      "favoritesList.book": "Journey to the West"
    }
  }, {
    $project: {
      favoritesList: {
        $filter: {
          input: '$favoritesList', 
          as: 'favorite', 
          cond: {
            $eq: ['$$favorite.book', 'Journey to the West']
          }
        }
      }, 
      _id: 0
    }
  }, {
    $unwind: {
      path: '$favoritesList'
    }
  }
])

"executionTimeMillis" : 457