$accumulator (operador de acumulador)

{ _id: 'a' },
{ _id: 'b' },
{ _id: 'c' }
...
{ _id: 'z' }

[
   {
      $sort: { _id: 1 }
   },
   {
      $group: {
         _id: null,
         alphabet: {
            $accumulator: {
               init: function() {
                  return ""
               },
               accumulate: function(state, letter) {
                  return(state + letter)
               },
               accumulateArgs: [ "$_id" ],
               merge: function(state1, state2) {
                  return(state1 + state2)
               },
               lang: "js"
            }
         }
      }
   }
]

db.books.insertMany([
  { _id: 8751, title: "The Banquet", author: "Dante", copies: 2 },
  { _id: 8752, title: "Divine Comedy", author: "Dante", copies: 1 },
  { _id: 8645, title: "Eclogues", author: "Dante", copies: 2 },
  { _id: 7000, title: "The Odyssey", author: "Homer", copies: 10 },
  { _id: 7020, title: "Iliad", author: "Homer", copies: 10 }
])

db.books.aggregate([
{
  $group :
  {
    _id : "$author",
    avgCopies:
    {
      $accumulator:
      {
        init: function() {                        // Set the initial state
          return { count: 0, sum: 0 }
        },
        accumulate: function(state, numCopies) {  // Define how to update the state
          return {
            count: state.count + 1,
            sum: state.sum + numCopies
          }
        },
        accumulateArgs: ["$copies"],              // Argument required by the accumulate function
        merge: function(state1, state2) {         // When the operator performs a merge,
          return {                                // add the fields from the two states
            count: state1.count + state2.count,
            sum: state1.sum + state2.sum
          }
        },
        finalize: function(state) {               // After collecting the results from all documents,
          return (state.sum / state.count)        // calculate the average
        },
        lang: "js"
      }
    }
  }
}
])

db.restaurants.insertMany( [
  { _id: 1, name: "Food Fury", city: "Bettles", cuisine: "American" },
  { _id: 2, name: "Meal Macro", city: "Bettles", cuisine: "Chinese" },
  { _id: 3, name: "Big Crisp", city: "Bettles", cuisine: "Latin" },
  { _id: 4, name: "The Wrap", city: "Onida", cuisine: "American" },
  { _id: 5, name: "Spice Attack", city: "Onida", cuisine: "Latin" },
  { _id: 6, name: "Soup City", city: "Onida", cuisine: "Chinese" },
  { _id: 7, name: "Crave", city: "Pyote", cuisine: "American" },
  { _id: 8, name: "The Gala", city: "Pyote", cuisine: "Chinese" }
] )

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db.restaurants.aggregate([
2
{
3
  $group :
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  {
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    _id : { city: "$city" },
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    restaurants:
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    {
8
      $accumulator:
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      {
10
        init: function(city, userProfileCity) {        // Set the initial state
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          return {
12
            max: city === userProfileCity ? 3 : 1,     // If the group matches the user's city, return 3 restaurants
13
            restaurants: []                            // else, return 1 restaurant
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          }
15
        },
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        initArgs: ["$city", <userProfileCity>],        // Argument to pass to the init function
18
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        accumulate: function(state, restaurantName) {  // Define how to update the state
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          if (state.restaurants.length < state.max) {
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            state.restaurants.push(restaurantName);
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          }
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          return state;
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        },
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        accumulateArgs: ["$name"],                     // Argument required by the accumulate function
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        merge: function(state1, state2) {
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          return {
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            max: state1.max,
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            restaurants: state1.restaurants.concat(state2.restaurants).slice(0, state1.max)
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          }
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        },
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        finalize: function(state) {                   // Adjust the state to only return field we need
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          return state.restaurants
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        }
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        lang: "js"
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      }
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    }
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  }
43
}
44
])