$accumulator (aggregation)

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([
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{
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  $group :
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  {
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    _id : { city: "$city" },
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    restaurants:
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    {
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      $accumulator:
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      {
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        init: function(city, userProfileCity) {        // Set the initial state
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          return {
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            max: city === userProfileCity ? 3 : 1,     // If the group matches the user's city, return 3 restaurants
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            restaurants: []                            // else, return 1 restaurant
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          }
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        },
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        initArgs: ["$city", <userProfileCity>],        // Argument to pass to the init function
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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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  }
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}
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])