use mark


db.books.drop()
db.step1.drop()

db.books.insertMany( [
       {name : "book1", locations : [ { "store1" : 1}, { "store2" : 2}, { "store3" : 3} ], type : 'fiction' } ,
       {name : "book2",  locations : [ { "store1" : 4}, { "store3" : 5}, { "store4" : 6} ], type : 'fiction' } ,
       {name : "book2",  locations : [ { "store6" : 13}, { "store9" : 14}, { "store13" : 15} ], type : 'fiction' } ,
       {name : "book3", locations : [ { "store2" : 7}, { "store4" : 8}, { "store5" : 9} ], type : 'nonfiction'  }
       {name : "book3", locations : [ { "store5" : 10}, { "store6 : 11}, { "store5" : 12} ], type : 'nonfiction'  }
] );


print ("First aggregation just filters 'book1' out of documents.")
db.books.aggregate([
   {$match : { 'name' : 'book1'}},
   { $out : "step1" }
])

db.step1.find().forEach(printjson);

print ("Second aggregation counts total for each book entry.")
db.books.aggregate([
   {
      { _id: "$name", totalQuantity: { $sum: "$quantity" } }
   }
   { $out : "step1" }
])

db.step1.find().forEach(printjson);

print (' many more examples will be added')