https://docs.mongodb.com/manual/reference/operator/aggregation-pipeline/

$match
$unwind
$group
$lookup
$count

db.packages.insertMany([
{
 name: "Package no. 2534",
 destination: "Alabama",
 items: [
  {name: "watch", type: "electronic", quantity: 6},
  {name: "baseball bat", type: "carpentry", quantity: 8},
  {name: "basil", type: "plant", quantity: 3}
 ]
},
{
 name: "Package no. 1",
 items: [
  {name: "handheld", type: "electronic", quantity: 8},
 ]
},
{
 name: "Packie",
 items: [
  {name: "Maybach", type: "car", quantity: 1},
  {name: "pitching machine", type: "electromechanical", quantity: 5},
  {name: "dasy", type: "plant", quantity: 401}
 ]
},
{
 name: "Big Box",
 items: [
  {name: "bath tub", type: "home appliance", quantity: 600},
  {name: "AC unit", type: "electric", quantity: 8645},
  {name: "football", type: "sport accesory", quantity: 34673}
 ]
}
]}

//pakiety bez destination
db.packages.find({destination:{$exists:false}})

//
> db.packages.find({destination:{$exists:0},"items.type":{$in:["electric", "electronic", "electromechanical"]}})

// items: quantity is divisible by 4
> db.packages.find( { "items.quantity": { $mod: [ 4, 0 ] } } ).pretty()

//

db.packages.find({destination:{$exists:0},"items.type":{$in:["electric", "electronic", "electromechanical"]}}{"items.name"})

//bez id i z nazwa tylko
> db.packages.find({destination:{$exists:0},"items.type":{$in:["electric", "electronic", "electromechanical"]}},{"_id":0,name:1})
{ "name" : "Package no. 1" }
{ "name" : "Packie" }
{ "name" : "Big Box" }



> db.inventory.updateMany({type:"other"}, {$set:{type:"other type"}, $inc:{quantity:1 }})
{ "acknowledged" : true, "matchedCount" : 2, "modifiedCount" : 2 }
>
>
>
>
>
>
>
>
>
> db.inventory.deleteMany({})
{ "acknowledged" : true, "deletedCount" : 6 }
> db.packgages.deleteMany({})
{ "acknowledged" : true, "deletedCount" : 0 }



> db.sillinesses.insertMany([
... {
...  silly: "steps",
...  bob: ["marley", "harley", "builder"],
...  coolness: 10.01
... },
... {
...  silly: "faces",
...  bob: ["bip", "pob", "goose", "smoose"],
...  coolness: 9.99
... },
... {
...  silly: "ideas",
...  bob: ["president", "minister", "senator", "congresman"],
...  coolness: -1000
... },
... {
...  silly: "hats",
...  bob: ["bean", "bread", "buffet", "barbeque"],
...  coolness: 11.3
... }])


//za punkty:
db.sillinesses.deleteMany({$and: [{bob: {$size:4}}, {coolness:{$gt:10}}]})

//final task:
> db.orders.insert([
... {
...   client:  {
...      firstName: "John",
...      lastName: "Smith" },
...   orderedItems: [
...      { name: "itemA", qty: 5},
...      { name: "itemS", qty: 3} ]
... },
... {
...   client:  {
...      firstName: "John",
...      lastName: "Jacobie" },
...   orderedItems: [
...      { name: "itemA", qty: 2},
...      { name: "itemB", qty: 4} ]
... },
... {
...   client:  {
...      firstName: "BOb",
...      lastName: "Smith" },
...   orderedItems: [
...      { name: "itemC", qty: 5},
...      { name: "itemD", qty: 7} ]
... },
... {
...   client:  {
...      firstName: "John",
...      lastName: "Scott" },
...   orderedItems: [
...      { name: "itemA", qty: 1},
...      { name: "itemD", qty: 3} ]
... },
... {
...   client:  {
...      firstName: "Scot",
...      lastName: "John" },
...   orderedItems: [
...      { name: "itemF", qty: 2},
...      { name: "itemE", qty: 8} ]
... },
... {
...   client:  {
...      firstName: "John",
...      lastName: "Scatman" },
...   orderedItems: [
...      { name: "itemF", qty: 13},
...      { name: "itemG", qty: 28} ]
... },
... {
...   client:  {
...      firstName: "Johny",
...      lastName: "B." },
...   orderedItems: [
...      { name: "itemE", qty: 5},
...      { name: "itemG", qty: 3} ]
... }])
BulkWriteResult({
        "writeErrors" : [ ],
        "writeConcernErrors" : [ ],
        "nInserted" : 7,
        "nUpserted" : 0,
        "nMatched" : 0,
        "nModified" : 0,
        "nRemoved" : 0,
        "upserted" : [ ]
})

> db.stock.insert([
... { item: "itemA", qty: 15},
... { item: "itemB", qty: 15},
... { item: "itemC", qty: 15},
... { item: "itemD", qty: 15},
... { item: "itemE", qty: 15},
... { item: "itemF", qty: 15},
... { item: "itemG", qty: 15}])
BulkWriteResult({
        "writeErrors" : [ ],
        "writeConcernErrors" : [ ],
        "nInserted" : 7,
        "nUpserted" : 0,
        "nMatched" : 0,
        "nModified" : 0,
        "nRemoved" : 0,
        "upserted" : [ ]
})

//1. step
db.orders.aggregate([
	{
		$unwind: "$orderedItems"
	},
	{
		$group:
		{
			_id:"$orderedItems.name",
			required:{$sum:"$orderedItems.qty"}
		}
	},
	{
		$lookup:
		{
			from:"stock",
			localField:"_id",
			foreignField:"item",
			as: "supply"
		}
	},
		$project:
		{
			_id: 0,
			item:"$_id",
			missing:
			{
				$let:
				{
					vars:
					{
						sup:
						{
							$arrayElemAt:["$supply",0]
						}
					}
					in:
					{
							//JEST  NA TEL
					}
				}
			}
		}
])




//2. ste[