/*    1. Query Operators*/db.posts.find({    viewsCount: {$get: 1000, $lte: 3000}}, {_id: 0, viewsCount: 1, title: 1})// $indb.posts.find({    categories: {$in: [‘ios‘]}}, {categories: 1})//$wheredb.posts.find({    $where: function(){        return this.categories.length>15    }}, {categories: 1, title: 1})//see how many categories from previous query:db.posts.find({    $where: function(){        return this.categories.length>15    }}, {categories: 1, title: 1})[0].categories.length

/*    2. Update*/var d = db.posts.findOne();//change the author email:d.author.email = "new@new.com"//update the databasedb.post.update({_id: d._id}, d);//update mutli-documentdb.posts.update({viewsCount: {$gt: 3000}}, {$set: {newKey: true}}, {multi:true}) //add new key, set mutli

/*    3. Performance and indexes*///create the datadb.numbers.drop();var num = 5000000,i = 0; for(i = 0; i < num; i ++){    var randomNumber = Math.floor(Math.random()*10000);    db.numbers.insert({number:randomNumber});    //print(i); }db.number.find({number: {$gt: 1000, $lt: 3000}, {_id: 0, number: 1}}).explain(); //it scan all the doucment, took 3500ms //add index:db.numbers.ensureIndex({number: 1});//took 1046 ms, scan 1m instead of 5m 

/*    4. $group*///sum up the viewCount, rename the result as ‘totalViewsCount‘db.posts.aggregate({    $group: {        _id:null,        totalViewsCount: {$sum: ‘$viewsCount‘}    }})//adding 1 for each documentdb.posts.aggregate({    $group: {        _id:null,        totalNumberOfDocuments: {$sum: 1}    }})// =db.posts.count()