[MongoDB] Query, update, index and group

/*
    1. Query Operators
*/
db.posts.find({
    viewsCount: {$get: 1000, $lte: 3000}
}, {_id: 0, viewsCount: 1, title: 1})

// $in
db.posts.find({
    categories: {$in: ['ios']}
}, {categories: 1})

//$where
db.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

Update:

/*
    2. Update
*/
var d = db.posts.findOne();
//change the author email:
d.author.email = "[e-mail]"
//update the database
db.post.update({_id: d._id}, d);

//update mutli-document
db.posts.update({viewsCount: {$gt: 3000}}, {$set: {newKey: true}}, {multi:true}) //add new key, set mutli

Performance and indexes:

/*
    3. Performance and indexes
*/
//create the data
db.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 

Group:

/*
    4. $group
*/

//sum up the viewCount, rename the result as 'totalViewsCount'
db.posts.aggregate({
    $group: {
        _id:null,
        totalViewsCount: {$sum: '$viewsCount'}
    }
})

//adding 1 for each document
db.posts.aggregate({
    $group: {
        _id:null,
        totalNumberOfDocuments: {$sum: 1}
    }
})

// =
db.posts.count()