So it is Mongoose

Connections
An application can only connect up to one database in such a way, mongoose.connection Obtain a database connection

var mongoose = require('mongoose');
var db = mongoose.connection;
db.once('open', function () {
    console.log('connect mongodb successful...');
});
db.on('error', function (err) {
    console.error('err', err);
});
mongoose.connect('mongodb://localhost:27017/Lyf');

An application can connect multiple databases in such a way, mongoose.createConnection returns the database connection

var mongoose =require('mongoose');
var db1=mongoose.createConnection('mongodb://localhost:27017/Lyf');
var db2=mongoose.createConnection('mongodb://localhost:27017/blog');
db1.once('open', function () {
    console.log('connect Lyf mongodb successful...');
});
db1.on('error', function (err) {
    console.error('err', err);
});
db2.once('open', function () {
    console.log('connect blog mongodb successful...');
});
db2.on('error', function (err) {
    console.error('err', err);
});

Schema
Types when building schema: Number [] {} String Boolean ObjectId Mixed Schema Buffer Schema.methods.fn can used by instance of Model Schema.statics.fn can used by Schema Schema.virtual('vName').get(fn) , Schema.virtual('vName').set(fn) invalidate validate data legality, enum:[] min max required:true validate:[fn,'errMes'] unique:true

var mongoose =require('mongoose');
var Schema=mongoose.Schema;
options={
    autoIndex: false,//Default true, indexes are added automatically
    strict: false,//Default true, Strictly check the data
    capped: 100,//Operational data limit
    versionKey: '_name',// Version lock, the default _v
    collection: 'name'//The name of the collection in the database
    }
var IphoneSchema=new Schema({
    iphone:Number
});
var UserSchema=new Schema({
    name:{type:String,validate:[function(value){
    if(value.lenght<2)
       return false
    else
       return true
    },'path name field Invalid']},
    age:{type:Number,min:0},
    teen:Boolean,
    interesting:Schema.Types.Mixed,
    id_card:{type:Schema.Types.ObjectId,required:true},
    child:[Schema.Types.ObjectId],
    location:{
        country:{type:String,enum:['china','us']},
        province:String,
        city:String
    },
    iphone:IphoneSchema,
    file:Buffer
},options);
UserSchema.methods = {
    say: function () {
        console.log(`say->${this.name}`);
    }
}
UserSchema.statics = {
    say: function () {
        console.log(`say static`);
    }
}
UserSchema.virtual('basicInfo').get(function () {
    return this.name + ',' + this.age;
});
UserSchema.virtual('basicInfo').set(function (value) {
    var split = value.split(',');
    this.name = split[0];
    this.age = split[1];
});

model
compiling model by db.model('name',Schema)
use the instance of model -> save
use the Model class static -> create , can insert multi doc by once

var mongoose = require('mongoose');
var db = mongoose.connection;
db.once('open', function () {
    console.log('connect mongodb successful...');
});
db.on('error', function (err) {
    console.error('err', err);
});
mongoose.connect('mongodb://localhost:27017/Lyf');
var UserSchema = new mongoose.Schema({
    name: String
})
UserModel = db.model('user', UserSchema)
new UserModel({name: 'Lyf'}).save((err)=> {
    if (!err)
        console.log('ok')
})
UserModel.create([{name:'L'},{name:'yf'}],(err)=>{
    if (!err)
        console.log('ok')
})

Middleware
ps:The following is the middleware I used pre is used before save or remove post is used after save or remove

Schema.pre('save',function(next){
//...
next();
})
Schema.post('save',function(doc){
//...
})

populate populate(options) options={ path match select options } there are no joins in MongoDB,so use populate+ref

var mongoose = require('mongoose');
var db = mongoose.connection;
db.once('open', function () {
    console.log('connect mongodb successful...');
});
db.on('error', function (err) {
    console.error('err', err);
});
mongoose.connect('mongodb://localhost:27017/Lyf');
var BookSchema = new mongoose.Schema({
    _id: Number,//`@ _id:Number `
    title: String
});
var UserSchema = new mongoose.Schema({
    name: String,
    i_book: {type: Number, ref: 'book'}//`@book` `@Number`
})
UserModel = db.model('user', UserSchema);
BookModel = db.model('book', BookSchema);//`@book `
UserModel
    .findOne({name: 'Lyf'})
    .populate({path: 'i_book', select: {title: 1}, match: {title: '1001 '}})
    .exec(function (err, docs) {
        console.log(docs);
    });

Inline Array
As long as auth = Lyf or age = 24 {} in the array returns the doc

LModel.findOne({'names.auth': 'Lyf', 'names.age': 24}, {names: 1, _id: 0}, function (err, docs) {});

As long as auth=Lyf and age=23 in the array returns the doc

LModel.findOne({names: {$elemMatch: {auth: 'Lyf', age: 23}}}, {}, function (err, docs) {});

Inserts a record into the array of doc

LModel.update({}, {$push: {names: {auth: 'Wtl', age: 23}}}, {multi: true}, function (err, count) {});

Removes the matching record from the array of doc

LModel.update({}, {$pull: {names: {age: 23}}}, function (err, count) {})

Insert a unique record into the array of doc

LModel.update({}, {$addToSet: {names: {auth: 'Lyf', age: 23}}}, function (err, count) {});

Removes a record from the header (1) or trailer (-1) of the array in the document

LModel.update({}, {$pop: {names: -1}}, function (err, count) {})

Update a record in the document array .$. as a locator

LModel.update({'names.auth': 'Lyf'}, {$set: {'names.$.age': 24}}, function (err, count) {})

Frequently used keywords find update remove count gt gte lt lte ne set limit skip sort exists elemMatch in nin and or size multi upsert