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small notes in mongo
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‎General-Soft/Give-me-a-challenging-assignment-you-have-handled.md

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### A challenging case or assignment or technical problem you have solved

‎MongoDB/delete-single-document-from-collection.md

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### Delete a single document from collection
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Say I have the following in `users` collections (i.e. when I run `db.users.find().pretty()` I get the below)
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```
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{
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"_id" : ObjectId("5bd982dea18ceecf4bba3c47"),
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"username" : "test-user-2",
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"post" : "123456"
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}
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{
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"_id" : ObjectId("5bd983e4a18ceecf4bba3c48"),
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"username" : "p@gmail.com",
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"password" : "$2a$10$HyXCD5.4U/0CvZHq9SDQ0uxD12BQ46yVAHu18lRRVEQZB3uyHXgy.",
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"__v" : 0
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}
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```
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And to delete just the top-most document with id of "5bd982dea18ceecf4bba3c47" run the below command
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```
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try {
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db.users.deleteOne( { "_id" : ObjectId("5bd982dea18ceecf4bba3c47") } );
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} catch (e) {
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print(e);
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}
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```
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[https://docs.mongodb.com/manual/reference/method/db.collection.deleteOne/](https://docs.mongodb.com/manual/reference/method/db.collection.deleteOne/)
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And now running `db.users.find().pretty()` will give me just
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```
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{
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"_id" : ObjectId("5bd983e4a18ceecf4bba3c48"),
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"username" : "p@gmail.com",
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"password" : "$2a$10$HyXCD5.4U/0CvZHq9SDQ0uxD12BQ46yVAHu18lRRVEQZB3uyHXgy.",
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"__v" : 0
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}
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```

‎MongoDB/deleting-collection-and-whole-document.md

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### To remove/delete all data under field name “messages” within an existing database - $ db.messages.remove({})
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1> To remove/delete all data under field name (or collection name ) “messages” within any existing databae -
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`$ db.messages.remove({})`
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But the above will NOT completely remove this collection. It will only empty this collection of all the documents in it.
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### TO DROP THE COLLECTION COMPLETELY
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`$ db.mesages.drop()`
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drop() method will return true, if the selected collection is dropped successfully, otherwise it will return false.
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To REMOVE / DELETE the whole database
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`$ show dbs`
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I will get the below in my terminal
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```
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employee 0.000GB
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mern-book-library 0.000GB
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mern-crud 0.000GB
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```
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then first move into that database before deleting
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`$ use employee`
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`$ db.dropDatabase()`

‎MongoDB/populate-method-mongoose.md renamed to ‎MongoDB/populate-method-mongoose-referencing-other-model.md

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@@ -78,7 +78,7 @@ function getUserWithPosts(username) {
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}
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```
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.populate() needs a query to attach itself to, so we are using User.findOne() to find a user who matches the username we provide in the argument. This returns our user document. This is when .populate() takes over.
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`.populate()` needs a query to attach itself to, so we are using User.findOne() to find a user who matches the username we provide in the argument. This returns our user document. This is when .populate() takes over.
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#### You’ll notice I am providing ‘posts’ to our .populate(). By providing the ‘posts’ argument, we’ve told .populate() what property in our user document we want it to work with. Calling .exec() just executes something once .populate() has done it’s thing. The log prints this:
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‎MongoDB/quick-mongodb-snippets.md

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### Inserting new attribute to an existing document in MongoDB
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Say first I have the below user collection with only one document, and I can see that by running `db.users.find().pretty()` . Now I want to add another attribute or property to this document named 'port' and given this attribute is of `Schema.Types.ObjectId` type in `user` collection, I have to add the corresponding `port's` \_id field to it as its value. So lets say that field is - "5bd983e4a18ceecf4bba3c48"
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```
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{
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"_id" : ObjectId("5bd983e4a18ceecf4bba3c48"),
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"username" : "p@gmail.com",
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"password" : "$2a$10$HyXCD5.4U/0CvZHq9SDQ0uxD12BQ46yVAHu18lRRVEQZB3uyHXgy.",
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"__v" : 0
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}
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```
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So the command to add `port` value is
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```
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db.users.update({'_id' : ObjectId("5bd983e4a18ceecf4bba3c48")},
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{'$set' : {'port' : "5bdaadc9ebae106aaa8ca247" }})
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```

‎Node-Express/bcrypt-How-it-works.md renamed to ‎Node-Express/bcrypt-How-it-works-de-hashing.md

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@@ -28,29 +28,30 @@ So from [official doc](https://github.com/dcodeIO/bcrypt.js#usage---async) the b
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```js
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var salt = bcrypt.genSaltSync(10);
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var hash = bcrypt.hashSync("B4c0/\/", salt);
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var hash = bcrypt.hashSync("B4c0//", salt);
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// And to hash a password the ES6 Async way is combine them both without blocking any other operation
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bcrypt.genSalt(10, function(err, salt) {
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bcrypt.hash("B4c0/\/", salt, function(err, hash) {
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// Store hash in your password DB.
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});
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bcrypt.genSalt(10, function(err, salt) {
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bcrypt.hash("B4c0//", salt, function(err, hash) {
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// Store hash in your password DB.
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});
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});
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```
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Bcrypt allows us to choose the value of saltRounds, which gives us control over the cost of processing the data. The higher this number is, the longer it takes for the machine to calculate the hash associated with the password. It is important when choosing this value, to select a number high enough that someone who tries to find the password for a user by brute force, requires so much time to generate all the possible hash of passwords that does not compensate him. And on the other hand, it must be small enough so as not to end the user’s patience when registering and logging in (this patience is not usually very high). By default, the saltRounds value is 10.
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### One example of the first step of generating the salt and hashing - [https://github.com/rohan-paul/Tiny-Twitter-Clone/blob/master/models/user.js](https://github.com/rohan-paul/Tiny-Twitter-Clone/blob/master/models/user.js)
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```js
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UserSchema.pre('save', function(next) {
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UserSchema.pre("save", function(next) {
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let user = this; // This is how I access UserSchema object
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// I shall only hash the password if it has been modified (or is new). So, in below line I make sure if there was already a password and isModified in not true, then move-on with next()
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if(!user.isModified('password')) return next();
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if (!user.isModified("password")) return next();
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// and for new password
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if(user.password) {
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if (user.password) {
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bcrypt.genSalt(10, function(err, salt) {
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if (err) return next(err);
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bcrypt.hash(user.password, salt, null, function(err, hash) {
@@ -63,87 +64,84 @@ UserSchema.pre('save', function(next) {
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});
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```
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Explanation of ``UserSchmea.pre`` in above - Its the middleware - also known as “pre” and “post” hooks that tie particular functions to particular lifecycle and query events. This middleware is defined on the schema level and can modify the query or the document itself as it is executed. Middleware is invoked with two arguments: the event trigger (as a string) and the callback function that is triggered for that particular event. The callback itself takes in an argument of a function, which we typically call next , and when invoked — advances the document/query to the next awaiting middleware.
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Explanation of `UserSchmea.pre` in above - Its the middleware - also known as “pre” and “post” hooks that tie particular functions to particular lifecycle and query events. This middleware is defined on the schema level and can modify the query or the document itself as it is executed. Middleware is invoked with two arguments: the event trigger (as a string) and the callback function that is triggered for that particular event. The callback itself takes in an argument of a function, which we typically call next , and when invoked — advances the document/query to the next awaiting middleware.
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So what the below function does is - before (i.e. pre) user saves the normal text password into the database, making sure it encrypts it first
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### Example from my [dev-book app - https://github.com/rohan-paul/Developer-Profile-App/blob/master/routes/api/users.js](https://github.com/rohan-paul/Developer-Profile-App/blob/master/routes/api/users.js)
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### First hashing and then save the hashed password into mongo
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```js
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// and then get other details of the new user from the post request
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const newUser = new User({
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name: req.body.name,
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email: req.body.email,
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avatar,
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password: req.body.password
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});
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bcrypt.genSalt(10, (err, salt) => {
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bcrypt.hash(newUser.password, salt, (err, hash) => {
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if (err) throw err;
85-
newUser.password = hash;
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newUser
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.save()
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.then(user => res.json(user))
89-
.catch(user => console.log(err))
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});
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});
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const newUser = new User({
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name: req.body.name,
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email: req.body.email,
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avatar,
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password: req.body.password
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});
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bcrypt.genSalt(10, (err, salt) => {
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bcrypt.hash(newUser.password, salt, (err, hash) => {
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if (err) throw err;
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newUser.password = hash;
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newUser
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.save()
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.then(user => res.json(user))
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.catch(user => console.log(err));
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});
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});
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```
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### Next step before signing-in a new user, it compares the password with that saved in the database
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Example [https://github.com/rohan-paul/Developer-Profile-App/blob/master/routes/api/users.js](https://github.com/rohan-paul/Developer-Profile-App/blob/master/routes/api/users.js)
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```js
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router.post('/login', (req, res) => {
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/* I pass the data to the validateRegisterInput() function. The data (i.e. req.body) includes
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router.post("/login", (req, res) => {
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/* I pass the data to the validateRegisterInput() function. The data (i.e. req.body) includes
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all the information that the user puts in while registering.
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And get the function's return values assigned to const { errors, isValid }. */
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const { errors, isValid } = validateLoginInput(req.body);
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// Check Validation
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if(!isValid) {
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return res.status(400).json(errors);
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}
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const { errors, isValid } = validateLoginInput(req.body);
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// Check Validation
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if (!isValid) {
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return res.status(400).json(errors);
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}
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const email = req.body.email;
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const password = req.body.password;
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const email = req.body.email;
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const password = req.body.password;
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//Check if the user email exists at all in database
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User.findOne({ email }).then(user => {
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if (!user) {
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errors.email = 'User not found';
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return res.status(404).json({email: 'User not found'});
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}
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//Check if the user email exists at all in database
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User.findOne({ email }).then(user => {
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if (!user) {
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errors.email = "User not found";
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return res.status(404).json({ email: "User not found" });
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}
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// Compare password hash saved in database with the password provided in the req.body
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bcrypt.compare(password, user.password).then(isMatch => {
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if(isMatch) {
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// if User is matched, then create JWT Payload
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const payload = { id: user.id, name: user.name, avatar: user.avatar };
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// signed JWT token to be sent to server
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jwt.sign(
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payload,
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keys.secretOrKey,
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{ expiresIn: 3600 },
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(err, token) => {
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res.json({
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success: true,
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token: 'Bearer ' + token
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});
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}
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);
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} else {
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errors.password = 'Password Incorrect';
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return res.status(400).json(errors);
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}
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});
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// Compare password hash saved in database with the password provided in the req.body
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bcrypt.compare(password, user.password).then(isMatch => {
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if (isMatch) {
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// if User is matched, then create JWT Payload
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const payload = { id: user.id, name: user.name, avatar: user.avatar };
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// signed JWT token to be sent to server
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jwt.sign(
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payload,
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keys.secretOrKey,
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{ expiresIn: 3600 },
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(err, token) => {
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res.json({
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success: true,
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token: "Bearer " + token
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});
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}
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);
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} else {
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errors.password = "Password Incorrect";
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return res.status(400).json(errors);
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}
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});
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});
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});
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```
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### Authentication at the time of signing in a user
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```js
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bcrypt.compare = function(s, hash, callback, progressCallback) {
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function _async(callback) {
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if (typeof s !== "string" || typeof hash !== "string") {
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nextTick(
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callback.bind(
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this,
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Error("Illegal arguments: " + typeof s + ", " + typeof hash)
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)
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);
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return;
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}
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if (hash.length !== 60) {
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nextTick(callback.bind(this, null, false));
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return;
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}
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bcrypt.hash(
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s,
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hash.substr(0, 29),
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function(err, comp) {
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if (err) callback(err);
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else callback(null, safeStringCompare(comp, hash));
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},
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progressCallback
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);
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}
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function _async(callback) {
162-
if (typeof s !== "string" || typeof hash !== "string") {
163-
nextTick(callback.bind(this, Error("Illegal arguments: "+(typeof s)+', '+(typeof hash))));
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return;
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}
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if (hash.length !== 60) {
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nextTick(callback.bind(this, null, false));
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return;
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}
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bcrypt.hash(s, hash.substr(0, 29), function(err, comp) {
171-
if (err)
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callback(err);
173-
else
174-
callback(null, safeStringCompare(comp, hash));
175-
}, progressCallback);
183+
if (callback) {
184+
if (typeof callback !== "function")
185+
throw Error("Illegal callback: " + typeof callback);
186+
_async(callback);
187+
} else
188+
return new Promise(function(resolve, reject) {
189+
_async(function(err, res) {
190+
if (err) {
191+
reject(err);
192+
return;
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}
177-
178-
if (callback) {
179-
if (typeof callback !== 'function')
180-
throw Error("Illegal callback: "+typeof(callback));
181-
_async(callback);
182-
} else
183-
return new Promise(function(resolve, reject) {
184-
_async(function(err, res) {
185-
if (err) {
186-
reject(err);
187-
return;
188-
}
189-
resolve(res);
190-
});
191-
});
192-
};
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194+
resolve(res);
195+
});
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});
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};
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```
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### Online bcrypt hashing and de-hashing generator and checker
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[https://bcrypt-generator.com/](https://bcrypt-generator.com/)
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Just put the rounds (which is the salt length to generate, i.e. the function wheere I am hashing the plain-text password )
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``bcrypt.hashSync(plainTextPassword, 10)`` So the number 10 is the rounds in the above online tool
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After hashing a plaintext password, for checking I will just put the hashed password from the mongo database - i.e. after running terminal command something like ``db.users.find()`` which will give all the users saved in the mongo database.
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So an example is this hashed password - ``$2a$10$m0mq4PYOOvm74Gukml4FN.T0Ntobhzi42T6b5v1WIsJ5aZkVzJz3a`` And then put the round as 10 and I will get ``123`` which was my plaintext password in this case.
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Just put the rounds (which is the salt length to generate, i.e. the function where I am hashing the plain-text password )
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`bcrypt.hashSync(plainTextPassword, 10)` So the number 10 is the rounds in the above online tool
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After hashing a plaintext password, for checking I will just put the hashed password from the mongo database - i.e. after running terminal command something like `db.users.find()` which will give all the users saved in the mongo database.
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So an example is this hashed password - `$2a$10$m0mq4PYOOvm74Gukml4FN.T0Ntobhzi42T6b5v1WIsJ5aZkVzJz3a` And then put the round as 10 and I will get `123` which was my plaintext password in this case.

‎ONLY-Questions-Collections/Java-Mongo-DS-SAP-Interview.md renamed to ‎ONLY-Questions-Collections-Yet-to-draft-ans/Java-Mongo-DS-SAP-Interview.md

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‎ONLY-Questions-Collections/MongoDB-Questions.md renamed to ‎ONLY-Questions-Collections-Yet-to-draft-ans/MongoDB-Questions.md

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‎ONLY-Questions-Collections/React-Redux-Questions-To-Prepare-ans-for.md renamed to ‎ONLY-Questions-Collections-Yet-to-draft-ans/React-Redux-Questions-To-Prepare-ans-for.md

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‎ONLY-Questions-Collections/web-development.-general.md renamed to ‎ONLY-Questions-Collections-Yet-to-draft-ans/web-development.-general.md

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