Posted on Nov 10, 2024

Today’s new knowledge #1

The first thing I do today is refresh my mongodb knowledge as it's an essential part of my Advanced Web Development course curriculum. and the key concepts i learned today are:

Simple CURD Operation on MongoDB

insert single document --> db.collection.insertOne();
insert multiple document --> db.collection.insertMeny();
find single document --> db.collection.findOne();
find multiple document --> db.collection.find();
update single document --> db.collection.updateOne();
update multiple document --> db.collection.updateMeny();
delete single document --> db.collection.deleteOne();
delete multiple document --> db.collection.deleteMany();

MongoDB Comparison Operators

MongoDB comparison operators allow you to filter documents by comparing field values.
$eq: Matches values that are equal to a specified value.
$ne: Matches values that are not equal to a specified value.
$gt: Matches values that are greater than a specified value.
$gte: Matches values that are greater than or equal to a specified value.
$lt: Matches values that are less than a specified value.
$lte: Matches values that are less than or equal to a specified value.
$in: Matches any of the values specified in an array.
$nin: Matches none of the values specified in an array.
Suppose we have a collection called users, with documents structured like this:

{ "_id": 1, "name": "Alice", "age": 25, "status": "A" } { "_id": 2, "name": "Bob", "age": 30, "status": "B" } { "_id": 3, "name": "Charlie", "age": 35, "status": "A" } { "_id": 4, "name": "David", "age": 28, "status": "C" } { "_id": 5, "name": "Eve", "age": 40, "status": "B" } { "_id": 6, "name": "Frank", "age": 22, "status": "A" }

Now, let’s make a query that show each comparison operator.

//method one db.users.find({ $and: [ // Find documents where "age" is exactly 25 { age: { $eq: 25 } }, // and where "age" is NOT 30 { age: { $ne: 30 } }, // and where "age" is greater than 20 { age: { $gt: 20 } }, // and where "age" is 25 or higher { age: { $gte: 25 } }, // and where "age" is less than 40 { age: { $lt: 40 } }, // and where "age" is 35 or lower { age: { $lte: 35 } }, // and where "status" is either "A" or "B" { status: { $in: ["A", "B"] } }, // and where "status" is NOT "C" or "D" { status: { $nin: ["C", "D"] } } ] }) //method two db.users.find({age: {$eq:25, $ne:30, $gt: 20, $gte: 25, $lt:40, $lte: 35}, status:{$in:["A", "B"], $nin:["C","D"]} }) //output: { "_id": 1, "name": "Alice", "age": 25, "status": "A" }

method one is explicit and method
method two is inplicit and method

MongoDB’s logical operators

MongoDB’s logical operators allow you to combine and manipulate conditions within a query.

The $and operator in MongoDB allows you to combine multiple conditions in a single query.
The $or operator in MongoDB allows you to return documents that meet at least one of the conditions.
The $nor operator in MongoDB allows you to return documents that do not match the specified condition.
The $not operator in MongoDB Inverts the effect of a condition, returning documents that do not match the specified condition.

Now, let’s make a query that show each logical operator.

db.users.find({ $and: [ { $or: [{ status: "A" }, { age: { $lt: 25 } }] }, { age: { $gt: 20 } }, { age: { $not: { $gt: 35 } } }, { $nor: [{ status: "B" }, { age: 30 }] } ] }) //output { "_id": 1, "name": "Alice", "age": 24, "status": "A" } { "_id": 3, "name": "Charlie", "age": 28, "status": "A" } { "_id": 6, "name": "Frank", "age": 22, "status": "A" }

MongoDB’s element operators

$exists: Checks if a field exists in the document.
$type: checks if the mensioned typed exits in the document.

Now, let’s make a query that show each logical operator.

 db.users.find({ "age": { $type: "int" } }) db.users.find({ "status": { $exists: true } }) //output [ { "_id": 1, "name": "Alice", "age": 25, "status": "A" }, { "_id": 2, "name": "Bob", "age": 30, "status": "B" }, { "_id": 3, "name": "Charlie", "age": 35, "status": "A" }, { "_id": 4, "name": "David", "age": 28, "status": "C" }, { "_id": 5, "name": "Eve", "age": 40, "status": "B" }, { "_id": 6, "name": "Frank", "age": 22, "status": "A" } ]

MongoDB’s Array operators

$all: Matches arrays that contain all elements specified.
$elemMatch: Matches elements within an array based on specified criteria.
$size: Matches arrays with the specified number of elements.

Suppose we have a collection called students , with documents structured like this:

[ { "_id": 1, "name": "Alice", "grades": [85, 90, 92], "subjects": ["Math", "English", "History"] }, { "_id": 2, "name": "Bob", "grades": [78, 80, 85], "subjects": ["Math", "Science", "English"] }, { "_id": 3, "name": "Charlie", "grades": [95, 98, 93], "subjects": ["Math", "History"] }, { "_id": 4, "name": "David", "grades": [60, 70, 68], "subjects": ["Math", "Science"] } ]

Now, let’s make a query that show each Array operator.

db.students.find({ "grades": { $elemMatch: { $gt: 90 } } }) //output  [ { "_id": 1, "name": "Alice", "grades": [85, 90, 92], "subjects": ["Math", "English", "History"] }, { "_id": 3, "name": "Charlie", "grades": [95, 98, 93], "subjects": ["Math", "History"] } ] db.students.find({ "subjects": { $size: 3 } }) //output  [ { "_id": 1, "name": "Alice", "grades": [85, 90, 92], "subjects": ["Math", "English", "History"] }, { "_id": 2, "name": "Bob", "grades": [78, 80, 85], "subjects": ["Math", "Science", "English"] } ] db.students.find({ "subjects": { $all: ["Math", "History"] } }) //output  [ { "_id": 1, "name": "Alice", "grades": [85, 90, 92], "subjects": ["Math", "English", "History"] }, { "_id": 3, "name": "Charlie", "grades": [95, 98, 93], "subjects": ["Math", "History"] } ]

Those operator are helpful in case of array and object filltering

MongoDB’s update operators

MongoDB provides several update operators that allow you to modify existing documents in a collection.
$set: Used to update the value of a field. If the field does not exist, it will be created.
$unset: Used to remove a field from a document.
$addToSet: Adds a value to an array only if the value does not already exist in the array (similar to a set).
$push: Adds a value to an array field. If the field does not exist, it will be created as an array.
$each: operator is used in conjunction with the $push and $addToSet operators to add multiple elements to an array field in a single update operation.
$pop: Removes the first or last element of an array.1 is used to remove last element and -1 is used to remove first element.
$pull: Removes all instances of a value from an array that match a specified condition.
$pullAll: operator is used to remove multiple specific values from an array field.
$rename: Renames a field in a document.

Now, let’s make a query that show each update operator.

db.students.updateOne( { "_id": 2 },{ $set: { "graduated": false } } ) //output  { "_id": 2, "name": "Bob", "grades": [78, 80, 85], "subjects": ["Math", "Science", "English"], "graduated": false }, db.students.updateOne( { "_id": 2 }, { $unset: { "subjects": "" }} ) //output { "_id": 2, "name": "Bob", "grades": [78, 80, 85], "graduated": false }, db.students.updateOne( { "_id": 2 }, { $addToSet: { "subjects": "Geography" }} ) //output { "_id": 1, "name": "Alice", "grades": [85, 90, 92], "subjects": ["Math", "English", "History","Geography"] }, db.students.updateOne( { "_id": 1 }, { $push: { "grades": 88 }} ) //output { "_id": 1, "name": "Alice", "grades": [85, 90, 92, 88], "subjects": ["Math", "English", "History","Geography"] } db.students.updateOne( { "_id": 1 }, { $push: { "grades": { $each: [ 91, 82] }}} ) //output { "_id": 1, "name": "Alice", "grades": [85, 90, 92, 88, 91, 82], "subjects": ["Math", "English", "History","Geography"] } db.students.updateOne( { "_id": 1 },{ $pop: { "grades": 1 } // 1 for the last element, -1 for the first element } ) //output { "_id": 1, "name": "Alice", "grades": [85, 90, 92, 88, 91], "subjects": ["Math", "English", "History","Geography"] } db.students.updateOne( { "_id": 1 },{ $pull: { "grades": 90 }} ) //output { "_id": 1, "name": "Alice", "grades": [85, 92, 88, 91], "subjects": ["Math", "English", "History","Geography"] } db.students.updateOne( { "_id": 1 }, { $pullAll: { "grades": [80, 92] }} ) //output { "_id": 1, "name": "Alice", "grades": [85, 88, 91], "subjects": ["Math", "English", "History","Geography"] } db.students.updateOne( { "_id": 1 },{ $rename: { "name": "firstName" }} ) //output { "_id": 1, "firstName": "Alice", "grades": [85, 88, 91], "subjects": ["Math", "English", "History","Geography"] }