MongoDB Coding Practice
Q. Mention the command to insert a document in a database called company and collection called employee?
use company;
db.employee.insert( { name: "John", email: "john.k@gmail.com" } )
Q. Mention the command to check whether you are on the master server or not?
db.isMaster()
Q. How to combine data from multiple collections into one collection?
$lookup:
Performs a left outer join to an unsharded collection in the same database to filter in documents from the “joined” collection for processing. To each input document, the $lookup stage adds a new array field whose elements are the matching documents from the “joined” collection. The $lookup stage passes these reshaped documents to the next stage.
Syntax:
{
$lookup:
{
from: <collection to join>,
localField: <field from the input documents>,
foreignField: <field from the documents of the "from" collection>,
as: <output array field>
}
}
Q. How do I perform the SQL JOIN equivalent in MongoDB?
// Sample Records
comments
{ uid:12345, pid:444, comment="blah" }
{ uid:12345, pid:888, comment="asdf" }
{ uid:99999, pid:444, comment="qwer" }
users
{ uid:12345, name:"john" }
{ uid:99999, name:"mia" }
Answers
{
$lookup:
{
from: <collection to join>,
localField: <field from the input documents>,
foreignField: <field from the documents of the "from" collection>,
as: <output array field>
}
}
Q. How to query MongoDB with “like”?
db.users.insert({name: 'paulo'})
db.users.insert({name: 'patric'})
db.users.insert({name: 'pedro'})
db.users.find({name: /a/}) //like '%a%'
db.users.find({name: /^pa/}) //like 'pa%'
db.users.find({name: /ro$/}) //like '%ro'
Q. Find objects between two dates MongoDB?
Operator $gte and $lt is used to find objects between two dates in MongoDB.
Example: Creating a collection
>db.order.insert({"OrderId":1,"OrderAddrees":"US","OrderDateTime":ISODate("2020-02-19")};
WriteResult({ "nInserted" : 1 })
>db.order.insert({"OrderId":2,"OrderAddrees":"UK","OrderDateTime":ISODate("2020-02-26")};
WriteResult({ "nInserted" : 1 })
Display all documents from the collection using find() method.
> db.order.find().pretty();
// Output
{
"_id" : ObjectId("5c6c072068174aae23f5ef57"),
"OrderId" : 1,
"OrderAddrees" : "US",
"OrderDateTime" : ISODate("2020-02-19T00:00:00Z")
}
{
"_id" : ObjectId("5c6c073568174aae23f5ef58"),
"OrderId" : 2,
"OrderAddrees" : "UK",
"OrderDateTime" : ISODate("2020-02-26T00:00:00Z")
}
Here is the query to find objects between two dates:
> db.order.find({"OrderDateTime":{ $gte:ISODate("2020-02-10"), $lt:ISODate("2020-02-21") }
}).pretty();
// Output
{
"_id" : ObjectId("5c6c072068174aae23f5ef57"),
"OrderId" : 1,
"OrderAddrees" : "US",
"OrderDateTime" : ISODate("2020-02-19T00:00:00Z")
}
Q. Is it possible to update MongoDB field using value of another field?
The aggregate function can be used to update MongoDB field using the value of another field.
Example
db.collection.<update method>(
{},
[
{"$set": {"name": { "$concat": ["$firstName", " ", "$lastName"]}}}
]
)
Q. How to check if a field contains a substring?
The $regex operator can be used to check if a field contains a string in MongoDB.
db.users.findOne({"username" : {$regex : ".*some_string.*"}});
Q. How to get the last N records from find?
// Syntax
db.<CollectionName>.find().sort({$natural:-1}).limit(value)
// Example
db.employee.find().sort({$natural:-1}).limit(100)
Q. How to remove a field completely from a MongoDB document?
How do I remove words completely from all the documents in this collection?
{
name: 'book',
tags: {
words: ['abc','123'], // <-- remove it comletely
lat: 33,
long: 22
}
}
Answer
db.example.update({}, {$unset: {words: 1}}, false, true);
Q. How to find MongoDB records where array field is not empty?
db.inventory.find({ pictures: { $exists: true, $ne: [] } })
Q. How to find document with array that contains a specific value?
Populate the inventory collection
db.inventory.insertMany([
{ item: "journal", qty: 25, tags: ["blank", "red"], dim_cm: [ 14, 21 ] },
{ item: "notebook", qty: 50, tags: ["red", "blank"], dim_cm: [ 14, 21 ] },
{ item: "paper", qty: 100, tags: ["red", "blank", "plain"], dim_cm: [ 14, 21 ] },
{ item: "planner", qty: 75, tags: ["blank", "red"], dim_cm: [ 22.85, 30 ] },
{ item: "postcard", qty: 45, tags: ["blue"], dim_cm: [ 10, 15.25 ] }
]);
To query if the array field contains at least one element with the specified value, use the filter { <field>: <value> } where <value> is the element value.
db.inventory.find( { tags: "red" } )
Q. Mention the command to remove indexes and list all the indexes on a particular collection?
List all Indexes on a Collection
// To view all indexes on the people collection
db.people.getIndexes()
List all Indexes for a Database
// To list all the collection indexes in a database
db.getCollectionNames().forEach(function(collection) {
indexes = db[collection].getIndexes();
print("Indexes for " + collection + ":");
printjson(indexes);
});
Remove Indexes
MongoDB provides two methods for removing indexes from a collection:
db.collection.dropIndex()db.collection.dropIndexes()
1. Remove Specific Index
db.accounts.dropIndex( { "tax-id": 1 } )
// Output
{ "nIndexesWas" : 3, "ok" : 1 }
2. Remove All Indexes
// The following command removes all indexes from the accounts collection
db.accounts.dropIndexes()
Q. Select only selected elements from given collection?
Consider a books collection with the following document:
{
"_id" : 1,
title: "abc123",
isbn: "0001122223334",
author: { last: "zzz", first: "aaa" },
copies: 5
}
The following $project stage adds the new fields isbn, lastName, and copiesSold:
db.books.aggregate([
{
$project : {
title:1,
isbn: {
prefix: { $substr: [ "$isbn", 0, 3 ] },
group: { $substr: [ "$isbn", 3, 2 ] },
publisher: { $substr: [ "$isbn", 5, 4 ] },
title: { $substr: [ "$isbn", 9, 3 ] },
checkDigit: { $substr: [ "$isbn", 12, 1] }
},
lastName: "$author.last",
copiesSold: "$copies"
}
}
])
The operation results in the following document:
{
"_id" : 1,
"title" : "abc123",
"isbn" : {
"prefix" : "000",
"group" : "11",
"publisher" : "2222",
"title" : "333",
"checkDigit" : "4"
},
"lastName" : "zzz",
"copiesSold" : 5
}
Q. How to join 3 or more collections in MongoDB?
Let’s say we have 3 hypothetical collections in MongoDB: customers, orders, and orderItems.
Each customer has multiple orders, and each order has multiple order items.
Example:
// customers
[
{
customer_id: 1,
name: "Jim Smith",
email: "jim.smith@example.com"
},
{
customer_id: 2,
name: "Bob Jones",
email: "bob.jones@example.com"
}
]
// orders
[
{
order_id: 1,
customer_id: 1
},
{
order_id: 2,
customer_id: 1
}
]
// orderItems
[
{
order_item_id: 1,
name: "Foo",
price: 4.99,
order_id: 1
},
{
order_item_id: 2,
name: "Bar",
price: 17.99,
order_id: 1
},
{
order_item_id: 3,
name: "baz",
price: 24.99,
order_id: 2
}
]
Desired Result:
[
{
customer_id: 1,
name: "Jim Smith",
email: "jim.smith@example.com"
orders: [
{
order_id: 1,
items: [
{
name: "Foo",
price: 4.99
},
{
name: "Bar",
price: 17.99
}
]
},
{
order_id: 2,
items: [
{
name: "baz",
price: 24.99
}
]
}
]
},
{
customer_id: 2,
name: "Bob Jones",
email: "bob.jones@example.com"
orders: []
}
]
Answer
Do nested lookup using lookup with pipeline,
$lookupwith orders collection.let, define variable customer_id that is from main collection, to access this reference variable inside pipeline using$$like$$customer_id.pipelinecan add pipeline stages same as we do in root level pipeline$exprwhenever we match internal fields it requires expression match condition, so$$customer_idis parent collection field that declared in let and $customer_id is child collection’s/current collection’s field$lookupwith orderitems collection
db.customers.aggregate([
{
$lookup: {
from: "orders",
let: { customer_id: "$customer_id" },
pipeline: [
{ $match: { $expr: { $eq: ["$$customer_id", "$customer_id"] } } },
{
$lookup: {
from: "orderitems",
localField: "order_id",
foreignField: "order_id",
as: "items"
}
}
],
as: "orders"
}
}
])
Q. How to validate data in mongodb?
db.collection.validate(<documents>) validates a collection. The method scans a collection data and indexes for correctness and returns the result.
Syntax:
db.collection.validate( {
full: <boolean>, // Optional
repair: <boolean> // Optional, added in MongoDB 5.0
} )
Example:
// validate a collection using the default validation setting
db.myCollection.validate({ })
// perform a full validation of collection
db.myCollection.validate( { full: true } )
// repair collection
db.myCollection.validate( { repair: true } )