MongoDB

How Dates are saved

"date": new Date(2012,8,13) September not August since JavaScript counts month from zero Then MongoDB saves it in ISO format

Working with arrays... adding UNIQUE values to arrays

$addToSet update operator

Range queries on Arrays

$elemMatch - element match array query operator to make sure atleast 1 element matches the criteria specified

Query of comparisons in MongoDB What are the comparison query operators?

$gt - greater than $lt - less than $gte - greater than equal to $lte - less than equal to $ne - not equal to DONT FORGET THE $

More update operators

$max - updates if new value is greater than current $min - updates if new value is lesser than current $mul - updates field value * supplied value

Working with arrays... removing values from arrays

$pop update operator -1 pop first element 1 pop last element

How to remove values from an array

$pull update operator pull remove all instances of a matching value

Working with arrays... adding values to arrays

$push update operator

Collections and Documents

Collections are groups of documents documents exist independently and can have different fields. documents are JSON-like objects. documents are persisted in a format called BSON

What is NoSQL

Databases that are not relational and don't have a query language like SQL

Important note about update function

It only updates the FIRST MATCHING document

Can we have inclusion and exclusion specified in a projection?

No! you can have only inclusion or exclusion Except if you want to exclude ID field

Filtering in MongoDB

Passing multiple queries to find method

data types that can be stored are

String , boolean, null, numbers, Arrays , Object EXTRA : objectId, ISODate

What happens after we execute a query

We get a WriteResult object returned

How to access a value in an array when location of value is not known?

We have to use positional operator

What is the cursor object?

Whenever we search for documents, an object is returned from the find method known as the cursor object. Using find method you can only get the first 20 documents back We have to type 'it' for more documents

If update operators are not specified what happens?

Whole doc is overwritten with the update object

What can you do with projections

You can SELECT the fields you want OR EXCLUDE the fields you do not want

MongoDB Vs SQL

both store data in a database SQL uses tables | MongoDB uses collections Tables - rows | Collections - documents fixed schema | dynamic schema

Cursor methods

count() sort() skip() limit()

Command to know which database your using

db

how to retrieve all documents in a collection

db.<collection name>.find()

How to find particular documents in a collection

db.<collection name>.find({"key":"value"}) AKA "query of equality"

how to insert document

db.<collection name>.insert( { "<key>":"value" } ) Note : if the collection does not exist it will be created

Example of projections - exclusion

db.<collection-name>.find( {"grade" : {"$gte":80}}, //fields that you don't want to include in the output {"vendor" : false , "price" : false} )

Example for Range queries on Arrays

db.<collection-name>.find( {"sizes :{ "$elemMatch":{ "$gt":8 , "$lt":16} } })

Range queries on Arrays - odd behavior without $elemMatch

db.<collection-name>.find( {"sizes :{ "$gt":8 , "$lt":16} }) each value in the array is checked individually if atleast one array value is true for each criteria the entire document matches

Example of query using comparison operators for range of values

db.<collection-name>.find( { "price" : { "$gt" : 10 , "$lt" : 20 }})

Example of query using comparison operators

db.<collection-name>.find( { "price" : { "$lt" : 20 }})

Cursor methods for pagination

db.<collection-name>.find() .skip(<no. of docs you want to skip>) .limit(<no. of docs you want to retrieve>) //this approach becomes expensive for large collections

Example of sort() cursor method

db.<collection-name>.find().sort({"price":1}) 1 for ascending order -1 for descending order

How to remove documents from a collection

db.<collection-name>.remove({<specify key and value>});

How to update values in a document?

db.<collection-name>.update( {<query parameter>}, {<update parameter>})

Example using positional operator

db.potions.update( {"ingredients":"secret"}, \\ the value you want to update {"$set":{"ingredients . $ " : 42}}, {"multi" : true} )

Example of $pop update operator syntax

db.potions.update( {"name" : "Shrinking" }, {"$pop":{"category":1}} )

Example of $pull update operator

db.potions.update( {"name" : "Shrinking" }, {"$pull":{"category":"<value you want to remove>"}} )

Example of $push update operator syntax

db.potions.update( {"name" : "Shrinking" }, {"$push":{"category":"<value you want to push>"}} )

Example of $rename update operator syntax

db.potions.update( {}, {"$rename":{"old-field-name":"new-field-name"}}, {"multi":true} )

Example of $unset update operator syntax

db.potions.update( {}, // empty query parameter means select all documents {"$unset":{"color":""}}, {"multi": true} )

MongoDB unique id

each document must have a unique id "_id"

What are Projections?

find() method has a 2nd parameter that takes field-boolean_value pairs to specify which fields we want in the output By default document Ids are always returned

What WriteResult after an update

nMatched - number of docs matched nUpserted - no. of docs created nModified - no. of docs modifed

finding documents that contain embedded documents

need to use dot notation

What constraints MongoDB checks

no other doc has the same id no syntax errors document is less than 16 mb

What is MongoDB

opensource nosql document-oriented database . Good for lots of unstructured data. (access through shell using javascript)

What is positional operator

positional operator is a placeholder that will set the proper position of the value specified in the query parameter <field>.$ - placeholder

finding documents based on data in array

same as query of equality

command to see all databases and their sizes

show dbs

Command to Access or Create database

use <database name>

How to rename a field in a collection

using $rename update operator

how to remove fields from a collection

using $unset update operator

How to access a value in an array? Location of value is known

using dot notation and index position

What do you do in order to update all matching documents

we need to pass the Options parameter {"multi" : true}

What do you do if the query parameter does not find any matches but still want the update value to get inserted?

we need to pass the Options parameter {"upsert" : true} creates a document using the values in the query and update parameters

how to write update parameters

{"$<update operator>" : {<key>:<value>}} update operators always begin with $ Example $set

Which update operator to use to increment value of a field

{"$inc":{"<field>":<increment by>}}