2.md

https://www.tutorialspoint.com/mongodb/mongodb_query_document.htm

https://www.tutorialspoint.com/mongodb/mongodb_update_document.htm

Delete a Single Document

The remove() collection method will delete documents that match the provided query.

db.potions.remove( {"name": "Love"} )

WriteResult({ "nRemoved": 1 })

update command

is atomic . so no 2 clients may update the same document

Syntax : db.foo.update(query , update , options);

foo is the collection name , "query" for targeted document/documents , "update" parameter to mention what change you want to make

options like upsert , one-many ,

db.foo.update({a:1} , {$inc : {a:1}})

Updating a Document : only applies to 1st matching document

{"$set": {"price": 3.99 }}

db.potions.update( {"name": "Love"}, {"$set": {"price": 3.99 }} )

nMatched : how many doc were matched

WriteResult({ "nMatched": 1,
"nUpserted": 0, "nModified": 1 })

Update Without an Operator

If the update parameter consists of only field/value pairs, then everything but the _id is replaced in the matching document.

db.potions.update( {"name": "Love"}, {"price": 3.99 } )

Document replaced with the update parameter "price"

Updating Multiple Documents

{"multi": true}

db.potions.update( {"vendor": "KC"}, {"$set": { "vendor": "Kettlecooked" }} {"multi": true} )

Update a Document’s Count

We can use the $inc operator to increment the count of an existing log document.

db.logs.update( {"potion": "Shrinking"}, {"$inc": {"count": 1}} )

If field doesn’t exist, it gets created with the value.

If we run the update on a potion that doesn’t exist, then nothing will happen.

Find or Create With Upsert

{"upsert": true}

db.logs.update( {"potion": "Love"}, {"$inc": {"count": 1}}, {"upsert": true} )

WriteResult({ "nMatched": 0, "nUpserted": 1, "nModified": 0 })

Running same query again will result in :

WriteResult({ "nMatched": 1, "nUpserted": 0, "nModified": 1

})

Advanced Modification

Removing Fields From Documents

The $unset operator can be used to remove specified fields.

db.a.update({_id :1} , {$unset :{y: 0}})

rename

db.foo.update({_id :1} , {$rename: {'Naem' : 'Name'}})

Unset field : color db.potions.update( {}, {"$unset": {"color": ""}}, {"multi": true} )

Updating a Field Name With $rename

We can use $rename to change field names.

{"$rename": {fieldToRename: newField}}

db.potions.update( {}, {"$rename": {"score": "grade"}}, {"multi": true} )

The Dilemma of Updating an Array

"ingredients": ["hippo", "secret", "mouse feet"]

db.potions.update( {"ingredients": "secret"}, {"$set": {"ingredients": "42"}} )

Would overwrite the entire array and set it as 42

Updating Array Values by Location

Since array values are treated individually, we can update a single value by specifying its location in the array using dot notation.

ingredients.1 : Means 2nd array element

db.potions.update( {"name": "Shrinking"}, {"$set": {"ingredients.1" : 42}} )

Updating Values Without Knowing Position

We need to change “secret” in multiple documents

db.potions.update( {"ingredients": "secret"}, {"$set": {"ingredients.$" : 42}}, {"multi": true} )

The $ is a placeholder for the matched value

Updating an Embedded Value

"ratings": {

"strength": 1, "flavor": 5 }

db.potions.update( {"name": "Shrinking"}, {"$set": {"ratings.strength" : 5}} )

Useful Update Operators

$max , $min , $mul

Modifying Arrays

Removing the First or Last Value of an Array

Removes the first element

"categories": ["tasty", "effective"]

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

Result "categories": ["tasty"]

Adding Values to the End of an Array

The $push operator will add a value to the end of an array.

{"_id" :1 , "things" :["one"] }

db.foo.update({_id :1} , { $push : {things : "two"} })

Adding Unique Values to an Array

The $addToSet operator will add a value to the end of an array unless it is already present.

Removing Values From an Array

The $pull operator will remove any instance of a value from an array. i.e if there are 2 elements in an array with name "three" , then pull will remove all instances of three

{$pull : {things : 'three'}}

add to set only add once however times you run it

$addToSet

update multiple arrays with same fieldname

db.foo.update({} , {$push : {things : 4}})

{} means match any . bydefault only one document is affected.

making changes in all

db.foo.update({} , {$push : {things : 4}} , {multi : true })

find and modify : affect only one record