Basic queries for CRUD operations in MongoDB II

Posted By Akash Sharma | 01-May-2013

In this blog I am going to share some more CRUD operations in mongoDB.

Previously I have written a blog for   Basic queries for CRUD operations in MongoDB .

Let say we have a database “school” having collection “student”.

 

Example 1: How to rename a field in a collection.

The $rename operator updates the name of a field. The new field name must differ from the existing field name.

I have a field named “class” of a student.I want to rename it as “standard”.

> db.student.update( {} , {$rename:{"class" : "standard"}} ,false , true)

The first blank curly braces indicates that you have to update all the documents.

parameters false and true are for upsert and multi options respectively.

 

Example 2: How to get a formatted output of all the documents.

If I apply the command

>db.student.find()

I will get an output like this:

> db.student.find()

{ "_id" : ObjectId("517ce315ebdc79a0c6ad6948"), "age" : 15, "name" : "rahul", "s

tandard" : 9, "subjects" : [ "hindi", "maths", "science" ] }

{ "_id" : ObjectId("517ce328ebdc79a0c6ad6949"), "age" : 17, "name" : "ashok", "s

tandard" : 10, "subjects" : [ "hindi", "maths", "science" ] }

{ "_id" : ObjectId("517ce341ebdc79a0c6ad694a"), "age" : 14, "name" : "ankur", "s

tandard" : 6, "subjects" : [ "hindi", "maths", "science" ] }

> 

 

To make a formated output we can apply this command:

> db.student.find().pretty()

{

       "_id" : ObjectId("517ce315ebdc79a0c6ad6948"),

       "age" : 15,

       "name" : "rahul",

       "standard" : 9,

       "subjects" : [

               "hindi",

               "maths",

               "science"

       ]

}

{

       "_id" : ObjectId("517ce328ebdc79a0c6ad6949"),

       "age" : 17,

       "name" : "ashok",

       "standard" : 10,

       "subjects" : [

               "hindi",

               "maths",

               "science"

       ]

}

{

       "_id" : ObjectId("517ce341ebdc79a0c6ad694a"),

       "age" : 14,

       "name" : "ankur",

       "standard" : 6,

       "subjects" : [

               "hindi",

               "maths",

               "science"

       ]

}

> 

 

 

Example 3: How to get output for limited number of fields but not for all.

Let say I want to see only name, age and standard for all documents:

> db.student.find({},{name:1,standard:1,age:1}).pretty()

{

       "_id" : ObjectId("517ce315ebdc79a0c6ad6948"),

       "age" : 15,

       "name" : "rahul",

       "standard" : 9

}

{

       "_id" : ObjectId("517ce328ebdc79a0c6ad6949"),

       "age" : 17,

       "name" : "ashok",

       "standard" : 10

}

{

       "_id" : ObjectId("517ce341ebdc79a0c6ad694a"),

       "age" : 14,

       "name" : "ankur",

       "standard" : 6

}

> db.student.find({},{name:1,standard:1,age:1,_id:0}).pretty()

{ "age" : 15, "name" : "rahul", "standard" : 9 }

{ "age" : 17, "name" : "ashok", "standard" : 10 }

{ "age" : 14, "name" : "ankur", "standard" : 6 }

> 

The first curly braces in find() will filter the documents on some condition.

The second curly braces determines which fields you want to show.

If you want to see the value for a particular field you have to set its value as 1.

By default value for _id field is 1 and 0 for all other fields.


 

Example 4: How to get output for limited number of fields but not for all for a given condition.

I want to see only name, age and standard for documents having standard greater than 8:

> db.student.find({standard:{$gt:8}},{name:1,standard:1,age:1,_id:0})

{ "age" : 15, "name" : "rahul", "standard" : 9 }

{ "age" : 17, "name" : "ashok", "standard" : 10 }

> 

 

 

Example 5 : How to remove a field from all documents of a collection.

suppose we want to remove age field from all documents:

> db.student.update({age:{$exists:true}},{$unset:{age:1}},false,true)

Here $exists checks in those documents which contains age field.

$unset removes the field.

false and true are for upsert and multi options respectively.

 

Akash Sharma

Akash.sharma@oodlestechnologies.com

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