$project in MongoDB with example

$project use to reshape each document for the next pipeline, such as adding a new computed field or existing field from old document.

$project stage has the following prototype form

{ $project: { <specifications> } }

The $project takes a document that can specify the inclusion of fields, the suppression of the _id field, the addition of new fields, and the resetting the values of existing fields. The specifications have the following forms:

Syntax	Description
<Field>:<1 or true>	Specify the inclusion of the field.
_id:<1 or true>	Specify the suppression of _id field.
<field>: <expression>	Add a new field or reset the value of an existing field.

For example, I am creating a User Collection into Test database. See below scripts

Use Test

db.Users.insert( {     "fName" : "Dilip",     "lName" : "Singh",     "address" : "Noida",     "totalOrder" : 150,     "createdDate" : "2014/07/07" }) db.Users.insert( {     "fName" : "Vipul",     "lName" : "Bhatt",     "address" : "Delhi",     "totalOrder" : 50,     "createdDate" : "2013/07/07" }) db.Users.insert( {     "fName" : "Brijesh",     "lName" : "Kumar",     "address" : "Gorakhpur",     "totalOrder" : 70,     "createdDate" : "2012/07/07" }) db.Users.insert( {     "fName" : "Raj",     "lName" : "Kumar",     "address" : "Bokaro",     "totalOrder" : 40,     "createdDate" : "2010/07/07" }) db.Users.find()

it will returns following result.

/* 0 */ {     "_id" : ObjectId("55dac0b31c949abf34d5daed"),     "fName" : "Dilip",     "lName" : "Singh",     "address" : "Noida",     "totalOrder" : 150,     "createdDate" : "2014/07/07" } /* 1 */ {     "_id" : ObjectId("55dac0c21c949abf34d5daf1"),     "fName" : "Vipul",     "lName" : "Bhatt",     "address" : "Delhi",     "totalOrder" : 50,     "createdDate" : "2013/07/07" } /* 2 */ {     "_id" : ObjectId("55dac0d11c949abf34d5daf2"),     "fName" : "Brijesh",     "lName" : "Kumar",     "address" : "Gorakhpur",     "totalOrder" : 70,     "createdDate" : "2012/07/07" } /* 3 */ {     "_id" : ObjectId("55dac0e31c949abf34d5daf3"),     "fName" : "Raj",     "lName" : "Kumar",     "address" : "Bokaro",     "totalOrder" : 40,     "createdDate" : "2010/07/07" }

Remove unnecessary fields

Now I want to apply $project on document to remove some fields

db.Users.aggregate([{ $project :{fName:1,totalOrder:1}}])

output result

/* 0 */ {     "result" : [         {             "_id" : ObjectId("55dac0b31c949abf34d5daed"),             "fName" : "Dilip",             "totalOrder" : 150         },         {             "_id" : ObjectId("55dac0c21c949abf34d5daf1"),             "fName" : "Vipul",             "totalOrder" : 50         },         {             "_id" : ObjectId("55dac0d11c949abf34d5daf2"),             "fName" : "Brijesh",             "totalOrder" : 70         },         {             "_id" : ObjectId("55dac0e31c949abf34d5daf3"),             "fName" : "Raj",             "totalOrder" : 40         }     ],     "ok" : 1 }

In the above query, it in result returns the only fName, total order fields

Adding a new field

See the following query I am adding a filed "UserType" on the condition, if totalOrder>100 then "Gold" else "silver"

db.Users.aggregate( [{     $project :     {         fName:1,         totalOrder:1,         UserType:{             $cond:{                     if:{$gte:["$totalOrder",100]},                     then:"gold",                     else:"Silver"                  }                 }     } }])

output

/* 0 */ {     "result" : [         {             "_id" : ObjectId("55dac0b31c949abf34d5daed"),             "fName" : "Dilip",             "totalOrder" : 150,             "UserType" : "gold"         },         {             "_id" : ObjectId("55dac0c21c949abf34d5daf1"),             "fName" : "Vipul",             "totalOrder" : 50,             "UserType" : "Silver"         },         {             "_id" : ObjectId("55dac0d11c949abf34d5daf2"),             "fName" : "Brijesh",             "totalOrder" : 70,             "UserType" : "Silver"         },         {             "_id" : ObjectId("55dac0e31c949abf34d5daf3"),             "fName" : "Raj",             "totalOrder" : 40,             "UserType" : "Silver"         }     ],     "ok" : 1 }

Here we added a new field "UserType".

Suppress _id using $project

See following query to usppress _id from pipeline. _id:0

db.Users.aggregate( [{     $project :     {         _id:0,         fName:1,         totalOrder:1            } }])

output

/* 0 */ {     "result" : [         {             "fName" : "Dilip",             "totalOrder" : 150         },         {             "fName" : "Vipul",             "totalOrder" : 50         },         {             "fName" : "Brijesh",             "totalOrder" : 70         },         {             "fName" : "Raj",             "totalOrder" : 40         }     ],     "ok" : 1 }