The $addToSet operator adds value to an array unless the value is already present, in which case $addToSet does nothing to that array.
Let us create a collection with documents −
> db.demo533.insertOne({"ProjectName":"Online Hospital Management"});{ "acknowledged" : true, "insertedId" : ObjectId("5e8b4cfaef4dcbee04fbbbfc") } > db.demo533.insertOne({"ProjectName":"Online Library Management"});{ "acknowledged" : true, "insertedId" : ObjectId("5e8b4d02ef4dcbee04fbbbfd") } > db.demo533.insertOne({"ProjectName":"Online Hospital Management"});{ "acknowledged" : true, "insertedId" : ObjectId("5e8b4d04ef4dcbee04fbbbfe") } > db.demo533.insertOne({"ProjectName":"Online Customer Tracker"});{ "acknowledged" : true, "insertedId" : ObjectId("5e8b4d0def4dcbee04fbbbff") }
Display all documents from a collection with the help of find() method −
> db.demo533.find();
This will produce the following output −
{ "_id" : ObjectId("5e8b4cfaef4dcbee04fbbbfc"), "ProjectName" : "Online Hospital Management" } { "_id" : ObjectId("5e8b4d02ef4dcbee04fbbbfd"), "ProjectName" : "Online Library Management" } { "_id" : ObjectId("5e8b4d04ef4dcbee04fbbbfe"), "ProjectName" : "Online Hospital Management" } { "_id" : ObjectId("5e8b4d0def4dcbee04fbbbff"), "ProjectName" : "Online Customer Tracker" }
Following is the query to implement $addToSet and fetch the values of field ProjectName −
> db.demo533.aggregate( ... [ ... { ... $group: ... { ... _id:null, ... SetOfProject: { $addToSet: "$ProjectName" } ... } ... } ... ] ... )
This will produce the following output −
{ "_id" : null, "SetOfProject" : [ "Online Customer Tracker", "Online Library Management", "Online Hospital Management" ] }