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Implement a MongoDB $cond field in a projection pipeline based on the presence or absence of a field?
For this, you can use $cond along with $anyElementTrue. NULL values (absence of a field) would evaluate to FALSE. With that, an empty array also returns FALSE with $ anyElementTrue.
Let us first create a collection with documents −
> db.presenceDemo.insertOne({"StudentName":null});
{
"acknowledged" : true,
"insertedId" : ObjectId("5e06f70c25ddae1f53b621f3")
}
> db.presenceDemo.insertOne({"StudentName":"Chris"});
{
"acknowledged" : true,
"insertedId" : ObjectId("5e06f71425ddae1f53b621f4")
}
> db.presenceDemo.insertOne({"StudentName":null});
{
"acknowledged" : true,
"insertedId" : ObjectId("5e06f71825ddae1f53b621f5")
}
> db.presenceDemo.insertOne({"StudentName":"David"});
{
"acknowledged" : true,
"insertedId" : ObjectId("5e06f71e25ddae1f53b621f6")
}Following is the query to display all documents from a collection with the help of find() method −
> db.presenceDemo.find();
This will produce the following output −
{ "_id" : ObjectId("5e06f70c25ddae1f53b621f3"), "StudentName" : null }
{ "_id" : ObjectId("5e06f71425ddae1f53b621f4"), "StudentName" : "Chris" }
{ "_id" : ObjectId("5e06f71825ddae1f53b621f5"), "StudentName" : null }
{ "_id" : ObjectId("5e06f71e25ddae1f53b621f6"), "StudentName" : "David" }Here is the query to implement a $cond field based on the presence or absence of a field −
> db.presenceDemo.aggregate([
... { "$project": {
... "MyValue": {
... "$cond": [
... { "$anyElementTrue": [ [ "$StudentName" ] ] },
... 1,
... 0
... ]
... }
... }}
... ]);This will produce the following output −
{ "_id" : ObjectId("5e06f70c25ddae1f53b621f3"), "MyValue" : 0 }
{ "_id" : ObjectId("5e06f71425ddae1f53b621f4"), "MyValue" : 1 }
{ "_id" : ObjectId("5e06f71825ddae1f53b621f5"), "MyValue" : 0 }
{ "_id" : ObjectId("5e06f71e25ddae1f53b621f6"), "MyValue" : 1 }
Updated on: 31-Mar-2020
185 Views
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