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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 }
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