[{courseID:"Demo",materials:[{moduleNo:1,moduleMaterial:"A",},{moduleNo:2,moduleMaterial:"B",},{moduleNo:3,moduleMaterial:"C",},{moduleNo:4,moduleMaterial:"D",},{moduleNo:4,moduleMaterial:"E",}]}]

db.collection.aggregate([{$match:{courseID:"Demo"}},{"$project":{courseID:1,materials:{"$filter":{"input":"$materials","as":"material","cond":{"$eq":["$$material.moduleNo",4]}}}}}])