// Drop the collections db.testUsers.drop(); db.testCollection.drop(); // Create test data db.testUsers.insertMany( [ { _id:ObjectId( "54570f77968d6e492b0d68b0" ), email: "john@example.com" , password: "1z2j890" , name: { first: "John" , last: "Smith" } }, { _id:ObjectId( "54570f77968d6e492b0d68b1" ), email: "jane@example.com" , password: "5sc46dr" , name: { first: "Jane" , last: "Doe" } } ]); db.testCollection.insert( { _id:ObjectId( '5b186cc4d1ded8d06a7c3baf' ), user:ObjectId( "54570f77968d6e492b0d68b0" ), }); var pipeline = [ { $lookup: { from: 'testCollection' , localField: '_id' , foreignField: 'user' , as: 'nested' } }, { $project: { _id: true , // name: true , <---- this will cause an error email: true , role: true , nested: true } }, { $redact: { $cond: { if : { $eq: [ { $size: "$nested" } , 0 ] }, then: "$$DESCEND" , else : "$$PRUNE" } } } ]; // This works fine var status1 = db.testUsers.aggregate(pipeline); printjson(statistics.result); // try to project the name field pipeline[1].$project.name = true // this should error var status2 = db.testUsers.aggregate(pipeline); // I ran this in the new aggregate debugger in Mongodb Compass (1.14.0-beta.3) and it output this error: // The argument to $size must be an array, but was of type: missing