db={"parents":[{"_id":ObjectId("5a834e000102030405000000"),"child":ObjectId("5a934e000102030405000000")},{"_id":ObjectId("5a834e000102030405000001"),"child":ObjectId("5a934e000102030405000001")},{"_id":ObjectId("5a834e000102030405000002"),"child":ObjectId("5a934e000102030405000002")},],"children":[{"_id":ObjectId("5a934e000102030405000000"),"active":true,"grandchild":ObjectId("5a734e000102030405000000")},{"_id":ObjectId("5a934e000102030405000001"),"active":false,"grandchild":ObjectId("5a734e000102030405000001")},{"_id":ObjectId("5a934e000102030405000002"),"active":true,"grandchild":ObjectId("5a734e000102030405000002")}],"grandchildren":[{"_id":ObjectId("5a734e000102030405000000"),"name":"grandchild1"},{"_id":ObjectId("5a734e000102030405000001"),"name":"grandchild2"},{"_id":ObjectId("5a734e000102030405000002"),"name":"grandchild3"}]}

db.parents.aggregate([{$lookup:{from:"children",localField:"child",foreignField:"_id",as:"child",}},{$addFields:{child:{$arrayElemAt:["$child",0]}}},{"$lookup":{from:"grandchildren",localField:"child.grandchild",foreignField:"_id",as:"grandchild",}},{$addFields:{grandchild:{$arrayElemAt:["$grandchild",0]}}},{$match:{"grandchild.name":"grandchild1"}}])