[{type:"Data",data:{"content":[{"otherStructure":{"testData":{"keys":[{"values":[{"meta":{"importantObject":{"importantKey":ObjectId("5a934e000102030405000003")}}},{"meta":{"importantObject":{"importantKey":ObjectId("5a934e000102030405000004")}}}]}]}}}]}},{type:"Data",data:{"content":[{"otherStructure":{"testData":{"keys":[{"values":[{"meta":{"importantObject":{"importantKey":ObjectId("5a934e000102030405000005")}}},{"meta":{"importantObject":{"importantKey":ObjectId("5a934e000102030405000006")}}}]}]}}}]}},{type:"Data",data:{"content":[{"otherStructure":{"testData":{"keys":[{"values":[{"meta":{"importantObject":{"importantKey":ObjectId("5a934e000102030405000000")}}},{"meta":{"importantObject":{"importantKey":ObjectId("5a934e000102030405000001")}}}]}]}}}]}},]

db.collection.aggregate([{$addFields:{"data.content":{$map:{input:"$data.content",as:"data",in:{otherStructure:{testData:{keys:{$map:{input:"$$data.otherStructure.testData.keys",as:"testData",in:{"values":{$map:{input:"$$testData.values",as:"values",in:{"meta":{"importantObject":{"importantKey":{$toString:"$$values.meta.importantObject.importantKey"}}}}}}}}}}}}}}}}])