[{_id:ObjectId("5e3bad4a86620ba667d764b7"),uri:"http://tao.gcf.education/gcf.rdf#i1566561304546290602",explanation:"",item_uri:"http://tao.gcf.education/gcf.rdf#i1566561304546290602",option_list:[{option_id:"choice_1",},{option_id:"choice_2",},{option_id:"choice_3",},{option_id:"choice_4",}],question_id:"BQNazm0Xkv",response_identifier:"RESPONSE",title:"Q42",type:"Singleselect"}]

db.collection.aggregate([{"$project":{"uri":1,"options_list":{"$let":{"vars":{"string":{"$concat":["$response_identifier",":",{"$reduce":{"input":"$option_list.option_id","initialValue":"","in":{"$concat":["$$this",",","$$value"]}}}]}},"in":{"$substr":["$$string",0,{"$add":[{"$strLenCP":"$$string"},-1]}]}}}}}])