[{_id:"689000",category_id:"CC100",versions:[{version_id:"001",sub_versions:[{sub_version_code:"PS",verification_name:"test"},{sub_version_code:"PPS",verification_name:"test"},]},{version_id:"002",sub_versions:[{sub_version_code:"BS",verification_name:"test"},{sub_version_code:"BBS",verification_name:"test"},]},]},{_id:"689001",category_id:"CC100",versions:[{version_id:"001",sub_versions:[{sub_version_code:"PS",verification_name:"test"},{sub_version_code:"PPS",verification_name:"test"},]},{version_id:"002",sub_versions:[{sub_version_code:"BS",verification_name:"test"},{sub_version_code:"BBS",verification_name:"test"},]},]}]

db.collection.aggregate([{"$project":{_id:1,codes:{"$reduce":{"input":"$versions","initialValue":[],"in":{"$concatArrays":["$$value",{"$map":{"input":"$$this.sub_versions","as":"item","in":"$$item.sub_version_code"}}]}}}}}])