db={"c1":[{"_id":"60f83d3cbc3f0161da2141f7","id":17736625,"name":"OJ Simpson","asset_contract":{"name":"Hashmasks","address":"0xc2c747e0f7004f9e8817db2ca4997657a7746928"},"traits":[{"trait_type":"Character","value":"Male","display_type":null,"max_value":null,"trait_count":8659,"order":null},{"trait_type":"Mask","value":"Doodle","display_type":null,"max_value":null,"trait_count":2187,"order":null},{"trait_type":"Eye Color","value":"Dark","display_type":null,"max_value":null,"trait_count":7419,"order":null},{"trait_type":"Eye Color","value":"Blue","display_type":null,"max_value":null,"trait_count":7419,"order":null},{"trait_type":"Item","value":"No Item","display_type":null,"max_value":null,"trait_count":14533,"order":null},{"trait_type":"Skin Color","value":"Dark","display_type":null,"max_value":null,"trait_count":3784,"order":null},{"trait_type":"Token ID","value":3535,"display_type":"number","max_value":null,"trait_count":0,"order":null},{"trait_type":"Background","value":"Doodle","display_type":null,"max_value":null,"trait_count":5538,"order":null}],"token_id":"3535"}],"c2":[{}]}

db.c1.aggregate([{"$lookup":{"from":"c2","let":{"traits":"$traits"},"pipeline":[{"$addFields":{"traits":"$$traits"}},{"$project":{"traits":1}},{"$unwind":{"path":"$traits"}},{"$group":{"_id":"$traits.trait_type","values":{"$push":"$traits.value"}}},{"$replaceRoot":{"newRoot":{"$let":{"vars":{"r":{"$map":{"input":{"$objectToArray":"$$ROOT"},"as":"m","in":["$$m.k","$$m.v"]}}},"in":{"$let":{"vars":{"pair":[[{"$replaceAll":{"input":{"$concat":["traits_",{"$toLower":{"$arrayElemAt":[{"$arrayElemAt":["$$r",0]},1]}}]},"find":" ","replacement":"_"}},{"$let":{"vars":{"v":{"$arrayElemAt":[{"$arrayElemAt":["$$r",1]},1]}},"in":{"$cond":[{"$eq":[{"$size":"$$v"},1]},{"$arrayElemAt":["$$v",0]},"$$v"]}}}]]},"in":{"$arrayToObject":"$$pair"}}}}}}}],"as":"traits"}},{"$addFields":{"traits":{"$arrayToObject":{"$map":{"input":"$traits","as":"t","in":{"$arrayElemAt":[{"$map":{"input":{"$objectToArray":"$$t"},"as":"m","in":["$$m.k","$$m.v"]}},0]}}}}}},{"$replaceRoot":{"newRoot":{"$mergeObjects":["$traits","$$ROOT"]}}},{"$project":{"traits":0}}])