[{"id":"62cd5de762c595905e37821b","letterNo":"122233","letterDate":"2023-03-01","metaData":{"name":"test","family ":"test","customer":[{"firstName":"myFirstName","nationalId":"1234567890",}],}},{"id":"62cd5de762c595905e37821a","letterNo":"122233","letterDate":"2023-03-01","metaData":{"type":"xxxx","where":"test","customer":[{"personType":1,"customerName":"myName"}],}}]

db.collection.aggregate([{$project:{"metaData":{$map:{input:{$objectToArray:"$metaData"},as:"metaData",in:{$cond:{if:{$isArray:"$$metaData.v"},then:{"customer":{$reduce:{input:{$ifNull:["$$metaData.v",[]]},initialValue:"",in:{$map:{input:{$objectToArray:"$$this"},as:"cdata",in:{$concat:[{$toString:"$$cdata.k"},"\":\"",{$toString:"$$cdata.v"}]}}}}},},else:{$concat:["{ \"","$$metaData.k","\": \"","$$metaData.v","\" }"]}}}}}}}])