I want to generate JSON string from Account sObject field which also contains other sObject. Please help...
In my experience, the best way to go about this without using middleware is to create a custom metadata type for the API mapping. It will store the JSON key, SObject name, and SObject field.
You can use that data to generate a dynamic SOQL query and then create a map (Map) that can be serialized.
The complexity depends on the JSON you need to generate. If you need sub-objects or arrays of objects in the JSON, it can get complicated.