Test if a field is available on arbitrary SObject record without using Describe calls

Question

I have many Custom Objects in my package that have a common custom field Version__c. One part of my app serializes and deserializes records to/from Json. While deserializing I want to change this Version field for any record which is of a type that has this field.

The code looks like this:

    private SObject createVersion(SObject oldRecord) {
        SObject newRecord = oldRecord.clone();

        if(isFieldAvailableOn('Version__c', oldRecord)) {
            newRecord.put('Version__c', oldRecord.Version__c);
        }

        return newRecord;
    }

How can I implement isFieldAvailableOn(String fieldName, SObject record) without consuming Describe call?

Answer 1

You can simply wrap your attempt to read or write the Version field in a try-catch block. An exception means the field is not present, not writeable, or the wrong data type. The exception thrown will describe which scenario occurred.

Answer 2

How about a JSON roundtrip:

SObject so = new Account(Name='foo');
String s = JSON.serialize(so);
Map<String, Object> m = (Map<String, Object>) JSON.deserializeUntyped(s);
system.debug(m.keySet().contains('Name'));

This will not tell you if the sobject definition has the field, but only if the instance has it (has been set, or has been read form the db).