How to update reference field in sobject

Question

I'm trying to update a sobject (CollaborationGroupFeed) via apex, but I am unable to update a particular reference field (in this case: CreatedBy).

Here is my code:

CollaborationGroupFeed cgf = [SELECT Id, CreatedById
                                                 FROM CollaborationGroupFeed
                                                 WHERE Id =: feedElement.Id
                                                 LIMIT 1];

                    User u = [SELECT Id, Name FROM User WHERE FirstName = 'Steve' AND LastName = 'Jobs'];
                    cgf.CreatedById = u.Id;
                    update cgf;

Error message: Field is not writeable: CollaborationGroupFeed.CreatedById

Answer 1

As Adrian Larson pointed out, the CreatedById field is not writable.

It is a System Field, and from the documentation on System Fields (emphasis mine)

System Fields: The following fields are read-only fields found on most objects. These fields are automatically updated during API operations. For example, the ID field is automatically generated during a create operation and the LastModifiedDate is automatically updated during any operation on an object.

CreatedByIdis automatically set when the record is inserted.