Updating data on the server side

GORM Mapping
Use belongsTo or fkId (A.ID = B.A_ID):
- belongsTo
- in a Grails bulk creation transaction, IDs are created when the transaction flushes
- newly created item A doesn't have A.ID to be used as the fk B.A_ID
- using belongsTo allows object reference
- fkId
- A and B don't need to be created in a single transaction
- this allows flat object to convert to map for json