IN the old version I could call set on a field that did not exist (ie not declared in the fields array) and all was well, everything worked. In 4.1 that doesnt work. MY problem is that I dont know the fields I am going to get until very late - i get data from my server and it has fields in it; i just load them into the model object and off we go. I will admit that for some (not all) fields i did ext.apply(item.data, serverObject); which is not strictly speaking following the published interface but worked very nicely. Certainly there is no way that I can statically define a separate model item for my objects, I do not know in advance what they will be. Today I have just one 'abstract' class that has a well known set of control fields on it