You can do this by using propertyIsOptional:. Just return YES for the names of the properties you want to make Optional.
https://github.com/icanzilb/JSONModel#make-all-model-properties-optional-avoid-if-possible
+(BOOL)propertyIsOptional:(NSString*)propertyName
{
if ([propertyName isEqualToString: @"objId"]) return YES;
return NO;
}