处理用户定义的数据结构

Does anyone have any recommendations for dealing with user-defined data structures? i.e. your users have to be able to define extra fields, and sometimes tables, to associate with the 'fixed' entities in your system. I always seem to be unlucky enough to end up working on projects where this is a major component. Normally the solution ends up being dynamically generated SQL tables, with some terrifying SQL generation tacked on around the edges of the ORM to load and save the dynamic data.Does anyone have any recommendations for dealin