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    <p>It looks like you have two reasonable approaches here, but it will depend on the nuances of your requirements.</p> <p>You can use Single Table Inheritance (STI) to do what you want, where User is only the base class for others named SalesAgent or Client and so forth. Each of these sub-classes may define their own validations. All you need for this to work is a string column called "type" and ActiveRecord will do the rest:</p> <pre><code>class User &lt; ActiveRecord::Base end class Agent &lt; User end </code></pre> <p>The alternative is to have a number of free-form fields where you store various bits of related data and simply interpret them differently at run-time. You may structure it like this:</p> <pre><code>class User &lt; ActiveRecord::Base has_one :agent_role, :dependent =&gt; :destroy end class AgentRole &lt; ActiveRecord::Base belongs_to :user # Represents the agent-specific role fields end </code></pre> <p>That would have the advantage of allowing for multiple roles, and if you use has_many, then multiple roles of the same type.</p>
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    1. COI think STI was exactly what I was looking for. I'll experiment with it and see if it fits... Clarification, though: when you say that the latter method has "the advantage of allowing multiple roles"... what exactly do you mean here? Couldn't I still do `class Agent < User` and `class Client < User` ?? Still a bit confused.
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    2. COOn further investigation, it looks like Polymorphic Association would be a better fit for me. See this article: http://www.andygoh.net/2008/06/19/ruby-on-rails-polymorphic-association/ Still having trouble wrapping my head around actually using these associations, though... unfortunately, Ryan Bate's screencast on the subject is almost the exact reverse of what I want to accomplish (whereas he has a model that belongs to many other models, I want a model that has many other models)...?
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    3. COWhat I meant by the multiple roles comment was that a single User could have multiple roles associated with it, whereas with STI a user assumes a single role. Polymorphic associations can be good for some circumstances, but since they are impossible to enforce with foreign key constraints, and because joins with them always require a compound key, they are best left on the fringes of your database design. When part of the core they can be a nuisance. STI is "database friendly" since the type information is only relevant to the ORM, not the DB.
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