Polymorphic either or but not both

I am trying to create a generic person model. Then subclass it as
either a professor or an author. The only problem is that since some
professors are authors, I don't want duplicate entries. Any ideas on
how to handle this?

I wouldn’t use a polymorphic model for this, I would use single table inheritance.

Have a single table People with a type column, then have a model Person and subclasses for each type so:

class Person < ActiveRecord::Base

end

class Professor < Person
end

class Author < Person
end

I just went back and re-read your original post, single table won't
work either since you have the possibility of a person having two
types.

The solution is going to be a little more complex as you will need to
create a many to many relationship.

I would use 3 tables:

    create_table :people do |t|
      t.column :id, :integer
      t.column :name, :string
    end

    create_table :person_roles do |t|
      t.column :id, :integer
      t.column :person_id, :integer
      t.column :role_id, :integer
    end

    create_table :people do |t|
      t.column :id, :integer
      t.column :role, :string
    end

"People" would house the information about the person, "Roles" would
be a domain of roles a person could play (Author, professor, student),
and "Person_Roles" would allow the many to many.

You would then have:

class Person < ActiveRecord::Base
  has_many :roles, :through => :person_roles
  has_many :person_roles
end

Hope that helps, sorry for my hasty first post!

Joe

I should know never to post prior to my morning caffeine intake… There is a typo on the third table definition:

create_table :people do |t|
t.column :id, :integer
t.column :role, :string
end

Should be:
create_table :roles do |t|
t.column :id, :integer
t.column
:role, :string
end