It occurred to me that if I have a has_many join, where the foreign model does not have a belongs_to, and so the join is one way, then I don't actually need a foreign key.

We could have a column, category_ids, which stores a marshaled Array of IDs which we can pass to find.

So here is an untested example:

class page < AR

  def categories
    Category.find(self.category_ids)
  end

  def categories<<(category)
    # get id and append to category_ids
    save!
  end

  def category_ids
    @cat_ids ||= Marshal.load(read_attribute(:category_ids)) rescue []
  end

  def category_ids=(ids)
    @cat_ids = ids
    write_attribute(:category_ids, ids)
  end

end

page.category_ids => [1,4,12,3] page.categories => Array of Category

Is there accepted pattern for this already? Is it common or just not worth the effort?