How to create a model without primary key in rails?

Create a migration that looks like this:

class CreateQuestionsTags < ActiveRecord::Migration

  def self.up
   create_table :questions_tags, {:id => false, :force => true} do |t|
     ...
     t.timestamps
   end
  end

  def self.down
   drop_table :questions_tags
  end

end

If you're looking to create a pivot table, as it looks like from the table name, then AR will handle that in the background.

However, if you're looking to create a table with more feilds then: 1) rename your table to "realtions" please 2) use a primary key "id"

There's no good reason not to be using a primary key in a table, and it is very likely that you might well find yourself regretting it later.