simple role system through associative table

rubyonrails-talk
1

So I have the Ninja model which has many Hovercrafts through ninja_hovercrafts (which stores the ninja_id and the hovercraft_id).

It is of my understanding that this kind of arrangement should be set in a way that the associative table stores only enough information to bind two different classes.

But I'd like to use the associative table to work as a very streamlined authorization hub on my application. So i'd also like this table to inform my system if this binding makes the ninja the pilot or co-pilot of a given hovercraft, through a "role" field in the table.

My questions are:

Is this ugly?

Is this normal?

Are there methods built into rails that would help me to automagically create Ninjas and Hovercrafts associations WITH the role? For exemple, could I have a nested form to create both ninjas and hcs in a way that the role field in ninjas_hovercrafts would be also filled?

If managing my application roles this way isn't a good idea, whats the non-resource heavy alternative (my app is being designed trying to avoid scalability problems such as excessive joins, includes, etc)

thank you

2

Daniel,

Adding attributes to the HMT model is an acceptable practice. Originally HABTM was used for many-to-many relationships and it assumed a simple table in the middle with the related ids. The problem was that the relationship table often needed to become a model of its own with additional attributes. This is why HMT came about and is the preferred method for many-to-many relationships.

I am not aware of a way to create all three models with the role in the field as you described. I believe you will have to code that in the controller.

If you do something like this and the association will set the role value for you…

class Pilot < ActiveRecord::Base

belongs_to :ninja

belongs_to :hovercraft

end

class Hovercraft < ActiveRecord::Base

has_many :pilots

has_many :piloted, :conditions => {:role => ‘pilot’}, :class_name => “Pilot”

has_many :copiloted, :conditions => {:role => ‘co-pilot’}, :class_name => “Pilot”

has_many :ninjas, :through => :pilots

has_many :copilot_ninjas, :through => :copiloted, :source => :ninja

has_many :pilot_ninjas, :through => :piloted, :source => :ninja

end

class Ninja < ActiveRecord::Base

has_many :pilots

has_many :piloted, :conditions => {:role => ‘pilot’}, :class_name => “Pilot”

has_many :copiloted, :conditions => {:role => ‘co-pilot’}, :class_name => “Pilot”

has_many :hovercrafts, :through => :pilots

has_many :copiloted_hovercrafts, :through => :copiloted, :source => :hovercraft

has_many :piloted_hovercrafts, :through => :piloted, :source => :hovercraft

end

Then you can do stuff like this…

Create hovercraft

hovercraft = …

Create pilot ninja

pilot_ninja = …

Create co-pilot ninja

copilot_ninja = …

Associate ninjas to hovercrafts

hovercraft.piloted.create(:ninja => pilot_ninja)

hovercraft.copiloted.create(:ninja => copilot_ninja)

Anthony Crumley

http://commonthread.com