Multiple scoped has_many through relationship in Rails 4 [duplicate] - ruby-on-rails

Question

This question already has an answer here:
Losing an Attribute When Saving Through an Association w/ Scope (Rails 4.0.0)
(1 answer)
Closed 9 years ago.
I want to model such a relationship between the models User and Event.
Therefore I have started with the following classes:
class User < ActiveRecord::Base
...
end
class Attendance < ActiveRecord::Base
# with columns user_id and event_id
...
end
class Event < ActiveRecord::Base
has_many :attendances
has_many :users, :through => :attendances
...
end
So far everything is okay: I can assign users and access attendances. But now I want to bring the state into play, such that I can distinguish e.g. between "attending", "unexcused absent", ... users. My first try was:
class Event < ActiveRecord::Base
has_many :attendances
has_many :users, :through => :attendances
has_many :unexcused_absent_users, -> { where :state => 'unexcused' },
:through => :attendances,
:source => :user
...
end
(:source has to be specified since otherwise it would search for a belongs to association named 'unexcused_absent_users')
The problem here is, that the where-predicate is evaluated on table 'users'.
I am clueless how to solve this 'correctly', without introducing new join tables/models for every state. Especially since every user can be just in one state for every event, I think a solution with one Attendance-model makes sense.
Have you an idea, how to get this right?

Answer 1

You can simply narrow the scope to look at the correct table:
has_many :unexcused_absent_users, -> { where(attendances: {state: 'unexcused'}) },
:through => :attendances,
:source => :user
Evem better, add this scope to the Attendance model and merge it in:
class Attendance < ActiveRecord::Base
def self.unexcused
where state: 'unexcused'
end
end
class Event < ActiveRecord::Base
has_many :unexcused_absent_users, -> { merge(Attendance.unexcused) },
:through => :attendances,
:source => :user
end

Answer 2

I have found a workaround, but I still think, this is ugly.
class Event < ActiveRecord::Base
has_many :user_attendances, :class_name => 'Attendance'
has_many :users, :through => :user_attendances, :source => :user
has_many :unexcued_absent_user_attendances, -> { where :state => 'unexcused'}, :class_name => 'Attendance'
has_many :unexcused_absent_users, :through => :unexcued_absent_user_attendances, :source => :user
end
In general: For every state that I want, I have to introduce a new has_many relationship with a scope and on top of that and an according has_many-through relationship.

Answer 3

this might work for you?
class Event < ActiveRecord::Base
has_many :attendances
has_many :users, :through => :attendances
def unexcused_absent_users
User.joins(:attendances)
.where(:state => 'unexcused')
.where(:event_id => self.id)
end
end
in rails 3+ methods are basically the same as scopes, just less confusing (in my opinion), they are chainable
event = Event.find(xxxx)
event.unexcused_absent_users.where("name LIKE ?", "Smi%")