rails Unizillaize constant in Association

Question

I'm trying to find the relations between Surgeon and Appointments.

2.0.0-p195 :001 > surgeon = Surgeon.first
  Surgeon Load (0.1ms)  SELECT "surgeons".* FROM "surgeons" ORDER BY "surgeons"."id" ASC LIMIT 1
 => #<Surgeon id: 5, name: "Micheal Gorbachev", created_at: "2014-04-13 07:15:42", updated_at: "2014-04-13 07:15:42"> 

2.0.0-p195 :008 > surgeon.appointments
NameError: uninitialized constant Surgeon::Appointment

but this works

2.0.0-p195 :007 > surgeon.association :appointments

  2.0.0-p195 :007 > surgeon.association :appointments
   => #<ActiveRecord::Associations::HasManyAssociation:0x00000002a46850 
   @reflection=#<ActiveRecord::Reflection::AssociationReflection:0x000000030dc4d0 
   @macro=:has_many, @name=:appointments, @scope=nil, @options={}, 
   @active_record=Surgeon(id: integer, name: string, created_at: datetime, 
   updated_at: datetime), @plural_name="appointments", @collection=true>, 
   @owner=#<Surgeon id: 5, name: "Micheal Gorbachev", created_at: "2014-0

this is my Model

class Surgeon < ActiveRecord::Base
       has_and_belongs_to_many :instruments

       #section 2
       has_many :appointments
       has_many :interventions, :through => :appointments
    end
    class Intervention < ActiveRecord::Base
      has_and_belongs_to_many :instruments

      #section 2
      has_many :appointments
      has_many :surgeons, :through => :appointments
    end
    class Instrument < ActiveRecord::Base
       has_and_belongs_to_many :surgeons
       has_and_belongs_to_many :interventions
    end
    class Appointments < ActiveRecord::Base

      #section 2
      belongs_to :surgeon
      belongs_to :intervention
    end

Answer 1

Models must be named in singular, not plural.

class Appointment < ActiveRecord::Base