模型方法的交互作用

Question

Inside my "Course" model:

One method is supposed to select all user_ids which belong to a certain course

def participants
  Course.joins(:click).pluck(:user_id)
end

The other method is supposed to pick a random user_id

def set_winner
  Course.participants.sample
end

However, I get the following error:

undefined method `participants' for #<Class:0x007fc639811468>

If somebody could explain to me, why this doesn't work, I'd be really grateful.

Answer 1

Your example doesn't work because you define instance methods. And then you try to run them on class as if they are class methods.To fix it you could write:

def self.participants
  Course.joins(:click).pluck(:user_id)
end

def self.set_winner
  Course.participants.sample
end

or, better

class Course < ActiveRecord::Base
  scope :participants, -> { joins(:click).pluck(:user_id) }
  scope :set_winner, -> { participants.sample }
end