Scoping through a has_many relationship in rails

Question

I have a model that I'd like to scope based on an associated model. Here are the models, ClassSection and ClassDate.

class ClassSection < ActiveRecord::Base
  has_many :class_dates
  accepts_nested_attributes_for :class_dates, allow_destroy: true

  def begins
    self.class_dates.order('start_time ASC').limit(1).first.start_time
  end

  def ends
    self.class_dates.order('end_time DESC').limit(1).first.end_time
  end

end

Each ClassDate has two datetime values, :start_time and :end_time.

class ClassDate < ActiveRecord::Base
  belongs_to :class_section
  validates :start_time, :presence => true
  validates :end_time, :presence => true
  validate  :end_time_must_be_greater_than_start_time,
            :end_time_must_not_be_greater_than_24_hours_after_start_time

  def end_time_must_be_greater_than_start_time
    if start_time > end_time
      errors.add(:end_time, "End time must be after start time.")
    end
  end

  def end_time_must_not_be_greater_than_24_hours_after_start_time
    if start_time < end_time - 1.day
      errors.add(:end_time, "End time cannot be more than 24 hours after start time.")
    end
  end
end

I want a scope called :in_session where today's date falls in between class_section.begins and class_section.ends. This does not work:

scope :in_session, where(class_section.begins < Time.now) && where(class_section.ends > Time.now)

Is it possible to scope using either the methods I've written or through the association with ClassDate? What would be the most efficient way to do this?

Answer 1

Yes it's possible. In Rails 4 however, you need to use lambda scope and not eagerly load the conditions as you've done.

Try:

scope :in_session, -> { joins(:class_section).where('? between class_sections.begins and class_sections.ends', Time.now) }

Your table name class_section is singular as you've shown your scope, it should be plural class_sections unless you've explicitly specified it to be singular. I've used plural class_sections in the scope above, update it to singular class_section if your table name is singular.