Question

Let’s say I’ve got the following (simplified) data model:

class Location
  include Mongoid::Document

  has_many :event_dates
end

class EventDate
  include Mongoid::Document

  has_many :event_sessions
  belongs_to :location

  def total_spots
    self.event_sessions.sum("max_participants")
  end

  def booked_spots
    self.event_sessions.sum("participants_count")
  end

  def available_spots
    self.total_spots - self.booked_spots
  end
end

class EventSession
  include Mongoid::Document
  belongs_to :event_date

  field :max_participants
  field :participants_count
end

I know can get a all of location’s event dates simply:

Location.event_dates

But how can I scope the query to just get, for example, event dates with available spots? i.e.

Location.event_dates.where(available_spots > 0)

Since available_spots is a calculated value and not an actual database field of EventDate, that doesn’t seem possible. I was hoping to avoid adding those calculation methods to the DB itself, but if it’s the only way….

Thanks in advance!

Answer 1

Ok, looks like the answer was hiding in plain sight (as usual), under the Extensions section of the Mongoid documentation:

Relations – Common Behavior: Extensions

Two part answer:

First, the method I want is select, not where, so while I can’t do what I proposed above, I can do:

Location.event_dates.select {|date| date.available_spots > 0}

To “scopify” that (in essense), what I want is to extend the relation as defined in Location as follows:

class Location
  has_many :event_dates do
    def with_available_spots
      @target.select {|date| date.available_spots > 0}
    end

So now I can call, simply:

Location.event_dates.with_available_spots

Et voila.