How to know if an Model has a relationship set in Ruby?

Question

I have a model like this:

class Year < ActiveRecord::Base
  extend FriendlyId
  friendly_id :year, use: :slugged

  has_many :events

I want in my controller, to get all the years that have at least one event. How can I do that?

I was trying to make this:

 Year.includes(:events).first.events
  Year Load (0.3ms)  SELECT  `years`.* FROM `years`  ORDER BY `years`.`id` ASC LIMIT 1
  Event Load (0.3ms)  SELECT `events`.* FROM `events` WHERE `events`.`year_id` IN (1)
 => #<ActiveRecord::Associations::CollectionProxy []>

But this is returning empty sets.

Ursus · Answer 1 · 2018-06-11T14:08:04.960

4

Join with events. This gives you all your years except those ones that don't have an event attached.

Year.joins(:events).distinct

edited Jun 11 '18 at 14:08

answered Jun 11 '18 at 14:00

Ursus

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This may return duplicates. One of my previous answer cover this issue: https://stackoverflow.com/questions/18234602/rails-active-record-querying-association-with-exists/18234998#18234998 – MrYoshiji Jun 11 '18 at 14:04
Yes. Its returning duplicates :( – Fernando Maymone Jun 11 '18 at 14:07
`Year.joins(:events).distinct` – Ursus Jun 11 '18 at 14:07
Or `Year.where(id: Event.select(:year_id).distinct)`. SQL `SELECT years.* FROM years WHERE years.id IN ( SELECT DISTINCT events.year_id FROM events)` – engineersmnky Jun 11 '18 at 14:11

How to know if an Model has a relationship set in Ruby?

1 Answers1