CODEWITHSUNDEEP
sqlruby-on-railsjoinarel
James Ostevik
James Ostevik

Reputation: 11

Arel join sub query

I'm new to Rails and Arel I created a join query but looks poorly written

join_query = 
ActiveRecord::Base.connection.execute <<~SQL.strip_heredoc
        (
          SELECT schools.name, sum(q.enrollment), count(*) FROM schools LEFT JOIN
            (
              SELECT sites.school_id, sites.enrollment  FROM sites JOIN projects ON
              sites.project_id = projects.id
              WHERE sites.school_id IS NOT NULL and projects.type IN ('Startup') AND projects.deleted_at IS NULL
            ) q
          ON q.school_id = schools.id
          GROUP BY schools.name
        )
      SQL
join_query.values

How can this be written using Arel?
Also, could you point me out to a good tutorial in this subject?
Thanks in advance

Upvotes: 1

Views: 920

Answers (1)

Son Dang
Son Dang

Reputation: 371

This is what I could come up with the given SQL:

schools = Arel::Table.new("schools")
sites = Arel::Table.new("sites")
projects = Arel::Table.new("projects")

join = sites.project(:school_id, :enrollment)
  .join(projects).on(sites[:project_id].eq(projects[:id]))   
  .where(
    sites[:school_id].not_eq(nil)
    .and(projects[:type].in("Startup"))
    .and(projects[:deleted_at].eq(nil))
  )

q = Arel::Table.new(join).alias(:q)

schools.project(:name, q[:enrollment].sum, Arel.star.count)
  .join(join, Arel::Nodes::OuterJoin).on(q[:school_id].eq(schools[:id]))
  .group(schools[:name])

Being a newbie, these are some of the references I often visit:

https://www.rubydoc.info/github/rails/arel (official docs)

https://devhints.io/arel (a very useful cheatsheet but not complete)

https://gist.github.com/mildmojo/3724189 (example of left outer join)

https://jpospisil.com/2014/06/16/the-definitive-guide-to-arel-the-sql-manager-for-ruby.html (comprehensive guide to Arel)

Upvotes: 2

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