How to fetch distinct values with arel/relational algebra

Question

I'm doing my best to bend my brain around arel and the relational algebra behind it, but how to represent a SELECT DISTINCT is consistently eluding my comprehension. Can anyone explain how to arel:

SELECT DISTINCT title FROM posts; 

Many thanks!

Answer 1

Post.select('DISTINCT title')

Update 1:

At the time of the post, this was not available in Arel. These days, ActiveRecord::QueryMethods has the uniq method (http://apidock.com/rails/ActiveRecord/QueryMethods/uniq), so you'd want:

Post.select(:title).uniq

Update 2: Looks like Arel now supports this behavior. @maerics has the correct answer. I'd delete this if it wasn't the accepted answer.

Answer 2

Using pure Arel (not Rails/ActiveRecord) there is a "distinct" method:

Arel::VERSION # => '3.0.2'
posts = Arel::Table.new(:posts)
posts.project(posts[:title])
posts.distinct
posts.to_sql # => 'SELECT DISTINCT "posts"."title" FROM "posts"'

Curiously, the "distinct" method is not chainable, per the other Arel methods.

Answer 3

The Arel way to do it is:

t = Arel::Table.new(:foo)
count_distinct = t[:field].count(true)
count_distinct.to_sql # => "COUNT(DISTINCT `foo`.`field`)"

Answer 4

The previous answer is the Rails way, no? Not the Arel way.

This works for arel 1.x:

posts = Table(:posts)
posts.project(Arel::Distinct.new(posts[:title]))

I'd guess there's another "more correct" way to do this via the API but I haven't figured that out yet.

Answer 5

If you are doing this using a scope:

  scope :recent, lambda {|count|
    select("DISTINCT posts.*").
    joins(:whatever).
    limit(count).
    order("posts.updated_at DESC")
  }

Answer 6

Since AREL always uses SET in it's operation, duplicate row results will be deleted automatically. Just use a normal Project (Phi) operation.