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

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 2

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 3

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.