GROUP BY and COUNT using ActiveRecord

Question

Refering to this: Is there any difference between GROUP BY and DISTINCT

Given a table that looks like this:

name
------
barry
dave
bill
dave
dave
barry
john
This query:

SELECT name, count(*) AS count FROM table GROUP BY name;
Will produce output like this:

name    count
-------------
barry   2
dave    3
bill    1
john    1

What is the correct Rails convention here for ActiveModel to perform a GROUP BY with COUNT?

Solution 1

Distinct and Group By are going to give you different results. To get the results you expect you'll want to use

Person.group(:name).count
(1.2ms)  SELECT COUNT(*) AS count_all, name AS name FROM "people" GROUP BY "people"."name"
=> {"Dan"=>3, "Dave"=>2, "Vic"=>1} 

Seen above, group will return things as a hash. While distinct just returns the number of people in total, seen below.

Person.distinct(:name).count
(0.4ms)  SELECT DISTINCT COUNT(DISTINCT "people"."id") FROM "people"
=> 6 

Solution 2

Note that the accepted answer will return a hash:

Tagging.joins(:tag).group(:name).size
   (0.4ms)  SELECT COUNT(*) AS count_all, `name` AS name FROM `taggings` INNER JOIN `tags` ON `tags`.`id` = `taggings`.`tag_id` GROUP BY `name`
 => {"applesauce"=>1, "oranges"=>2} 

But what if you want to return the count plus some columns from different tables in a join. Then you also need to use the select ActiveRecord query:

collection = Tagging.select('COUNT(*) as total, taggings.taggable_id, taggings.taggable_type').joins(:tag).where(taggable_type: 'LineItem', taggable_id: ['5cad0dcc3ed1496086000031', '5cad0dcd3ed1496086000081'] ).group(:name)

collection.each do |item|
  puts item.taggable_id
  puts item.total
end

5cad0dcc3ed1496086000031
1
5cad0dcc3ed1496086000031
2

With this second approach, you can fetch additional details about a join relationship without any additional queries or loop constructs.

Solution 3

another option:

Person.select(:name, 'COUNT(name)').group(:name)

this generate an array of people with attribute count