GROUP BY and COUNT using ActiveRecord

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?

4 Answers

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 

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.

another option:

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

this generate an array of people with attribute count

Another option:

Person.select(:name).distinct(:name).count