Group function in belongs_to association rails

Question

user.rb

enum gender: [:Male, :Female] 
belongs_to :qualification

qualification.rb

has_many :users

When I query

User.all.group(:gender).count.map { |k, v| [User.genders.key(k), v] }.to_h

It gives me

{"Male"=>18, "Female"=>1}

But I doesn't know to do same for qualification because association is involve in it.How I do it?

User.all.group(:qualification_id).count.map { ??? }

Answer 1

You can perform a join between your associated models by using joins() with the name of the association, and then group by any column of the joined model.

For example, if you want to group by the name column of qualification :

User.joins(:qualification).group('qualifications.name').count

Use the symbol :qualification describing the relation in the joins method, but use the table name 'qualifications' in the group method in order to specify the column. This will generate a SQL request like this :

SELECT COUNT(users.id)
FROM users
INNER JOIN qualifications ON users.qualification_id = qualifications.id
GROUP BY qualifications.name

• Documentation of ActiveRecord (search for joins) : http://guides.rubyonrails.org/active_record_querying.html
• Explanation of SQL JOINS : http://www.w3schools.com/sql/sql_join.asp