Question

Models:

class User < ActiveRecord::Base
  has_many :attendances
  has_many :courses, :through => :attendances
end

class Course < ActiveRecord::Base
  has_many :attendances
end

class Attendance < ActiveRecord::Base
  belongs_to :user
  belongs_to :course
end

Migrations:

create_table(:users) do |t|
  t.string :name
end

create_table(:courses) do |t|
  t.string :name
end

create_table(:attendances) do |t|
  t.references :user, :course
  t.date :date
end

Question

I would like to query:

• a list of all courses
• the date of a given user’s last attendance (if any) for each course

What is the best way to tie the following together into a single query?

@courses = Course.all
@user = User.first
@attendances = @user.attendances.group('course_id').order('date DESC')

Note that there is a requirement to include courses that a user has not yet attended.

Any advice much appreciated.

Update

The result I am looking for is as follows:

Course            Last attended
===============================
Some course       2011-03-09
More training     Not Attended
Another course    2010-12-25

In SQL, I would write this query as:

SELECT * FROM courses AS c
LEFT OUTER JOIN attendances AS a ON a.course_id=c.id
WHERE a.user_id=1
GROUP BY a.course_id
ORDER BY a.date DESC;

I could always execute this SQL from within Rails, but I would prefer to avoid this and do things “the Rails way” if possible.

Answer 1

The “Rails-Way” is to define small finders in the model and then to chain them in the controllers.

In class Attendance you could define a new method

def self.last_attendance
  maximum("date")
end

In class Corse …

def self.ordered
  order("name DESC")
end

And so on. In the controller then, you use them in different combinations. The big advantage of this approach is

• Reusability of finder methods
• Decoupling of classes
• Better readability of code