LEFT OUTER JOIN in Rails 4

Question

I have 3 models:

class Student < ActiveRecord::Base
  has_many :student_enrollments, dependent: :destroy
  has_many :courses, through: :student_enrollment         


        

Answer 1

There is actually a "Rails Way" to do this.

You could use Arel, which is what Rails uses to construct queries for ActiveRecrods

I would wrap it in method so that you can call it nicely and pass in whatever argument you would like, something like:

class Course < ActiveRecord::Base
  ....
  def left_join_student_enrollments(some_user)
    courses = Course.arel_table
    student_entrollments = StudentEnrollment.arel_table

    enrollments = courses.join(student_enrollments, Arel::Nodes::OuterJoin).
                  on(courses[:id].eq(student_enrollments[:course_id])).
                  join_sources

    joins(enrollments).where(
      student_enrollments: {student_id: some_user.id, id: nil},
      active: true
    )
  end
  ....
end

There is also the quick (and slightly dirty) way that many use

Course.eager_load(:students).where(
    student_enrollments: {student_id: some_user.id, id: nil}, 
    active: true
)

eager_load works great, it just has the "side effect" of loding models in memory that you might not need (like in your case)
Please see Rails ActiveRecord::QueryMethods .eager_load
It does exactly what you are asking in a neat way.