LEFT OUTER JOIN in Rails 4

You can pass a string that is the join-sql too. eg joins("LEFT JOIN StudentEnrollment se ON c.id = se.course_id")

Though I'd use rails-standard table naming for clarity:

joins("LEFT JOIN student_enrollments ON courses.id = student_enrollments.course_id")

If anyone came here looking for a generic way to do a left outer join in Rails 5, you can use the #left_outer_joins function.

Multi-join example:

Ruby:

Source. select('sources.id', 'count(metrics.id)'). left_outer_joins(:metrics). joins(:port). where('ports.auto_delete = ?', true). group('sources.id'). having('count(metrics.id) = 0'). all

SQL:

SELECT sources.id, count(metrics.id)  FROM "sources"  INNER JOIN "ports" ON "ports"."id" = "sources"."port_id"  LEFT OUTER JOIN "metrics" ON "metrics"."source_id" = "sources"."id"  WHERE (ports.auto_delete = 't')  GROUP BY sources.id  HAVING (count(metrics.id) = 0)  ORDER BY "sources"."id" ASC

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.