Rendering a JSON object of a join-model and its associated models

Question

In a Rails ( 4.1.5 / ruby 2.0.0p481 / win64 ) application I have a many-to-many relationship between Student and Course and a join model StudentCourse

Answer 1

Use JBuilder, it comes by default with Rails. Ignore the lines starting with '#':

Jbuilder.new do |j|

    # courses: [{
    j.courses <student.courses - replace this with whatever variable> do |course|

      # id: <course.id>
      j.id course.id 

      # name: <course.name>
      j.name course.name

      # students: [{
      j.students <course.students - replace with whatever variable> do |student|

          # name: <student.name>
          j.name student.name

      end
      # }]

   end
   # }]

end

Not a lot of code. Removing the comments and simplifying some features, it will look like:

student_courses = <...blah...>
json = Jbuilder.new do |j|
  j.courses student_courses do |course|
    j.(course, :id, :name)
    j.students <course.students - whatever method here>, :name
  end
end.target!

Check out their guide, its pretty awesome to generate JSON in plain Ruby DSL. So go ahead and use whatever ruby code you want to fetch students/courses/studentcourses.

Answer 2

Use scope to your advantage:

class Course < ActiveRecord::Base
  # ...
  scope :not_started, -> { joins(:student_courses) \
                                      .where(student_courses: {started: false}) }

  scope :with_classmates, -> { includes(:students) } # use eager loading
end

Then call:

@student.courses.not_started.with_classmates \
                                     .to_json(include: {students: {only: :name}})

Output:

[
    {
        "id": 1,
        "name": "english",
        "description": null,
        "students": [
            {"name": "Aiden"},
            {"name": "Alison"}]},
    {
        "id": 2,
        "name": "music",
        "description": null,
        "students": [
            {"name": "Aiden"},
            {"name": "Alison"}]},
    {
        "id": 3,
        "name": "dance",
        "description": null,
        "students": [
            {"name": "Aiden"}]}]