Exploiting abstraction while writing rails controllers

Abstraction is simple and beautiful while writing abstract code is an art.

While writing web apps apart from writing business logic most of the time we end up writing CRUD operations (Create, Read, Update Delete). There are a bunch of methods available to generate boilerplate code i.e. scaffolding, custom generators, etc. which have their own dos and don'ts.

I personally have been exploiting abstraction to write CRUD operations. Consider an example of developing a school management system where we have controllers like Student, Course, Section, Teacher, Exam, and many others. A typical student controller CRUD would be

class StudentsController < ApplicationController
  before_action :set_student, only: %i[ show edit update destroy ]

  def index
    @students = Student.all
  end

  def show
  end

  def new
    @student = Student.new
  end

  def edit
  end

  def create
    @student = Student.new(student_params)

    respond_to do |format|
      if @student.save
        format.html { redirect_to @student, notice: "Student was successfully created." }
        format.json { render :show, status: :created, location: @student }
      else
        format.html { render :new, status: :unprocessable_entity }
        format.json { render json: @student.errors, status: :unprocessable_entity }
      end
    end
  end

  def update
    respond_to do |format|
      if @student.update(student_params)
        format.html { redirect_to @student, notice: "Student was successfully updated." }
        format.json { render :show, status: :ok, location: @student }
      else
        format.html { render :edit, status: :unprocessable_entity }
        format.json { render json: @student.errors, status: :unprocessable_entity }
      end
    end
  end

  def destroy
    @student.destroy
    respond_to do |format|
      format.html { redirect_to students_url, notice: "Student was successfully destroyed." }
      format.json { head :no_content }
    end
  end

  private
    def set_student
      @student = Student.find(params[:id])
    end

    def student_params
      params.require(:student).permit(:first_name, :last_name, :roll_no, :date_of_birth)
    end
end

Sections controller CRUD would look like

class SectionsController < ApplicationController
  before_action :set_section, only: %i[ show edit update destroy ]

  def index
    @sections = Section.all
  end

  def show
  end

  def new
    @section = Section.new
  end

  def edit
  end

  def create
    @section = Section.new(section_params)

    respond_to do |format|
      if @section.save
        format.html { redirect_to @section, notice: "Section was successfully created." }
        format.json { render :show, status: :created, location: @section }
      else
        format.html { render :new, status: :unprocessable_entity }
        format.json { render json: @section.errors, status: :unprocessable_entity }
      end
    end
  end

  def update
    respond_to do |format|
      if @section.update(section_params)
        format.html { redirect_to @section, notice: "Section was successfully updated." }
        format.json { render :show, status: :ok, location: @section }
      else
        format.html { render :edit, status: :unprocessable_entity }
        format.json { render json: @section.errors, status: :unprocessable_entity }
      end
    end
  end

  def destroy
    @section.destroy
    respond_to do |format|
      format.html { redirect_to sections_url, notice: "Section was successfully destroyed." }
      format.json { head :no_content }
    end
  end

  private
    def set_section
      @section = Section.find(params[:id])
    end

    def section_params
      params.require(:section).permit(:name)
    end
end

And Course CRUD would be

class CoursesController < ApplicationController
  before_action :set_course, only: %i[ show edit update destroy ]

  def index
    @courses = Course.all
  end

  def show
  end

  def new
    @course = Course.new
  end

  def edit
  end

  def create
    @course = Course.new(course_params)

    respond_to do |format|
      if @course.save
        format.html { redirect_to @course, notice: "Course was successfully created." }
        format.json { render :show, status: :created, location: @course }
      else
        format.html { render :new, status: :unprocessable_entity }
        format.json { render json: @course.errors, status: :unprocessable_entity }
      end
    end
  end

  def update
    respond_to do |format|
      if @course.update(course_params)
        format.html { redirect_to @course, notice: "Course was successfully updated." }
        format.json { render :show, status: :ok, location: @course }
      else
        format.html { render :edit, status: :unprocessable_entity }
        format.json { render json: @course.errors, status: :unprocessable_entity }
      end
    end
  end

  def destroy
    @course.destroy
    respond_to do |format|
      format.html { redirect_to courses_url, notice: "Course was successfully destroyed." }
      format.json { head :no_content }
    end
  end

  private
    def set_course
      @course = Course.find(params[:id])
    end

    def course_params
      params.require(:course).permit(:title, :credit_hours, :code, :year)
    end
end

If you notice most of the time we are doing same kind of operation i.e. in the case of the show we first set the resource which could be Student, Course, Section, etc. In general, we are dealing with a resource or resources everywhere in the controllers.

In the Ruby on Rails world, we often talk about resources, we model every real-world object into Rails model as we did with the school management system Student, Course, Teacher, Exam. If we look at our above code we just specified resource name in reach controller for example in StudentsController we used Student, @student, @students, set_student, or student_params and the same is the case with CoursesController we have used Course, @course, @courses, set_course, or course_params, etc. In short, we are repeating the resource name in each file. We can come up with a  generic controller let say BaseController where we perform all operations on a resource rather than student, course, section, etc. Then, we just inherit StudentsController, CoursesController, TeachersController, ExamsController from a single source of truth BaseController.

Let's start with the show function and try to write BaseController.

class BaseController < ApplicationController
  before_action :set_resource, only: %i[ show ]

  def show
  end

  def set_resource
    resource ||= resource_class.find(params[:id])
    instance_variable_set("@#{resource_name}", resource)
  end

  def resource_class
    @resource_class ||= resource_name.classify.constantize
  end

  def resource_name
    @resource_name ||= controller_name.singularize
  end
end

Writing create function is a bit more interesting

  def create
    @resource = resource_class.new(resource_params)

    respond_to do |format|
      if @resource.save
        format.html { redirect_to @resource, notice: "#{resource_name} was successfully created." }
        format.json { render :show, status: :created, location: @resource }
      else
        format.html { render :new, status: :unprocessable_entity }
        format.json { render json: @resource.errors, status: :unprocessable_entity }
      end
    end
  end

update and destroy functions are somewhat similar

  def update
    @resource = resource_class.find(params[:id])

    respond_to do |format|
      if @resource.update(resource_params)
        format.html { redirect_to @resource, notice: "#{resource_name} was successfully updated." }
        format.json { render :show, status: :ok, location: @resource }
      else
        format.html { render :edit, status: :unprocessable_entity }
        format.json { render json: @resource.errors, status: :unprocessable_entity }
      end
    end
  end

  def destroy
    @resource = resource_class.find(params[:id])

    @resource.destroy
    respond_to do |format|
      format.html { redirect_to "#{controller_name}_url", notice: "#{resource_name} was successfully destroyed." }
      format.json { head :no_content }
    end
  end

In all of the above rewrites, we just simply used resources as a generic variable attribute. destroying an object is the same no matter its Student, Course, Section, or Exam so resource.destroy works the same for each resource.

Our final refactoring will look like this

Since controllers are inherited from BaseController we can always override any method in the respective controller. For example, instead of directly creating a Student object we have a service written which takes student params and creates Student object after applying a bunch of business logic and pre-checks. For such scenarios, we can override create the function in StudentsController.

We can use the same pattern while writing our APIs and all other scenarios where we have a similar pattern repetition.

Thank You so much for the read.