Implemented add_user in courses controller

app/controllers/api/v1/courses_controller.rb

@@ -30,7 +30,38 @@ def destroy
       end
 
       def add_user
-        render :json => 'The add_user method of CoursesControler us not yet implemented'.to_json, status: 501
+        user_id = params[:user_id]
+        course_id = params[:course_id]
+        role = params[:role]
+
+        # Check if the provided course_id is valid i.e. exists in courses table
+        if !Course.find_by(id: course_id)
+          render json: { error: "The course does not exist." }, status: :not_found
+          return
+        end
+
+        # Check if the provided user is valid i.e. exists in users table
+        if !User.find_by(id: user_id)
+          render json: { error: "The user does not exist." }, status: :not_found
+          return
+        end
+
+        # Check if the user has been already added to the course
+        existing_user_to_course = UserToCourse.find_by(course_id: course_id, user_id: user_id)
+        if existing_user_to_course
+          render json: { error: "The user is already added to the course."}, status: :unprocessable_entity
+          return
+        end
+
+        # Add the user to the course with the desired role
+        new_user_to_course = UserToCourse.new(course_id: course_id, user_id: user_id, role: role)
+        if new_user_to_course.save
+          flash[:success] = "User added to the course successfully."
+          render json: new_user_to_course, status: :created
+        else
+          flash[:error] = "Failed to add the user the to course."
+          render json: new_user_to_course.errors, status: :unprocessable_entity
+        end
       end
     end
   end

app/models/user_to_course.rb

@@ -0,0 +1,10 @@
+class UserToCourse < ApplicationRecord
+  # Associations
+  belongs_to :user
+  belongs_to :course
+
+  # Validations
+  validates :user_id, presence: true
+  validates :course_id, presence: true
+  validates :role, presence: true
+end

spec/controllers/api/v1/courses_controller_spec.rb

@@ -44,9 +44,36 @@ module V1
       end
 
       describe 'add_user' do
-        it 'throws a 501 error' do
-          put :add_user, params: { course_id: 16, user_id: 16 }
-          expect(response.status).to eq(501)
+        let(:test_course) { Course.create(course_name: "Test Course") }
+        let(:test_user) { User.create(email: "[email protected]") }
+
+        context "Provided parameters are valid" do
+          it "adds an existing user to an existing course" do
+            post :add_user, params: { course_id: test_course.id, user_id: test_user.id, role: "ta" }
+            expect(response).to have_http_status(:created)
+            expect(flash["success"]).to eq("User added to the course successfully.")
+          end
+        end
+
+        context "Provided parameter are invalid" do
+          it "returns an error if course is not existed in the courses table" do
+            post :add_user, params: { course_id: 123456, user_id: test_user.id, role: "ta" }
+            expect(response).to have_http_status(:not_found)
+            expect(JSON.parse(response.body)["error"]).to eq("The course does not exist.")
+          end
+
+          it "returns an error if user is not existed in the users table" do
+            post :add_user, params: { course_id: test_course.id, user_id: 123456, role: "ta" }
+            expect(response).to have_http_status(:not_found)
+            expect(JSON.parse(response.body)["error"]).to eq("The user does not exist.")
+          end
+
+          it "returns an error if the user is already associated with the course" do
+            post :add_user, params: { course_id: test_course.id, user_id: test_user.id, role: "student" }
+            post :add_user, params: { course_id: test_course.id, user_id: test_user.id, role: "student" }
+            expect(response).to have_http_status(:unprocessable_entity)
+            expect(JSON.parse(response.body)["error"]).to eq("The user is already added to the course.")
+          end
         end
       end
     end