Adds JoinedUserActivity model to associate users to every activities they created

app/controllers/api/v1/activities_controller.rb

@@ -13,6 +13,10 @@ class Api::V1::ActivitiesController < ApplicationController
   def create
     activity = current_user.created_activities.build(activity_params)
     if activity.save
+      JoinedUserActivity.create!(
+        user_id: current_user.id,
+        activity_id: activity.id
+      )
       render json: activity, status: :created
     else
       render json: { errors: activity.errors }, status: :unprocessable_entity

app/models/activity.rb

@@ -1,6 +1,9 @@
 class Activity < ApplicationRecord
   belongs_to :author, class_name: "User"
   
+  has_one :joined_user_activities
+  has_one :user, :through => :joined_user_activities
+
   validates :name, presence: true
   validates :author, presence: true
 end

app/models/joined_user_activity.rb

@@ -0,0 +1,4 @@
+class JoinedUserActivity < ApplicationRecord
+  belongs_to :user
+  belongs_to :activity
+end

app/models/user.rb

@@ -5,5 +5,9 @@ class User < ApplicationRecord
   validates :password_digest, presence: true
 
   has_many :created_activities, foreign_key: 'author_id', class_name: 'Activity', dependent: :destroy
+
+  has_many :joined_user_activities
+  has_many :activities, :through => :joined_user_activities
+
   has_secure_password
 end