Init Record model

app/models/activity.rb

 class Activity < ApplicationRecord
   belongs_to :author, class_name: "User"
-  
   has_one :joined_user_activity
   has_one :user, :through => :joined_user_activity
-
   has_many :tasks
+  has_many :records
 
   validates :name, presence: true
   validates :author, presence: true

app/models/record.rb

+class Record < ApplicationRecord
+  belongs_to :user
+  belongs_to :task
+end

app/models/user.rb

   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_many :tasks
-
+  has_many :records
+  
   has_secure_password
 end

db/migrate/20210511212634_create_records.rb

+class CreateRecords < ActiveRecord::Migration[6.1]
+  def change
+    create_table :records do |t|
+      t.integer :duration, null: false
+      t.boolean :is_handled, default: false
+      t.references :handled_by, default: :null
+      t.references :user, null: false
+      t.references :task, null: false
+
+      t.timestamps
+    end
+  end
+end

db/schema.rb

 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 2021_05_10_133939) do
+ActiveRecord::Schema.define(version: 2021_05_11_212634) do
 
   create_table "activities", force: :cascade do |t|
     t.string "name", null: false
     t.index ["user_id"], name: "index_joined_user_activities_on_user_id"
   end
 
+  create_table "records", force: :cascade do |t|
+    t.integer "duration", null: false
+    t.boolean "is_handled", default: false
+    t.integer "handled_by_id"
+    t.integer "user_id", null: false
+    t.integer "task_id", null: false
+    t.datetime "created_at", precision: 6, null: false
+    t.datetime "updated_at", precision: 6, null: false
+    t.index ["handled_by_id"], name: "index_records_on_handled_by_id"
+    t.index ["task_id"], name: "index_records_on_task_id"
+    t.index ["user_id"], name: "index_records_on_user_id"
+  end
+
   create_table "tasks", force: :cascade do |t|
     t.string "name", null: false
     t.text "description"

test/fixtures/records.yml

+# Read about fixtures at https://api.rubyonrails.org/classes/ActiveRecord/FixtureSet.html
+
+one:
+  duration: 2021-05-11 23:26:34
+  handled: false
+  user: one
+  task: one
+
+two:
+  duration: 2021-05-11 23:26:34
+  handled: false
+  user: two
+  task: two

test/models/record_test.rb

+require "test_helper"
+
+class RecordTest < ActiveSupport::TestCase
+  # test "the truth" do
+  #   assert true
+  # end
+end