Add activity model and unit tests

app/models/activity.rb

+class Activity < ApplicationRecord
+  belongs_to :author, class_name: "User"
+  
+  validates :name, presence: true
+  validates :author, presence: true
+end

app/models/user.rb

   validates :email, format:{ with: /\S+@\S+[.]\S+/, message: "Must be a valid email format." }
   validates :password_digest, presence: true
 
+  has_many :created_activities, foreign_key: 'author_id', class_name: 'Activity', dependent: :destroy
   has_secure_password
 end

db/migrate/20210413142821_create_activities.rb

+class CreateActivities < ActiveRecord::Migration[6.1]
+  def change
+    create_table :activities do |t|
+      t.string :name, index: true, null: false
+      t.string :client
+      t.string :description
+      t.references :author, null: false, foreign_key: { to_table: :users }
+
+      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_04_13_093535) do
+ActiveRecord::Schema.define(version: 2021_04_13_142821) do
+
+  create_table "activities", force: :cascade do |t|
+    t.string "name", null: false
+    t.string "client"
+    t.string "description"
+    t.integer "author_id", null: false
+    t.datetime "created_at", precision: 6, null: false
+    t.datetime "updated_at", precision: 6, null: false
+    t.index ["author_id"], name: "index_activities_on_author_id"
+    t.index ["name"], name: "index_activities_on_name"
+  end
 
   create_table "users", force: :cascade do |t|
     t.string "email", null: false
     t.index ["username"], name: "index_users_on_username", unique: true
   end
 
+  add_foreign_key "activities", "users", column: "author_id"
 end

test/fixtures/activities.yml

+# Read about fixtures at https://api.rubyonrails.org/classes/ActiveRecord/FixtureSet.html
+
+one:
+  name: RandomActivity
+  client: MyClient
+  description: RandomActivity
+  author: one

test/fixtures/users.yml

 one:
   email: one@one.com
   username: OneUsername
-  password_digest: <%= BCrypt::Password.create('g00d_pa$$') %>
+  password_digest: <%= BCrypt::Password.create('g00d_pa$$') %>

test/models/activity_test.rb

+require "test_helper"
+
+class ActivityTest < ActiveSupport::TestCase
+  test "activity's author should exist" do
+    user = User.new(email: "test@test.com", username: "UserTest", password_digest: 'password')
+    activity = Activity.new(name: "Pyheatpump", description: "A software to control heat pumps", author: user)
+    assert activity.valid?
+  end
+
+  test "not valid if author does not exist" do
+    activity = Activity.new(name: "Pyheatpump", description: "A software to control heat pumps", author: User.find_by_id(666))
+    assert_not activity.valid?
+  end
+end

test/models/user_test.rb

   user = User.new(email: other_user.email, username: "UserTest", password_digest: "test_password")
   assert_not user.valid?
   end
+
+  test 'destroy user should destroy linked activity' do
+    assert_difference('Activity.count', -1) do
+      users(:one).destroy
+    end
+  end
 end