Add specs configuration

spec/dummy/app/models/application_record.rb

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+# frozen_string_literal: true
+
+class ApplicationRecord < ActiveRecord::Base
+  self.abstract_class = true
+
+  scope :published, -> {}
+end

spec/dummy/app/models/author.rb

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+# frozen_string_literal: true
+
+class Author < ApplicationRecord
+  has_many :posts
+  has_many :published_posts, -> { published }, class_name: 'Post'
+  has_many :recent_posts, -> { recents }, class_name: 'Post'
+
+  has_many :tags, through: :posts
+
+  has_one :profile, inverse_of: :author, dependent: :destroy
+
+  has_one_attached :avatar
+
+  accepts_nested_attributes_for :profile, allow_destroy: true
+
+  validates :email, format: { with: /\A[A-Z0-9._%+-]+@[A-Z0-9.-]+\.[A-Z]{2,}\z/i, message: 'Invalid email' }
+
+  validate -> {
+    errors.add( :base, 'Invalid age' ) if !age || age.to_i % 3 == 1
+  }
+
+  def to_s
+    "#{name} (#{age})"
+  end
+end

spec/dummy/app/models/post.rb

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+# frozen_string_literal: true
+
+class Post < ApplicationRecord
+  enum state: %i[available unavailable arriving]
+
+  belongs_to :author, inverse_of: :posts, autosave: true
+
+  has_one :author_profile, through: :author, source: :profile
+
+  has_many :post_tags, inverse_of: :post, dependent: :destroy
+  has_many :tags, through: :post_tags
+
+  validates :title, allow_blank: false, presence: true
+
+  scope :published, -> { where(published: true) }
+  scope :recents, -> { where('created_at > ?', Date.today - 8.month) }
+
+  def short_title
+    title.truncate 10
+  end
+
+  def upper_title
+    title.upcase
+  end
+end

spec/dummy/app/models/post_tag.rb

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+# frozen_string_literal: true
+
+class PostTag < ApplicationRecord
+  belongs_to :post, inverse_of: :post_tags
+  belongs_to :tag,  inverse_of: :post_tags
+
+  validates :post, presence: true
+  validates :tag,  presence: true
+end

spec/dummy/app/models/profile.rb

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+# frozen_string_literal: true
+
+class Profile < ApplicationRecord
+  belongs_to :author, inverse_of: :profile, touch: true
+
+  def to_s
+    description
+  end
+end

spec/dummy/app/models/tag.rb

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+# frozen_string_literal: true
+
+class Tag < ApplicationRecord
+  has_many :post_tags, inverse_of: :tag, dependent: :destroy
+  has_many :posts, through: :post_tags
+end