Add database support and model for tips

app/models/exercise.rb

@@ -24,6 +24,8 @@ class Exercise < ApplicationRecord
   has_many :tags, through: :exercise_tags
   accepts_nested_attributes_for :exercise_tags
   has_many :user_exercise_feedbacks
+  has_many :exercise_tips
+  has_many :tips, through: :exercise_tips
 
   has_many :external_users, source: :user, source_type: 'ExternalUser', through: :submissions
   has_many :internal_users, source: :user, source_type: 'InternalUser', through: :submissions

app/models/exercise_tip.rb

@@ -0,0 +1,18 @@
+# frozen_string_literal: true
+
+class ExerciseTip < ApplicationRecord
+  belongs_to :exercise
+  belongs_to :tip
+  belongs_to :parent_exercise_tip, class_name: 'ExerciseTip', optional: true
+  attr_accessor :children
+
+  # Ensure no parent tip is set if current tip has rank == 1
+  validates :rank, exclusion: {in: [1]}, if: :parent_exercise_tip_id?
+
+  validate :tip_chain?, if: :parent_exercise_tip_id?
+
+  def tip_chain?
+    # Ensure each referenced parent exercise tip is set for this exercise
+    errors.add :parent_exercise_tip, I18n.t('activerecord.errors.messages.together', attribute: I18n.t('activerecord.attributes.exercise_tip.tip')) unless ExerciseTip.exists?(exercise: exercise, tip: parent_exercise_tip)
+  end
+end

app/models/tip.rb

@@ -0,0 +1,23 @@
+# frozen_string_literal: true
+
+class Tip < ApplicationRecord
+  include Creation
+
+  has_many :exercise_tips
+  has_many :exercises, through: :exercise_tips
+  belongs_to :file_type, optional: true
+  validates_presence_of :file_type, if: :example?
+  validate :content?
+
+  def content?
+    errors.add :description, I18n.t('activerecord.errors.messages.at_least', attribute: I18n.t('activerecord.attributes.tip.example')) unless [description?, example?].include?(true)
+  end
+
+  def to_s
+    if title?
+      "#{I18n.t('activerecord.models.tip.one')}: #{title} (#{id})"
+    else
+      "#{I18n.t('activerecord.models.tip.one')} #{id}"
+    end
+  end
+end