Add database support and model for tips

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