app.kluk.fr/app/models/mood.rb

class Mood < ApplicationRecord
  belongs_to :user
  belongs_to :mode

  validates :recorded_at, presence: true
  validate :unique_per_day_and_user

  private

  def unique_per_day_and_user
    return unless recorded_at && user

    existing = user.moods.where.not(id: id).any? do |mood|
      mood.recorded_at.to_date == recorded_at.to_date
    end

    errors.add(:recorded_at, :taken) if existing
  end
end