How to Use Callback to Set :Level Value? - ruby-on-rails

Question

I'm creating a habit tracking app.
When a User creates a habit he will put in its :date_started and which days he is :committed to doing the habit.
Then after X amount of :committed days the habit moves onto the following :levels until eventually it hits "mastery" (days in the levels are broken down in the model below).
I already figured out date_started.
Now how can we use committed days, instead of just any day, to calculate what level a habit is on?
:levels is a dynamic getter method, which currently has no effect on the database.
I was told I need an appropriate callback to set the :levels attribute before the record is saved, but I don't know how.
Will you accept this challenge?
form
<%= f.label "Committed to:" %>
<%= f.collection_check_boxes :committed, Date::DAYNAMES, :downcase, :to_s %>
class Habit < ActiveRecord::Base
belongs_to :user
validates :action, presence: true
serialize :committed, Array
scope :missed, -> { where(missed: 1) }
scope :nonmissed, -> { where(missed: 0) }
def levels
committed_wdays = committed.map { |day| Date::DAYNAMES.index(day) }
n_days = (date_started..Date.today).count { |date| committed_wdays.include? date.wday }
case n_days
when 0..9
1
when 10..24
2
when 25..44
3
when 45..69
4
when 70..99
5
else
"Mastery"
end
end
end
controller
class HabitsController < ApplicationController
before_action :set_habit, only: [:show, :edit, :update, :destroy]
before_action :correct_user, only: [:edit, :update, :destroy]
before_action :authenticate_user!, except: [:index, :show]
def index
#habits = Habit.all.order("date_started DESC")
#missed_habits = current_user.habits.missed
#nonmissed_habits = current_user.habits.nonmissed
end
def show
end
def new
#habit = current_user.habits.build
end
def edit
end
def create
#habit = current_user.habits.build(habit_params)
if #habit.save
redirect_to #habit, notice: 'Habit was successfully created.'
else
render action: 'new'
end
end
def update
if #habit.update(habit_params)
redirect_to #habit, notice: 'Habit was successfully updated.'
else
render action: 'edit'
end
end
def destroy
#habit.destroy
redirect_to habits_url
end
private
def set_habit
#habit = Habit.find(params[:id])
end
def correct_user
#habit = current_user.habits.find_by(id: params[:id])
redirect_to habits_path, notice: "Not authorized to edit this habit" if #habit.nil?
end
def habit_params
params.require(:habit).permit(:missed, :left, :level, :date_started, :trigger, :action, :target, :positive, :negative, :committed => [])
end
end
Github: https://github.com/RallyWithGalli/ruletoday
Thanks in advance for your expertise =]