Calculation in model or controller

Question

I'm builing a weight loss app. For this in my app each user has_one :profile and has_many :weights. Each profile belongs_to :pal. For my app to work I need a value called SMR which basically is a formula that takes as variables the user's size, age and gender (all from profiles table), the user's current weight (from weights table) as well as a float number from pal table.

I am able to calculate SMR in profiles_controller.rb show action and show it in the profiles show.html.erb.

I have two questions now:

1. Is it correct to do this calculation in the profiles_controller.rb show action or should I do it in the profile.rb model? If I should do it in the model: how can I do it (how should the code look like)?
2. I will need the SMR value later on in my app as a variable for other calculations as well. How can I achieve this (if it is calculated in the profile controller/model but needed somewhere else later on)?

I'm fairly new to the Rails world so maybe my questions are really noob questions.

profile.rb

class Profile < ActiveRecord::Base
  belongs_to :user
  belongs_to :pal
  belongs_to :goal


  def age
    if birthdate != nil
      now = Time.now.utc.to_date
      now.year - birthdate.year - (birthdate.to_date.change(:year => now.year) > now ? 1 : 0)
    else
      nil
    end
  end
end 

weight.rb

class Weight < ActiveRecord::Base
  belongs_to :user
end

pal.rb

class Pal < ActiveRecord::Base
  has_many :profiles
end

profiles_controller.rb (show action only)

  def show
    @pal = @profile.pal
    @goal = @profile.goal
    @current_weight = Weight.where(:user_id => current_user.id).order(:day).last

    if @profile.gender == 0
      @smr = (10*@current_weight.kilograms+6.25*@profile.size-5*@profile.age+5)*@pal.value
    elsif @profile.gender == 1
      @smr = (10*@current_weight.kilograms+6.25*@profile.size-5*@profile.age-161)*@pal.value
    else
      nil
    end
  end

Answer 1

I think you should create a separate class or you can do on profile model as well

class SmrCalculator
  def initialize(profile, user)
     @profile = profile
     @user = user
  end

  def get_smr
    @pal = @profile.pal
    @goal = @profile.goal
    @current_weight = Weight.where(:user_id => @user.id).order(:day).last

    if @profile.gender == 0
      @smr = (10*@current_weight.kilograms+6.25*@profile.size-5*@profile.age+5)*@pal.value
    elsif @profile.gender == 1
      @smr = (10*@current_weight.kilograms+6.25*@profile.size-5*@profile.age-161)*@pal.value
    else
      nil
    end
  @smr
  end

end

And call this class on your controller show method like this:

@smr_calculator = SmrCalculator.new(@profile, current_user)
@smr = @smr_calculator.get_smr

And add this class as smr_calculator.rb in models folder

so anywhere in the app you need @smr you can call this class with profile and current user