Rendering a simple calculation from a controller - ruby-on-rails

Question

I'm sorry if this looks like a very easy (too easy) question but I'm struggling to find a way into working this out.
I need to calculate the Basal Metabolic Rate according to the data enter into a Body model and render it in my show view. The problem is that I can run the calculation in the view but, of course, this is not a proper way.
<%= ((13.397*#user.bodies.last.weight)+(479.9 * #user.bodies.last.height)-(5.677 * #user.bodies.last.age)+(88.362)) * (1 - (#user.bodies.last.fat / 100.00)) %>
this code snippet is in a partial that feeds Body#show view. Of course, I want to have this done at the controller level.
So I wrote a method in my bodies_controller that looks like this:
# Calculate Basal Metabolic Rate for Males
def bmr
#user = User.find(params[:user_id])
#bmr = ((13.397 * #user.bodies.last.weight) + (479.9 *
#user.bodies.last.height) - (5.677 * #user.bodies.last.age) +
88.362) * (1 - (#user.bodies.last.fat / 100.00))
end
When trying to pull the result of this simple calculation into my Body#show view like this: <%= #bmr %> nothing shows. Furthermore typing #bmr into the console returns NIL. How do I feed the Body attributes (Weight, age, height....) into the method?
Sorry if this question sounds stupid but I'm still learning.
Thanks a million in advance!

Answer 1

you can move bmr method to body model as follow
body.rb
class Body < ApplicationRecord
# your other codes
def bmr
((13.397 * self.weight) +
(479.9 * self.height) -
(5.677 * self.age) + 88.362) *
(1 - (self.fat / 100.00))
end
end
for your note: self is equal to last bodies object
and from your view you can call the result as follow
show.html.erb
<%= #user.bodies.last.bmr %>

Answer 2

There are a few pointers I'd give you in solving this more elegantly. Breakdown the problem into more manageable parts, and try to stay away from magic numbers.
class Body
def bmr
#bmr ||= Bmr.calculate(self)
end
end
(below) obviously, replace num1 etc. with descriptive names for those numbers so we have an idea of what they mean in this calculation. Also, consider using constants NUM1 defined at top of class instead of methods.
class Bmr
attr_reader :body
private :body
def initialize(body)
#body = body
end
def self.calculate(body)
new(body).calculate
end
def calculate
((num1 * body.weight) +
(num2 * body.height) -
(num3 * body.age) +
(num4) *
(num5 - (body.fat / 100.00))
end
def num1
13.397
end
def num2
479.9
end
def num3
5.677
end
def num4
88.362
end
def num5
1
end
end
On top of that, you could further break the inner calculations down..
def calculated_weight
num1 * body.weight
end
# then
def calculate
calculated_weight + calculated_height - calculated_age
# etc
end
This gives you a great opportunity to describe the overall calculation and makes it much easier to grasp at a glance, and dig into if necessary.