How to use scopes to filter data by partial variable - ruby

Question

I have some working script filtering my results with Active Record Scoping. Everything works fine when i want to filter by comparing params with data from database.
But i have some function counting car price inside _car.html.erb partial, the result of this function depends on params result.
How can i scope search results by result of this function and show only cars which are under some price (defined in params).
Some code to make it more clear:
car.rb (model file)
scope :price_leasing, -> (price_leasing) { where('price_leasing <= ?', price_leasing) }
# for now price_leasing is getting price from database
scope :brand, -> (brand) { where brand: brand }
scope :car_model, -> (car_model) { where car_model: car_model }
scope :category, -> (category) { where category: category }
cars_controller.rb (controller file)
def index
#cars = Car.where(nil)
#cars = #cars.price_leasing(params[:price_leasing]) if params[:price_leasing].present?
#cars = #cars.brand(params[:brand]) if params[:brand].present?
#cars = #cars.car_model(params[:car_model]) if params[:car_model].present?
#cars = #cars.category(params[:category]) if params[:category].present?
#brands = Brand.all # importing all car brands into filters
end
in index.html.erb i have "render #cars" code
<%=
if #cars.size > 0
render #cars.where(:offer_status => 1)
else
render html: '<p>Nie znaleziono pasujących wyników.</p>'.html_safe
end
%>
inside _car.html.erb file i have function from helper
<h3 class="car-cell__price"><%= calculate_car_price(car.price, car.id) %> <span class="car-cell__light-text">zł/mc</span></h3>
my calculate_car_price() function inside helper
def calculate_car_price(car_price, car_id)
car = Car.find(car_id)
fullprice = car_price
if params[:price_leasing].present?
owncontribution = params[:price_leasing].to_i
else
owncontribution = car.owncontribution
end
pv = fullprice - owncontribution + (0.02 * fullprice)
if params[:period].present?
carperiod = params[:period].to_i
carprice = (Exonio.pmt(0.0522/60, carperiod, pv)) * -1
else
carprice = (Exonio.pmt(0.0522/60, 60, pv)) * -1
end
p number_with_precision(carprice, precision: 0)
end
i would love to scope by the result of this function. Is it possible?

Answer 1

The thing about scopes are that they are implemented at the database level. You want to select records that have a value
To do what you want at the DB level, you would need to a virtual column in the database, the implementation will change based on which database product you're using (I would not expect a virtual column definition in postgreSQL to be the same as a virtual column definition in mySQL)
So I'm not sure there's an optimal way to do this.
I would suggest you build your own class method and instance method in your model Car. It would be less performant but easier to understand and implement.
def self.car_price_under(target_price, params)
select { |v| v.car_price_under?(target_price, params) }
end
def car_price_under?(target_price, params)
full_price = price
if params[:price_leasing].present?
my_own_contribution = params[:price_leasing].to_i
else
my_own_contribution = owncontribution
end
pv = full_price - my_own_contribution + (0.02 * full_price)
if params[:period].present?
car_period = params[:period].to_i
new_car_price = (Exonio.pmt(0.0522/60, car_period, pv)) * -1
else
new_car_price = (Exonio.pmt(0.0522/60, 60, pv)) * -1
end
new_car_price <= target_price
end
This would let you do...
#cars = Car.where(nil)
#cars = #cars.brand(params[:brand]) if params[:brand].present?
#cars = #cars.car_model(params[:car_model]) if params[:car_model].present?
#cars = #cars.category(params[:category]) if params[:category].present?
#cars = #cars.car_price_under(target_price, params) if target_price.present?
Note that this is happening in rails, NOT in the database, so the car_price_under method should be called after all other scopes to minimise the number of records that need to be examined. You cannot chain additional scopes as #cars would be an array... if you want to be able to chain additional scopes (or you want an active record relation, not an array) you could do something like #cars = Car.where(id: #cars.pluck(:id))