example_controller.rb

class CarsController < ApplicationController
    # ...

    # GET /cars/1
    def show
        @car = Car.find(params[:id])
        # ...
    end

    # GET /cars/1/edit
    def edit
        @car = Car.find(params[:id])
        # ...
    end

    # ... and so on ...
end

initialize_vars_from_param_ids.rb

def initialize_vars_from_param_ids
    params.keys.each do |param_key|
        model_klass_name = case param_key
        when 'id' then
            self.class.name.gsub(/Controller$/, '').singularize
        else
            next unless param_key =~ /(.+)_id$/
            $1.singularize.camelize
        end 
        model_klass = model_klass_name.constantize rescue next
     
        self.instance_variable_set(
            "@#{model_klass_name.tableize.singularize}",
            model_klass.find(params[param_key])
        ) if model_klass.respond_to?(:find)
    end 
end