Ruby on Rails - Virtual Attributes -

i have following model:

  create_table "material_costs", :force => true |t|     t.string   "material"     t.integer  "height"     t.integer  "width"     t.decimal  "cost",       :precision => 4, :scale => 2     t.datetime "created_at"     t.datetime "updated_at"   end 

how create virtual attribute in model give me cost per square inch of each material?

also have model holds vat value:

  create_table "taxes", :force => true |t|     t.string   "name"     t.decimal  "rate",       :precision => 10, :scale => 0     t.datetime "created_at"     t.datetime "updated_at"   end 

how use model give me total price per square inch each material item ie need add on vat rate?

edit - store vat value in following model:

  create_table "app_options", :force => true |t|     t.string   "name"     t.string   "value"     t.datetime "created_at"     t.datetime "updated_at"   end 

edit - controller code:

  def calculate_quote     @moulding = moulding.find( params[:id], :select => 'cost, width' )     @mount = materialcost.find(1).total_cost_per_square_mm     @glass = materialcost.find(2).total_cost_per_square_mm     @backing_board = materialcost.find(3).total_cost_per_square_mm     @wastage = appoption.find( 2, :select => 'value' )     @markup = appoption.find( 3, :select => 'value' )      respond_to |format|       format.json { render :json => { :moulding => @moulding, :mount => @mount, :glass => @glass, :backing_board => @backing_board, :wastage => @wastage, :markup => @markup } }     end   end 

it doesn't make sense put in table, or need recalculated on every update.

as matchu suggests, should define method in model class.

note: i've added class variable hold tax value.

class materialcost < activerecord::base   # initialize tax rate on initialization of class   @@tax = appoptions.find(:first, :name => 'vat').value.to_f    # ...    def base_cost_per_square_inch     cost / (height * width)   end    def total_cost_per_square_inch     base_cost_per_square_inch * (1 + @@tax)   end end 

and controller code:

class materialscontroller < applicationcontroller    def calculate_full_price     # /materials/calculate_full_price/5      # material in question using id param.     material = materialcost.find(:first, :id => params[:id])      # calculate total price     @total_price_per_sq_in_with_tax = material.total_cost_per_square_inch      # done (fall off render normal view)   end  end 

i'm not quite sure tax use case exactly, make little more sense?