Model Methods

As with ordinary ruby classes, you can add methods to your product class. Before we do that, let's add a way to track how many items of each product are left in our inventory. Do that below and in your app.

Instance Method

Once you've added a quantity column, you can set the quantity of your products. Next, let's create a method purchase for purchasing a specific item.

class Product < ActiveRecord::Base
  #...

  def purchase
  end

end

Now we want to fill in the above purchase method so it decreases quantity by one.

We could use the standard update_attributes method:

 update_attributes(quantity: quantity - 1)

Rails also provides a useful method decrement to make this more concise:

  def purchase
    decrement(:quantity)
  end

Trying it out
Now that you have a purchase method, start your console and set the quantity of some products. For example:

 prod = Product.first # this returns the first product
 prod.quantity = 3
 prod.save

Now, try out the purchase method:

prod.purchase

output:

(0.3ms)  begin transaction
  SQL (0.5ms)  UPDATE "products" SET "quantity" = ?, "updated_at" = ? WHERE "products"."id" = ?  [["quantity", 2]....
   (2.1ms)  commit transaction
 => true 

Now prod's quantity is only 2, which you see by entering prod.quantity.

You can purchase it a few more times, and it will go down to 0:

prod.purchase
prod.purchase

What happens if you purchase it again?

prod.purchase
prod.quantity
=> -1

Oops. We don't want to allow a negative quantity. Let's adjust the purchase method to prevent that:

def purchase
    if quantity > 0            # line 2
        decrement(:quantity)   # line 3
        return true
    end
end

Now if you reload your console, you'll find that the quantity doesn't change when you try purchasing a product that's already at 0.

:symbols and variables

Question: In the purchase method above, we referred to quantity on its own in line 2, but we used the symbol :quantity in line 3. What's going on? And how could we refer to quantity if no variable like that had been set earlier in the code?

Answer: Rails automatically provides methods to get and set the columns of a model. So if you have a column in your model, you can use methods to get and set its values.