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.

When you enter the following in your console:

prod = Product.first
prod.quantity 

prod.quantity calls the quantity 'getter' method and returns the value of quantity of prod. The same thing happens in the method above when it says if quantity > 0. The getter method is called to get the value of quantity of your current product object. In fact, to make your code clearer, you could change it to:

if self.quantity > 0

self here refers to the current object, and you can see it's calling quantity just like prod.quantity in the command line.

In the next line of code, we use a symbol:

decrement(:quantity)

Why? In this case it wouldn't make sense to say quantity or self.quantity, since that would just return a number value without telling decrement what to change. The Rails decrement method takes in a symbol of the column/field you want to change, so that's what you need to pass it. Symbols are used as identifiers throughout Rails.

If you're not referring to an existing variable or calling a method, you'll probably want to use a symbol in Rails to identify a specific column.

Question: What about the following line of code?

 update_attributes(quantity: quantity - 1)

Answer: In this case, Rails is using another shortcut. The method takes in a hash that consists of a symbol and the new value to set it to. You can rewrite it like this to be clearer:

 update_attributes(:quantity => quantity - 1)

And here's a more explicit version of the entire method:

def purchase
    if self.quantity > 0
        self.update_attributes({:quantity => self.quantity - 1})
        return true
    end
end

Ruby has many ways to write the same thing, which can initially be a little confusing when you're reading code written in a different way!

Class methods

The purchase method was called on specific product instances of the Product class. You can also create class methods that are called on the entire Product class.

Add the following method to your product class:

def self.available
end

Now let's get all the products whose quantity is not 0. You can do this in Rails 4 with the following code:

def self.available
    Product.where.not(quantity: 0)
end

Finally, let's create one more class method to retrieve the most recently created product. Use .order to order items in rails, and pass in the field you want to order them by: Product.order(:created_at). To get the last item in a collection, use .last. Putting it all together, we have:

def self.newest
    Product.order(:created_at).last
end

Challenges

Adding more class methods

• Create a class method oldest that returns the product with the oldest created_at time. You can copy and adjust your method for getting the newest item.
• (optional) Create a class methods sold_out that returns all the products with a quantity of 0. See the Rails Guide to Queries for help on the code to use.