Refactoring Ruby: Replace Temp with Query

Let’s look at another simple yet powerful refactoring method, Replace Temp with Query. You can use this when you have a temporary variable holding the result of an expression. I will reach for this when temporary variables add too much to a method’s size/complexity, or when the result of an expression needs to be used in other methods. It's a fantastic next step after the last approach we look at: Extract Variable

Let’s use Replace Temp with Query in the Movie#total_price method.

Before:

class Movie TAX_PERCENTAGE = 0.5 def initialize(price:, discount_percentage: 0) @price = price @discount_percentage = discount_percentage end def total_price # Temp variables holding expressions tax = price * TAX_PERCENTAGE discount = price * (discount_percentage / 100.0) price + tax - discount end private attr_reader :price, :discount_percentage end 

After:

class Movie TAX_PERCENTAGE = 0.5 def initialize(price:, discount_percentage: 0) @price = price @discount_percentage = discount_percentage end def total_price # Temp variables replaced with query methods price + tax - discount end private attr_reader :price, :discount_percentage def tax price * TAX_PERCENTAGE end def discount price * (discount_percentage / 100.0) end 

The method before refactoring looks like:

def total_price # Temp variables holding expressions tax = price * TAX_PERCENTAGE discount = price * (discount_percentage / 100.0) price + tax - discount end 

The method after refactoring looks like:

def total_price # Temp variables replaced with query methods price + tax - discount end 

With the temp variables extracted to private query methods, there’s even less to grok when understanding the role of total_price.

Let’s run through the exact steps to take when applying Replace Temp with Query:

1. Extract the expression into a private method. Try to use a private method as we don’t want to change the public API of an object if we don’t have to.

2. Replace all references to the temp with the method name. Additionally, use the new query methods in any other places that need them.

Reach for this when you need to share the result of an expression or to reduce the size/complexity of a method.