On Apr 14, 2006, at 7:46 AM, Ruby Q. wrote:
by Pat Eyler
This week’s quiz is a bit of a departure from the normal. Instead
of submitting
different implementations of the same code, we’d like you to submit
different
implementations of the same process – Refactoring.
I’ll start us off this week. (Thanks for the easy but educational
quiz Pat!)
James Edward G. II
Replace Conditional with Polymorphism
This is a traditional refactoring pattern located on page 255 of my
copy of the book this quiz is named after. It can also be found on
the [Refactoring Catalog site](Catalog of Refactorings
replaceConditionalWithPolymorphism.html).
Smelly Code
This is actual code from a Rails application we are building at
work. I noticed the problematic method when bug hunting in the
system the same day I had been reading in Refactoring. (This method
is not related to the bug I was after, it works fine.)
Here are the relevant pieces of the three classes involved:
class TimePeriod < ActiveRecord::Base
#------------------
# VALIDATIONS
#------------------
validates_presence_of :name, :start_date, :end_date
#------------------
# INSTANCE METHODS
#------------------
#
# Returns +true+ if the passed +date+ is in the TimePeriod,
but not in any
# attached closed day ranges.
#
def include?( date )
case self
when ClosedPeriod
(start_date…end_date).include?(date)
else
(start_date…end_date).include?(date) and
not closed_periods.any? { |closed| closed.include?(date) }
end
end
end
class SchedulePeriod < TimePeriod
#------------------
# ASSOCIATIONS
#------------------
has_many :closed_periods, :dependent => :destroy
end
class ClosedPeriod < TimePeriod
end
These classes are used to represent simple time spans in our
application. A SchedulePeriod might represent a semester at a
university, for example, which could have a related ClosedPeriod for
Spring Break. (These are stored in the database in the time_periods
table, making use of ActiveRecord’s Single Table Inheritance.)
What’s Wrong Here and Why “Fix” It?
Obviously, the method I intend to refactor is the only one shown
TimePeriod#include?. This method displays the code smell of a case
statement used to determine action based on object types. Message
dispatch like this is Ruby’s domain and not suited for case
statements. I will use Replace Conditional with Polymorphism to
eliminate this.
It might be interesting to note that this method got to the current
state through a very natural process of code decay. When the
database was first being established, we threw together a quick
TimePeriod class that looked something like:
class TimePeriod < ActiveRecord::Base
#------------------
# ASSOCIATIONS
#------------------
has_many :closed_periods, :class_name => "TimePeriod",
:foriegn_key => "closed_period_id",
:dependent => :destroy
#------------------
# VALIDATIONS
#------------------
validates_presence_of :name, :start_date, :end_date
#------------------
# INSTANCE METHODS
#------------------
#
# Returns +true+ if the passed +date+ is in the TimePeriod,
but not in any
# attached closed day ranges.
#
def include?( date )
(start_date…end_date).include?(date) and
not closed_periods.any? { |closed| closed.include?(date) }
end
end
This is the same thing, but tracked using only one object that can
have “closed_periods” of its own type. This worked just like the
current version does. However, when we began working through the
controllers to create, find, and relate these objects, it became
apparent that it would be easier on us programmers to keep everything
straight if we broke them down into separate objects. Thus the
change and the introduction of the smelly method.
Refactored Code
Looking at the refactoring check list, I see that I first need to
cover the method with tests to ensure that I won’t break anything.
Luckily, these were already in place for the code. Here is one such
test, just to give you an idea of how they look:
spring_break = TimePeriod.find_by_name("Spring Break")
current = Date.civil(2006, 3, 1)
loop do
# all days in March should validate, except for Spring Break
assert_equal( !spring_break.include?(current),
time_periods(:march).include?(current) )
current += 1
break if current.month == 4
end
Now according to the refactoring catalog, I need to do two things.
The first is: “Move each leg of the conditional to an overriding
method in a subclass.” That doesn’t sound too tough. Here’s the
first such move:
class ClosedPeriod < TimePeriod
#------------------
# INSTANCE METHODS
#------------------
# Returns +true+ if the passed +date+ is in the ClosedPeriod.
def include?( date )
(start_date..end_date).include?(date)
end
end
I ran the tests and nothing broke. I’m now ready to move the second
leg:
class SchedulePeriod < TimePeriod
#------------------
# ASSOCIATIONS
#------------------
has_many :closed_periods, :dependent => :destroy
#------------------
# INSTANCE METHODS
#------------------
#
# Returns +true+ if the passed +date+ is in the
SchedulePeriod, but not in
# any attached closed day ranges.
#
def include?( date )
(start_date…end_date).include?(date) and
not closed_periods.any? { |closed| closed.include?(date) }
end
end
Again the tests gave the green light. All is good.
The second piece to this refactoring is to: “Make the original
method abstract.” I considered that for a brief instance, mentally
translating the Javaism into Ruby. My decision made, I highlighted
TimePeriod#include? and tapped the delete key on my keyboard. I
briefly considered having it raise an exception, but Ruby is going to
do that for me anyway and this was less work.
The tests still pass. Refactoring complete.
You may want to do another refactoring to move the duplicated date
range test ((start_date..end_date).include?(date)) out of the two
methods, but that’s a different pattern and probably not too critical
in this example.
