Recently after introducing a new value object to a project I ran mutant to verify my test coverage quite early. It turned out that I missed a few places when it comes to tests, but also technical design of production code. In this post, I'll show you my development process for the Country Value Object.
When you think about Value Object it's important to get the difference between eql?, equal? and == operators. Those differences were quite important in the class design process.
What is Value Object?
So long story short, a value object is an object whose equality is based on its value, not its identity.
Code sample
This is a simple country object. Its purpose is to protect the application from using countries that are not supported.
class Country
SUPPORTED_COUNTRIES = [PL = "PL", NO = "NO"].freeze
protected attr_reader :iso_code
def initialize(iso_code)
raise unless SUPPORTED_COUNTRIES.include?(iso_code.to_s.upcase)
@iso_code = iso_code
end
def to_s
iso_code.to_s
end
def eql?(other)
other.instance_of?(Country) && iso_code.eql?(other.iso_code)
end
alias == eql?
def hash
iso_code.hash
end
end
Besides that, as you can see in the tests below, no matter how we use the country object, we want it to always get the proper value of the country's iso code.
class CountryTest < TestCase
cover Country
def test_returns_no
assert_equal "NO", Country.new("NO").to_s
assert_equal "NO", Country.new(:NO).to_s
assert_equal "NO", Country.new(Country::NO).to_s
end
def test_returns_pl
assert_equal "PL", Country.new("PL").to_s
assert_equal "PL", Country.new(:PL).to_s
assert_equal "PL", Country.new(Country::PL).to_s
end
def test_equality
assert Country.new(Country::PL).eql? Country.new(Country::PL)
assert Country.new(Country::NO).eql? Country.new(Country::NO)
assert Country.new(Country::PL) == Country.new(Country::PL)
assert Country.new(Country::NO) == Country.new(Country::NO)
end
def test_only_supported_countries_allowed
assert_raises { Country.new("NL") }
assert_raises { Country.new("ger") }
assert_nothing_raised { Country.new("pl") }
end
end
This is our starting point. Looks ok, doesn't it? Before finishing the job of designing this class, let's run mutant tests and verify the results.
Let's kill some mutants
We'll focus on increasing the mutant coverage of equality-related methods.
Let's look at result of first bundle exec mutant run
def eql?(other)
- other.instance_of?(Country) && iso_code.eql?(other.iso_code)
+ other.instance_of?(Country)
end
def eql?(other)
- other.instance_of?(Country) && iso_code.eql?(other.iso_code)
+ iso_code.eql?(other.iso_code)
end
def eql?(other)
- other.instance_of?(Country) && iso_code.eql?(other.iso_code)
+ other.instance_of?(Country) || iso_code.eql?(other.iso_code)
end
def eql?(other)
- other.instance_of?(Country) && iso_code.eql?(other.iso_code)
+ other && iso_code.eql?(other.iso_code)
end
def eql?(other)
- other.instance_of?(Country) && iso_code.eql?(other.iso_code)
+ true && iso_code.eql?(other.iso_code)
end
def eql?(other)
- other.instance_of?(Country) && iso_code.eql?(other.iso_code)
+ Country && iso_code.eql?(other.iso_code)
end
def eql?(other)
- other.instance_of?(Country) && iso_code.eql?(other.iso_code)
+ self.instance_of?(Country) && iso_code.eql?(other.iso_code)
end
def eql?(other)
- other.instance_of?(Country) && iso_code.eql?(other.iso_code)
+ other.instance_of?(Country) && iso_code
end
def eql?(other)
- other.instance_of?(Country) && iso_code.eql?(other.iso_code)
+ other.instance_of?(Country) && true
end
def eql?(other)
- other.instance_of?(Country) && iso_code.eql?(other.iso_code)
+ other.instance_of?(Country) && other.iso_code
end
def eql?(other)
- other.instance_of?(Country) && iso_code.eql?(other.iso_code)
+ other.instance_of?(Country) && iso_code.eql?(self.iso_code)
end
def hash
- iso_code.hash
+ raise
end
def hash
- iso_code.hash
+ super
end
def hash
- iso_code.hash
end
def hash
- iso_code.hash
+ nil
end
def hash
- iso_code.hash
+ iso_code
end
def hash
- iso_code.hash
+ self.hash
end
The - sign symbolizes removed line of code. The + sign symbolizes line of code introduced by mutant. So even though there are tests that look quite good, the result is poor. This causes false sense of security.
This is the summarized score that we'll start with:
Integration: minitest
Jobs: 1
Includes: ["test"]
Requires: ["./config/environment", "./test/support/mutant"]
Subjects: 4
Total-Tests: 523
Selected-Tests: 4
Tests/Subject: 1.00 avg
Mutations: 72
Results: 72
Kills: 55
Alive: 17
Timeouts: 0
Runtime: 26.23s
Killtime: 23.41s
Overhead: 12.09%
Mutations/s: 2.74
Coverage: 76.39%
Let's increase that coverage!
This is a good point in time to copy the code and try to increase it's mutant coverage 😉
Heal the code
At a first glance it looks like our test suite is not complete. Let's try to increase mutant coverage by adding missing tests.
def test_values_equality
refute Country.new(Country::PL).eql? Country.new(Country::NO)
refute Country.new("PL").eql? "PL"
end
So in this test we expect that
-
Countryobjects of two different countries are not equal - Value object is not the same thing as simple string
All right so this test removes most of the problems. Actually, there is 6 more issues left:
def hash
- iso_code.hash
+ raise
end
def hash
- iso_code.hash
+ super
end
def hash
- iso_code.hash
end
def hash
- iso_code.hash
+ nil
end
def hash
- iso_code.hash
+ iso_code
end
def hash
- iso_code.hash
+ self.hash
end
How can we kill those mutants?
Making hash method more robust
def hash
Country.hash ^ iso_code.hash
end
And run mutant again
def hash
- Country.hash ^ iso_code.hash
+ raise
end
def hash
- Country.hash ^ iso_code.hash
+ super
end
def hash
- Country.hash ^ iso_code.hash
end
def hash
- Country.hash ^ iso_code.hash
+ nil
end
def hash
- Country.hash ^ iso_code.hash
+ Country.hash
end
def hash
- Country.hash ^ iso_code.hash
+ nil ^ iso_code.hash
end
def hash
- Country.hash ^ iso_code.hash
+ Country ^ iso_code.hash
end
def hash
- Country.hash ^ iso_code.hash
+ self.hash ^ iso_code.hash
end
def hash
- Country.hash ^ iso_code.hash
+ iso_code.hash
end
def hash
- Country.hash ^ iso_code.hash
+ Country.hash ^ nil
end
def hash
- Country.hash ^ iso_code.hash
+ Country.hash ^ iso_code
end
def hash
- Country.hash ^ iso_code.hash
+ Country.hash ^ self.hash
end
Well... not good, not bad. Different mutants were injected in the code. Still, there are some survivors.
Step back. What are we trying to achieve?
We're trying to design Value Object.
Two Value Objects are equal when:
- they have the same hash values, we have such a test
- when their classes are the same
Once again it looks like we are missing some tests.
Let's write a test that will check if the hash value of two value objects are equal.
def test_hash_equality
assert Country.new(Country::PL).hash.eql? Country.new(Country::PL).hash
end
And now let's run mutant and see the results.
def hash
- Country.hash ^ iso_code.hash
end
def hash
- Country.hash ^ iso_code.hash
+ nil
end
def hash
- Country.hash ^ iso_code.hash
+ Country.hash
end
def hash
- Country.hash ^ iso_code.hash
+ nil ^ iso_code.hash
end
def hash
- Country.hash ^ iso_code.hash
+ iso_code.hash
end
So what is mutant trying to tell us?
If you're following along, modify the hash method to one of the suggestions and see what happens. Yep. The tests are still passing! And they shouldn't be, right?
I think we're missing a test to make sure the modification that we just did would be detected if the hash method was changed. Specifically I mean the step that we just did, so adding the class of the Value Object to the equation.
Let's fix this by extending the hash_equality test case by few negative scenarios testing the hash method.
def test_hash_equality
assert Country.new(Country::PL).hash.eql? Country.new(Country::PL).hash
# new cases below
assert_not_equal Country.new(:PL).hash, (Country.hash ^ "NO".hash)
assert_not_equal Country.new(:PL).hash, (Country.hash ^ "PL".hash)
assert_not_equal Country.new(:PL).hash, (Country.new(:NO).hash)
refute Country.new(Country::PL).hash == "PL".hash
end
Now after running the mutant we're good :)
Integration: minitest
Jobs: 1
Includes: ["test"]
Requires: ["./config/environment", "./test/support/mutant"]
Subjects: 4
Total-Tests: 525
Selected-Tests: 6
Tests/Subject: 1.50 avg
Mutations: 78
Results: 78
Kills: 78
Alive: 0
Timeouts: 0
Runtime: 37.15s
Killtime: 33.76s
Overhead: 10.03%
Mutations/s: 2.10
Coverage: 100.00%
Why did we use eql? instead of ==
The == operator compares two objects based on their value. For example
1 == 1 # true
1 == 1.0 # true
1.hash == 1.0.hash #false
For simple class:
class Klass
attr_accessor :code
def initialize(code)
@code = code
end
end
The test fails
def test_klass
assert Klass.new("a") == Klass.new("a")
end
The eql? method compares two objects based on their hash.
2.eql? 2 # true
2.eql? 2.0 # false
Couldn't we just do it like this...?
def test_klass
assert Klass.new("a").eql? Klass.new("a")
end
Nope.
Two objects with the same value. But! The hash is different. When the hash method is not overwritten, it's based on the object's identity. So it's something that we don't want when we think about Value Objects.
In general, it's better to use .eql? method for the Value Object if you want to make sure that there's no hash colision.
What about equal?
Why isn't the equal? method also aliased to the eql? operator? The reason is the fact that the equal? method checks the identity of the object.
Let's look at an example.
def test_equality
first = "a"
second = "a"
assert first.equal? second
end
The test fails. Check the identity of those two objects, they're different.
first.__id__ != second.__id__
Besides that, overwriting equal? is not recommended.
Final Value Object
class Country
SUPPORTED_COUNTRIES = [PL = "PL", NO = "NO"].freeze
protected attr_reader :iso_code
def initialize(iso_code)
raise unless SUPPORTED_COUNTRIES.include?(iso_code.to_s.upcase)
@iso_code = iso_code
end
def to_s
iso_code.to_s
end
def eql?(other)
other.instance_of?(Country) && iso_code.eql?(other.iso_code)
end
alias == eql?
def hash
Country.hash ^ iso_code.hash
end
end
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