How to test a model validation with a regex in RSpec? - ruby-on-rails

Question

After a bit of hacking I have come up with the following tests to make sure the regex pattern on my model validator is working correctly. I am wondering if there is a better way to test these conditions instead of building a bad string. I want to account for any and all characters outside the approved regex pattern. Different columns may have different validators too.
Model
validates :provider_unique_id,
presence: true,
length: { maximum: 50 },
format: { with: /\A[A-Za-z0-9]+\z/ }
Spec
describe 'provider unique id' do
let(:bad_string) { (0..255).map(&:chr).select { |x| x != /\A[A-Za-z0-9]+\z/ }.sample(20).join }
it 'should exist' do
shop.provider_unique_id = nil
expect(shop.valid?).to be_falsey
end
it 'passes regex rules' do
shop.provider_unique_id = bad_string
expect(shop.valid?).to be_falsey
end
end

Answer 1

Here's what I'd write if I were being extremely thorough. Imagine test-driving the validations, adding one test at a time and adding to the validations to make it pass.
describe '#provider_unique_id' do
%w(a z).each do |letter|
it "can be letter #{letter}" do
expect_to_be_valid letter
end
end
# after making this pass, I'd change the regex to use the i flag so I wouldn't need to test for Z
it "can be uppercase" do
expect_to_be_valid 'A'
end
[0, 9].each do |digit|
it "can be digit #{digit}" do
expect_to_be_valid digit
end
end
it "can be more than one character" do
expect_to_be_valid '00'
end
it "isn't nil" do
expect_to_be_invalid nil
end
it "isn't blank" do
expect_to_be_invalid ""
end
it "can be 50 characters long" do
expect_to_be_valid('0' * 50)
end
it "can't be longer than 50 characters" do
expect_to_be_invalid('0' * 51)
end
# I chose _ as a non-alphanumeric since it's the only non-alphanumeric word character.
# That is, it's as close to a valid character as it can be without be valid.
it "can't contain a non-alphanumeric character" do
expect_to_be_invalid '_'
end
# this example forces you to add \A
it "can't begin with a non-alphanumeric character" do
expect_to_be_invalid '_0'
end
# this example forces you to add \z
it "can't end with a non-alphanumeric character" do
expect_to_be_invalid '0_'
end
def expect_to_be_valid(provider_unique_id)
shop.provider_unique_id = provider_unique_id
expect(shop).to be_valid
end
def expect_to_be_invalid(provider_unique_id)
shop.provider_unique_id = provider_unique_id
expect(shop).to_not be_valid
end
end
I wouldn't randomly generate a bad string, because it wouldn't force you to write any additional code. I think the tests with _ are sufficient. Note that there are many more characters than ASCII 0-255, and it would be impractical to test them all.
You could imagine boundary-checking the ranges in the regex (a-z, A-Z, 0-9) by testing characters that come immediately before and after each range, but it's unlikely that someone would write code that would incorrectly include those characters, so I wouldn't go that far.