tests: add normalize_attribute helper method.

Add a custom Shoulda matcher for testing that a model correctly normalizes an attribute.

Usage:

    subject { build(:wiki_page) }
    should normalize_attribute(:title).from(" Azur  Lane ").to("azur_lane")

test/test_helper.rb

@@ -27,6 +27,7 @@ class ActiveSupport::TestCase
   include IqdbTestHelper
   include UploadTestHelper
   extend StripeTestHelper
+  extend NormalizeAttributeHelper
 
   mock_post_version_service!
   mock_pool_version_service!

test/test_helpers/normalize_attribute_helper.rb

@@ -0,0 +1,45 @@
+# A custom Shoulda matcher for testing that a model correctly normalizes an attribute.
+#
+# Usage:
+#
+#   subject { build(:wiki_page) }
+#   should normalize_attribute(:title).from(" Azur  Lane ").to("azur_lane")
+#
+# https://thoughtbot.com/blog/shoulda-matchers
+
+module NormalizeAttributeHelper
+  def normalize_attribute(attribute)
+    NormalizeAttributeMatcher.new(attribute)
+  end
+
+  class NormalizeAttributeMatcher
+    attr_reader :attribute, :from_value, :to_value
+
+    def initialize(attribute)
+      @attribute = attribute
+    end
+
+    def matches?(subject)
+      subject.send("#{attribute}=", from_value)
+      subject.send(attribute) == to_value
+    end
+
+    def description
+      "normalize the `#{attribute}` attribute from `#{from_value}` to `#{to_value}`"
+    end
+
+    def failure_message
+      "expected `#{attribute}=` to normalize `#{from_value}` to `#{to_value}`"
+    end
+
+    def from(from_value)
+      @from_value = from_value
+      self
+    end
+
+    def to(to_value)
+      @to_value = to_value
+      self
+    end
+  end
+end

test/unit/wiki_page_test.rb

@@ -92,14 +92,13 @@ class WikiPageTest < ActiveSupport::TestCase
     end
 
     context "during title validation" do
-      # these values are allowed because they're normalized first
-      should allow_value(" foo ").for(:title).on(:create)
-      should allow_value("~foo").for(:title).on(:create)
-      should allow_value("_foo").for(:title).on(:create)
-      should allow_value("foo_").for(:title).on(:create)
-      should allow_value("foo__bar").for(:title).on(:create)
-      should allow_value("FOO").for(:title).on(:create)
-      should allow_value("foo bar").for(:title).on(:create)
+      should normalize_attribute(:title).from(" foo ").to("foo")
+      should normalize_attribute(:title).from("~foo").to("foo")
+      should normalize_attribute(:title).from("_foo").to("foo")
+      should normalize_attribute(:title).from("foo_").to("foo")
+      should normalize_attribute(:title).from("FOO").to("foo")
+      should normalize_attribute(:title).from("foo__bar").to("foo_bar")
+      should normalize_attribute(:title).from("foo bar").to("foo_bar")
 
       should_not allow_value("").for(:title).on(:create)
       should_not allow_value("___").for(:title).on(:create)