[Users] Fix unexpected error on creation with duplicate name

find_by_name is not the same as find_by(name:), it is overwritten in the model class.
Luckily there was still a db constraint preventing the name, which is
why the error was raised.

app/logical/user_name_validator.rb

@@ -2,7 +2,7 @@ class UserNameValidator < ActiveModel::EachValidator
   def validate_each(rec, attr, value)
     name = value
 
-    rec.errors.add(attr, "already exists") if User.find_by(name: name).present?
+    rec.errors.add(attr, "already exists") if User.find_by_name(name).present?
     rec.errors.add(attr, "must be 2 to 20 characters long") if !name.length.between?(2, 20)
     rec.errors.add(attr, "must contain only alphanumeric characters, hypens, apostrophes, tildes and underscores") unless name =~ /\A[a-zA-Z0-9\-_~']+\z/
     rec.errors.add(attr, "must not begin with a special character") if name =~ /\A[_\-~']/

test/functional/users_controller_test.rb

@@ -92,6 +92,19 @@ class UsersControllerTest < ActionDispatch::IntegrationTest
         end
       end
 
+      context "with a duplicate username" do
+        setup do
+          create(:user, name: "test123")
+        end
+
+        should "prevent creation" do
+          assert_no_difference(-> { User.count }) do
+            post users_path, params: { user: { name: "TEst123", password: "xxxxx1", password_confirmation: "xxxxx1" } }
+            assert_match(/Name already exists/, flash[:notice])
+          end
+        end
+      end
+
       context "with email validation" do
         setup do
           Danbooru.config.stubs(:enable_email_verification?).returns(true)