NonCaseSwappableValueError

This error is raised when using validate_uniqueness_of. This matcher, of course, tests that an attribute disallows a non-unique value -- and what constitutes as "unique" depends on whether the case-sensitivity of that value matters. If it does matter -- meaning that the uniqueness validation in your model isn't using case_sensitive: false and you haven't qualified the matcher with case_insensitive -- then the matcher will run the following test:

Creating first a record with a value of "A":

• A new record with a value of "A" should not be valid (failing the uniqueness validation)
• A new record with a value of "a" should be valid

The test value we're using is in this case "A", and this is what the matcher will use if an existing record is not already present in the database. But if a record already exists, then the matcher will use it as comparison -- it will read the attribute under test off of the record and use its value. So a better example might be:

Given an existing record with a value:

• A new record with the same value should not be valid (failing the uniqueness validation)
• A new record with the same value, but where the case is swapped (using String#swapcase), should be valid

Now, what happens if an existing record is there, but the value being used is not one whose case can be swapped, such as "123" or "{-#%}"? Then the second assertion cannot be made effectively.

So this is why you're getting this exception. What can you do about it? As the error message explains, you have two options:

1. If you want the uniqueness validation in the model to operate case-sensitively and you didn't mean to use a non-case-swappable value, then you need to provide an existing record with a different value, one that contains alpha characters. Here's an example:

   # Model
   class User < ActiveRecord::Base
     validates_uniqueness_of :username
   end
   
   # RSpec
   RSpec.describe User, type: :model do
     context "validations" do
       subject do
         # Note that "123" == "123".swapcase. This is a problem!
         User.new(username: "123")
       end
   
       it do
         # So you can either override it like this, or just fix the subject.
         user = User.create!(username: "john123")
         expect(user).to validate_uniqueness_of(:username)
       end
     end
   end
   
   # Minitest (Shoulda)
   class UserTest < ActiveSupport::TestCase
     context "validations" do
       subject do
         # Note that "123" == "123".swapcase. This is a problem!
         User.new(username: "123")
       end
   
       should "validate uniqueness of :username" do
         # So you can either override it like this, or just fix the subject.
         user = User.create!(username: "john123")
         assert_accepts validate_uniqueness_of(:username), record
       end
     end
   end
   

2. If you don't want the uniqueness validation to operate case-sensitively, then you need to add case_sensitive: false to the validation and add case_insensitive to the matcher:

   # Model
   class User < ActiveRecord::Base
     validates_uniqueness_of :username, case_sensitive: false
   end
   
   # RSpec
   RSpec.describe User, type: :model do
     context "validations" do
       subject do
         # Note that "123" == "123".swapcase, but it's okay
         User.new(username: "123")
       end
   
       it { should validate_uniqueness_of(:username).case_insensitive }
     end
   end
   
   # Minitest (Shoulda)
   class UserTest < ActiveSupport::TestCase
     context "validations" do
       subject do
         # Note that "123" == "123".swapcase, but it's okay
         User.new(username: "123")
       end
   
       should validate_uniqueness_of(:username).case_insensitive
     end
   end