Get validates_uniqueness_of to work better with `NOT NULL` columns

Instead of having to persist the subject, like so:

  describe 'validations' do
    subject { build(:model) }
    it { validates_presence_of :required_attr }
    context "persisted" do
      # Needs to be persisted to prevent NOT NULL db error
      subject { create(:model) }
      it { validates_uniqueness_of :unique_attr }
    end
  end

You can now just write:

  describe 'validations' do
    subject { build(:model) }
    it { validates_presence_of :required_attr }
    it { validates_uniqueness_of :unique_attr }
  end

When required the matcher saves the record to the database.

NEWS.md

@@ -18,6 +18,12 @@
 * Fix `delegate_method` so that it can be required independent of Active
   Support.
 
+* Fix `validate_uniqueness_of`. When used against an unpersisted record whose
+  model contained a non-nullable column other than the one being validated, the
+  matcher would break. Even if the test set that column to a value beforehand,
+  the record had to be persisted in order for the matcher to work. Now this is
+  no longer the case and the record can remain unpersisted.
+
 ### Improvements
 
 * `have_and_belongs_to_many` now checks to make sure that the join table

lib/shoulda/matchers/active_model/validate_uniqueness_of_matcher.rb

@@ -209,6 +209,7 @@ module Shoulda
         end
 
         def matches?(subject)
+          @original_subject = subject
           @subject = subject.class.new
           @expected_message ||= :taken
           set_scoped_attributes &&
@@ -253,16 +254,20 @@ module Shoulda
             value = 'a'
           end
 
-          @subject.class.new.tap do |instance|
+          @original_subject.tap do |instance|
             instance.__send__("#{@attribute}=", value)
-            if has_secure_password?
-              instance.password = 'password'
-              instance.password_confirmation = 'password'
-            end
+            ensure_secure_password_set(instance)
             instance.save(validate: false)
           end
         end
 
+        def ensure_secure_password_set(instance)
+          if has_secure_password?
+            instance.password = "password"
+            instance.password_confirmation = "password"
+          end
+        end
+
         def has_secure_password?
           @subject.class.ancestors.map(&:to_s).include?('ActiveModel::SecurePassword::InstanceMethodsOnActivation')
         end

spec/shoulda/matchers/active_model/validate_uniqueness_of_matcher_spec.rb

@@ -47,6 +47,26 @@ describe Shoulda::Matchers::ActiveModel::ValidateUniquenessOfMatcher do
         expect(Example.count).to eq 0
         expect(validating_uniqueness_with_other).to matcher
       end
+
+      context "and the table uses NOT NULL columns" do
+        let(:model_class) do
+          define_model(
+            :example,
+            attr: :string,
+            required_attr: { type: :string, options: { null: false } }
+          ) do
+            attr_accessible :attr, :required_attr
+            validates_presence_of :required_attr
+            validates_uniqueness_of :attr
+          end
+        end
+
+        it "works if the subject to sets the required attibutes" do
+          model = model_class.new(required_attr: "foo")
+          expect(Example.count).to eq 0
+          expect(model).to matcher
+        end
+      end
     end
 
     def define_model_with_other(options = {})