Adds assert_invalid

Allows asserting that the correct validations were added to a model.
Streamline the testing process for validations and make it easier for developers to understand the purpose of each test case.

activesupport/CHANGELOG.md

@@ -1,3 +1,9 @@
+*   Adds `assert_invalid` to to validate a model has the right validations
+    ```ruby
+    assert_invalid :name, :blank, :user
+    ```
+    *Daniela Velasquez*
+
 *   Remove deprecated `ActiveSupport::Notifications::Event#children` and  `ActiveSupport::Notifications::Event#parent_of?`.
 
     *Rafael Mendonça França*

activesupport/lib/active_support/testing/assertions.rb

@@ -261,6 +261,20 @@ def assert_no_changes(expression, message = nil, from: UNTRACKED, &block)
         retval
       end
 
+      # Assertion that an active model has a specific invalid field
+      #
+      #   assert_invalid :name, :blank, :user
+      def assert_invalid(attribute, type, obj, msg = nil)
+        raise ArgumentError.new("#{obj.inspect} does not respond to #validate") unless obj.respond_to?(:validate)
+
+        obj.validate
+        msg = message(msg) {
+          data = [attribute, type]
+          "Expected %s to be %s" % data
+        }
+        refute_empty obj.errors.where(attribute, type), msg
+      end
+
       private
         def _assert_nothing_raised_or_warn(assertion, &block)
           assert_nothing_raised(&block)

activesupport/test/active_model_test.rb

@@ -0,0 +1,42 @@
+# frozen_string_literal: true
+
+require_relative "abstract_unit"
+require "active_model"
+
+class ActiveModelTest < ActiveSupport::TestCase
+  def setup
+    @active_model = Class.new do
+      include ActiveModel::Model
+      include ActiveModel::Attributes
+
+      attribute :name, :string
+      validates :name, presence: true
+    end.new
+  end
+
+  test "#assert_invalid asserts active model validation is invalid" do
+    assert_invalid :name, :blank, @active_model
+  end
+
+  test "#assert_invalid raises ArgumentError with a non-active record" do
+    assert_raises(ArgumentError) do
+      assert_invalid :name, :blank, Object.new
+    end
+  end
+
+  test "#assert_invalid raises ArgumentError with message about the object nor responding to validate" do
+    error = assert_raises(ArgumentError) do
+      assert_invalid :name, :blank, Object.new
+    end
+
+    assert_includes error.message, "does not respond to #validate"
+  end
+
+  test "#assert_invalid raises a Minitest::Assertion when validation" do
+    @active_model.name = "h"
+    error = assert_raises(Minitest::Assertion) do
+      assert_invalid :name, :blank,  @active_model
+    end
+    assert_includes error.message, "Expected name to be blank"
+  end
+end