Adds assert_error_on and assert_no_error_on assertions

Introduces two new assertions, `assert_error_on` and `assert_no_error_on`, to simplify checking for specific validation errors on models.

Example usage:
  - assert_error_on user, :name, :blank
  - assert_no_error_on user, :name, :blank

This enhances test readability and makes validation testing more intuitive.

activesupport/CHANGELOG.md

@@ -1,3 +1,12 @@
+*   Added new test assertions  `assert_error_on` and `assert_no_error_on` to simplify testing for specific validation errors on models.
+    Example usage:
+    ```ruby
+    assert_error_on user :name, :blank
+    assert_no_error user, :name, :blank
+    ```
+
+    *Daniela Velasquez*
+
 *   Fix `ActiveSupport::HashWithIndifferentAccess#stringify_keys` to stringify all keys not just symbols.
 
     Previously:

activesupport/lib/active_support/testing/assertions.rb

@@ -282,6 +282,86 @@ def assert_no_changes(expression, message = nil, from: UNTRACKED, &block)
         retval
       end
 
+      # Asserts that an ActiveModel object has a specific error on a given attribute.
+      #
+      # This assertion checks whether a validation error of a specific type
+      # exists on the provided attribute of the object. If the error does not
+      # exist, the test will fail, displaying an optional custom error message or
+      # a default one indicating the expected and actual outcomes.
+      #
+      # Example:
+      #
+      #   assert_error_on(user, :name, :blank)
+      #   # Asserts that the `user` object has a `:blank` validation error on the `:name` attribute.
+      #
+      # Arguments:
+      #   obj: The ActiveModel object being validated. It must respond to `#errors`.
+      #   attribute: The attribute for which the validation error is expected (e.g., `:name`).
+      #   type: The specific type of validation error (e.g., `:blank`, `:invalid`, etc.).
+      #   msg: An optional custom message to display if the assertion fails. If not provided,
+      #        a default message is generated based on the provided attribute and error type.
+      #
+      # Raises:
+      #   ArgumentError: If the `obj` does not respond to `#errors`.
+      #
+      # Example Custom Error Message:
+      #
+      #   assert_error_on(user, :name, :blank, "Expected 'name' to be present but it's blank.")
+      #
+      # Failing Example:
+      #
+      #   assert_error_on(user, :email, :invalid)
+      #   # Fails if `user.errors` does not contain an `:invalid` error for the `:email` attribute.
+      def assert_error_on(obj, attribute, type, msg = nil)
+        raise ArgumentError.new("#{obj.inspect} does not respond to #errors") unless obj.respond_to?(:errors)
+
+        msg = message(msg) {
+          data = [type, attribute]
+          "Expected error %s on %s" % data
+        }
+
+        assert(obj.errors.added?(attribute, type), msg)
+      end
+
+      # Asserts that an ActiveModel object does not have a specific error on a given attribute.
+      #
+      # This assertion checks that a validation error of a specific type is not present
+      # on the provided attribute of the object. If the error exists, the test will fail,
+      # displaying an optional custom error message or a default one indicating the
+      # unexpected error on the attribute.
+      #
+      # Example:
+      #
+      #   assert_no_error_on(user, :name, :blank)
+      #   # Asserts that the `user` object does not have a `:blank` validation error on the `:name` attribute.
+      #
+      # Arguments:
+      #   obj: The ActiveModel object being validated. It must respond to `#errors`.
+      #   attribute: The attribute for which the absence of a specific validation error is expected (e.g., `:name`).
+      #   type: The specific type of validation error that is expected *not* to be present (e.g., `:blank`, `:invalid`).
+      #   msg: An optional custom message to display if the assertion fails. If not provided,
+      #        a default message is generated based on the provided attribute and error type.
+      #
+      # Raises:
+      #   ArgumentError: If the `obj` does not respond to `#errors`.
+      #
+      # Example Custom Error Message:
+      #
+      #   assert_no_error_on(user, :name, :blank, "Expected 'name' to be valid and not blank.")
+      #
+      # Failing Example:
+      #
+      #   assert_no_error_on(user, :email, :invalid)
+      #   # Fails if `user.errors` contains an `:invalid` error for the `:email` attribute.
+      def assert_no_error_on(obj, attribute, type, msg = nil)
+        raise ArgumentError.new("#{obj.inspect} does not respond to #errors") unless obj.respond_to?(:errors)
+        msg = message(msg) {
+          data = [attribute, type]
+          "Expected %s to not be %s" % data
+        }
+        assert_not(obj.errors.added?(attribute, type), msg)
+      end
+
       private
         def _assert_nothing_raised_or_warn(assertion, &block)
           assert_nothing_raised(&block)

activesupport/test/validation_errors_test.rb

@@ -0,0 +1,77 @@
+# frozen_string_literal: true
+
+require_relative "abstract_unit"
+require "active_model"
+
+class ValidationErrorsTest < ActiveSupport::TestCase
+  def setup
+    @active_model = Class.new do
+      include ActiveModel::Model
+      include ActiveModel::Attributes
+
+      attribute :name, :string
+      attribute :last_name, :string
+      validates :name, :last_name, presence: true
+      validate :name_doesnt_contain_numbers
+
+      private
+        def name_doesnt_contain_numbers
+          unless name.nil? || name.scan(/\d/).empty?
+            errors.add(:name, "shouldn't contain numbers")
+          end
+        end
+    end.new
+  end
+
+  test "#assert_no_error_on asserts active model does not have an error on a field" do
+    @active_model.name = "name"
+    @active_model.validate
+
+    assert_no_error_on @active_model, :name, :blank
+  end
+
+  test "#assert_no_error_on raises ArgumentError with an object that doesn't respond to errors" do
+    error = assert_raises(ArgumentError) do
+        assert_no_error_on Object.new, :name, :blank, Object.new
+      end
+
+    assert_includes error.message, "does not respond to #errors"
+  end
+
+  test "#assert_no_error_on raises a Minitest::Assertion when validation fails" do
+    @active_model.validate
+    error = assert_raises(Minitest::Assertion) do
+      assert_no_error_on @active_model, :name, :blank
+    end
+    assert_includes error.message, "Expected name to not be blank"
+  end
+
+  test "#assert_error_on asserts active model has an error on name field" do
+    @active_model.validate
+    assert_error_on @active_model, :name, :blank
+  end
+
+  test "#assert_error_on asserts active model has an error on a field with a string" do
+    error_message = "must start with H"
+    @active_model.errors.add(:name, error_message)
+
+    assert_error_on @active_model, :name, error_message
+  end
+
+  test "#assert_error_on raises ArgumentError on an object that doesn't respond to errors" do
+    error = assert_raises(ArgumentError) do
+      assert_error_on :name, :blank, Object.new
+    end
+
+    assert_includes error.message, "does not respond to #errors"
+  end
+
+  test "#assert_error_on raises a Minitest::Assertion when validation fails" do
+    @active_model.name = "h"
+    @active_model.validate
+    error = assert_raises(Minitest::Assertion) do
+      assert_error_on @active_model, :name, :blank
+    end
+    assert_includes error.message, "Expected error blank on name"
+  end
+end