Create have_error_on.rb

spec/support/matchers/have_error_on.rb

@@ -0,0 +1,12 @@
+# frozen_string_literal: true
+
+# This custom matcher checks that a model's errors on a given attribute include a specific phrase.
+RSpec::Matchers.define :have_error_on do |attribute, expected_phrase|
+  match do |model|
+    model.valid? && model.errors[attribute].any? { |msg| msg.include?(expected_phrase) }
+  end
+
+  failure_message do |model|
+    "expected errors on #{attribute} to include '#{expected_phrase}', but got: #{model.errors[attribute].join(', ')}"
+  end
+end