New #added? method on ActiveModel::Errors

The #added? method makes it possible to check if a specific error has been added, using the same parameters as for #add.

activemodel/CHANGELOG

@@ -1,3 +1,5 @@
+* Added ActiveModel::Errors#added? to check if a specific error has been added [Martin Svalin]
+
 * Add ability to define strict validation(with :strict => true option) that always raises exception when fails [Bogdan Gusiev]
 
 * Deprecate "Model.model_name.partial_path" in favor of "model.to_partial_path" [Grant Hutchins, Peter Jaros]

activemodel/lib/active_model/errors.rb

@@ -213,13 +213,7 @@ module ActiveModel
     # If +message+ is a symbol, it will be translated using the appropriate scope (see +translate_error+).
     # If +message+ is a proc, it will be called, allowing for things like <tt>Time.now</tt> to be used within an error.
     def add(attribute, message = nil, options = {})
-      message ||= :invalid
-
-      if message.is_a?(Symbol)
-        message = generate_message(attribute, message, options.except(*CALLBACKS_OPTIONS))
-      elsif message.is_a?(Proc)
-        message = message.call
-      end
+      message = normalize_message(attribute, message, options)
       if options[:strict]
         raise ActiveModel::StrictValidationFailed,  message
       end
@@ -244,6 +238,15 @@ module ActiveModel
       end
     end
 
+    # Returns true if an error on the attribute with the given message is present, false otherwise.
+    # +message+ is treated the same as for +add+.
+    #   p.errors.add :name, :blank
+    #   p.errors.added? :name, :blank # => true
+    def added?(attribute, message = nil, options = {})
+      message = normalize_message(attribute, message, options)
+      self[attribute].include? message
+    end
+
     # Returns all the full error messages in an array.
     #
     #   class Company
@@ -329,6 +332,19 @@ module ActiveModel
 
       I18n.translate(key, options)
     end
+
+  private
+    def normalize_message(attribute, message, options)
+      message ||= :invalid
+
+      if message.is_a?(Symbol)
+        generate_message(attribute, message, options.except(*CALLBACKS_OPTIONS))
+      elsif message.is_a?(Proc)
+        message.call
+      else
+        message
+      end
+    end
   end
 
   class StrictValidationFailed < StandardError

activemodel/test/cases/errors_test.rb

@@ -80,6 +80,49 @@ class ErrorsTest < ActiveModel::TestCase
     assert_equal ["can not be blank"], person.errors[:name]
   end
 
+  test "added? should be true if that error was added" do
+    person = Person.new
+    person.errors.add(:name, "can not be blank")
+    assert person.errors.added?(:name, "can not be blank")
+  end
+
+  test "added? should handle when message is a symbol" do
+    person = Person.new
+    person.errors.add(:name, :blank)
+    assert person.errors.added?(:name, :blank)
+  end
+
+  test "added? should handle when message is a proc" do
+    person = Person.new
+    message = Proc.new { "can not be blank" }
+    person.errors.add(:name, message)
+    assert person.errors.added?(:name, message)
+  end
+
+  test "added? should default message to :invalid" do
+    person = Person.new
+    person.errors.add(:name, :invalid)
+    assert person.errors.added?(:name)
+  end
+
+  test "added? should be true when several errors are present, and we ask for one of them" do
+    person = Person.new
+    person.errors.add(:name, "can not be blank")
+    person.errors.add(:name, "is invalid")
+    assert person.errors.added?(:name, "can not be blank")
+  end
+
+  test "added? should be false if no errors are present" do
+    person = Person.new
+    assert !person.errors.added?(:name)
+  end
+
+  test "added? should be false when an error is present, but we check for another error" do
+    person = Person.new
+    person.errors.add(:name, "is invalid")
+    assert !person.errors.added?(:name, "can not be blank")
+  end
+
   test 'should respond to size' do
     person = Person.new
     person.errors.add(:name, "can not be blank")