3 files changed, 6 insertions, 5 deletions
diff --git a/activemodel/lib/active_model/lint.rb b/activemodel/lib/active_model/lint.rb
index 957d1b9d70..b71ef4b22e 100644
--- a/activemodel/lib/active_model/lint.rb
+++ b/activemodel/lib/active_model/lint.rb
@@ -23,7 +23,7 @@ module ActiveModel
       def test_to_key
         assert model.respond_to?(:to_key), "The model should respond to to_key"
         def model.persisted?() false end
-        assert model.to_key.nil?
+        assert model.to_key.nil?, "to_key should return nil when `persisted?` returns false"
       end
 
       # == Responds to <tt>to_param</tt>
@@ -40,7 +40,7 @@ module ActiveModel
         assert model.respond_to?(:to_param), "The model should respond to to_param"
         def model.to_key() [1] end
         def model.persisted?() false end
-        assert model.to_param.nil?
+        assert model.to_param.nil?, "to_param should return nil when `persisted?` returns false"
       end
 
       # == Responds to <tt>valid?</tt>
diff --git a/activemodel/lib/active_model/validations/length.rb b/activemodel/lib/active_model/validations/length.rb
index 7af6c83460..72735cfb89 100644
--- a/activemodel/lib/active_model/validations/length.rb
+++ b/activemodel/lib/active_model/validations/length.rb
@@ -43,7 +43,8 @@ module ActiveModel
 
           value ||= [] if key == :maximum
 
-          next if value && value.size.send(validity_check, check_value)
+          value_length = value.respond_to?(:length) ? value.length : value.to_s.length
+          next if value_length.send(validity_check, check_value)
 
           errors_options = options.except(*RESERVED_OPTIONS)
           errors_options[:count] = check_value
diff --git a/activemodel/lib/active_model/validator.rb b/activemodel/lib/active_model/validator.rb
index 1c6123eb09..c5ed8d22d3 100644
--- a/activemodel/lib/active_model/validator.rb
+++ b/activemodel/lib/active_model/validator.rb
@@ -120,7 +120,7 @@ module ActiveModel #:nodoc:
     # Override this method in subclasses with validation logic, adding errors
     # to the records +errors+ array where necessary.
     def validate(record)
-      raise NotImplementedError
+      raise NotImplementedError, "Subclasses must implement a validate(record) method."
     end
   end
 
@@ -156,7 +156,7 @@ module ActiveModel #:nodoc:
     # Override this method in subclasses with the validation logic, adding
     # errors to the records +errors+ array where necessary.
     def validate_each(record, attribute, value)
-      raise NotImplementedError
+      raise NotImplementedError, "Subclasses must implement a validate_each(record, attribute, value) method"
     end
 
     # Hook method that gets called by the initializer allowing verification