Validation frontend for models.

View the Project on GitHub soveran/scrivener

Validation frontend for models

Scrivener removes the validation responsibility from models and acts as a filter for whitelisted attributes.

A model may expose different APIs to satisfy different purposes. For example, the set of validations for a User in a Sign up process may not be the same as the one exposed to an Admin when editing a user profile. While you want the User to provide an email, a password and a password confirmation, you probably don't want the admin to mess with those attributes at all.

In a wizard, different model states ask for different validations, and a single set of validations for the whole process is not the best solution.

Scrivener is Bureaucrat's little brother. It draws all the inspiration from it and its features are a subset of Bureaucrat's. For a more robust and tested solution, please check it.

This library exists to satify the need of extracting Ohm's validations for reuse in other scenarios. By doing this, all projects using Ohm::Validations will be able to profit from extra assertions such as those provided by ohm-contrib.


Using Scrivener feels very natural no matter what underlying model you are using. As it provides its own validation and whitelisting features, you can choose to ignore the ones that come bundled with ORMs.

This short example illustrates how to move the validation and whitelisting responsibilities away from the model and into Scrivener:

# We use Sequel::Model in this example, but it applies to other ORMs such
# as Ohm or ActiveRecord.
class Article < Sequel::Model

  # Whitelist for mass assigned attributes.
  set_allowed_columns :title, :body, :state

  # Validations for all contexts.
  def validate
    validates_presence :title
    validates_presence :body
    validates_presence :state

title = "Bartleby, the Scrivener"
body  = "I am a rather elderly man..."

# When using the model...
article = title, body: body)

article.valid?            #=> false
article.errors.on(:state) #=> ["cannot be empty"]

Of course, what you would do instead is declare :title and :body as allowed columns, then assign :state using the attribute accessor. The reason for this example is to show how you need to work around the fact that there's a single declaration for allowed columns and validations, which in many cases is a great feature and in others is a minor obstacle.

Now see what happens with Scrivener:

# Now the model has no validations or whitelists. It may still have schema
# constraints, which is a good practice to enforce data integrity.
class Article < Sequel::Model

# The attribute accessors are the only fields that will be set. If more
# fields are sent when using mass assignment, a NoMethodError exception is
# raised.
# Note how in this example we don't accept the status attribute.
class Edit < Scrivener
  attr_accessor :title
  attr_accessor :body

  def validate
    assert_present :title
    assert_present :body

edit = title, body: body)
edit.valid?               #=> true

article =

# And now we only ask for the status.
class Publish < Scrivener
  attr_accessor :status

  def validate
    assert_format :status, /^(published|draft)$/

publish = "published")
publish.valid?            #=> true


# If we try to change other fields...
publish = "published", title: "foo")
#=> NoMethodError: undefined method `title=' for #<Publish...>

It's important to note that using Scrivener implies a greater risk than using the model validations. Having a central repository of mass assignable attributes and validations is more secure in most scenarios.


Scrivener ships with some basic assertions. The following is a brief description for each of them:


The assert method is used by all the other assertions. It pushes the second parameter to the list of errors if the first parameter evaluates to false.

def assert(value, error)
   value or errors[error.first].push(error.last) && false


Checks that the given field is not nil or empty. The error code for this assertion is :not_present.


Checks that the given field matches the provided regular expression. The error code for this assertion is :format.


Checks that the given field holds a number as a Fixnum or as a string representation. The error code for this assertion is :not_numeric.


Provides a pretty general URL regular expression match. An important point to make is that this assumes that the URL should start with http:// or https://. The error code for this assertion is :not_url.


In this current day and age, almost all web applications need to validate an email address. This pretty much matches 99% of the emails out there. The error code for this assertion is :not_email.


Checks that a given field is contained within a set of values (i.e. like an ENUM).

def validate
  assert_member :state, %w{pending paid delivered}

The error code for this assertion is :not_valid


Checks that a given field's length falls under a specified range.

def validate
  assert_length :username, 3..20

The error code for this assertion is :not_in_range.


Checks that a given field looks like a number in the human sense of the word. Valid numbers are: 0.1, .1, 1, 1.1, 3.14159, etc.

The error code for this assertion is :not_decimal.


$ gem install scrivener