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Gem Version Test Status

Ruby gem for validating form data.


Most web apps built for end users will need to process url-encoded form data. Registration forms, profile forms, checkout forms, contact forms, and forms for adding/editing application specific data.

As developers we would like to process this data safely, to minimise the possibility of security holes within our application that could be exploited. Formeze adopts the approach of being "strict by default", forcing the application code to be explicit in what it accepts as input.


Using Bundler:

$ bundle add formeze

Using RubyGems:

$ gem install formeze


Here is a minimal example, which defines a form with a single field:

require 'formeze'

class ExampleForm < Formeze::Form
  field :title

You can then parse and validate form data in Rails or Sinatra like this:

form =

if form.valid?
  # do something with form data
  # display form.errors to user

Formeze will automatically ignore the Rails "utf8" and "authenticity_token" parameters.

If you prefer not to inherit from the Formeze::Form class then you can instead call the Formeze.setup method on your classes like this:

class ExampleForm

  field :title

Both styles of setup will include the formeze class methods and instance methods but will otherwise leave the object untouched (i.e. you can define your own initialization logic).

Validation errors

Formeze distinguishes between validation errors (which are expected in the normal running of your application), and key/value errors (which most likely indicate either developer error, or form tampering). For the latter case, the parse method that formeze provides will raise a Formeze::KeyError or a Formeze::ValueError exception if the structure of the form data does not match the field definitions.

After calling parse you can check that the form is valid by calling the valid? method. If it isn't you can call the errors method which will return an array of error messages to display to the end user. You can also use the errors_on? and errors_on methods to check for and select error messages specific to a single field.

Field options

By default fields cannot be blank, they are limited to 64 characters, and they cannot contain newlines. These restrictions can be overridden by setting various field options.

Defining a field without any options works well for a simple text input. If the default length limit is too big or too small you can override it by setting the maxlength option. For example:

field :title, maxlength: 200

Similarly there is a minlength option for validating fields that should have a minimum number of characters (e.g. passwords).

Fields are required by default. Specify the required option if the field is not required, i.e. the value of the field can be blank/empty. For example:

field :title, required: false

You might want to return a different value for blank fields, such as nil, zero, or a "null" object. Use the blank option to specify this behaviour. For example:

field :title, required: false, blank: nil

If you are dealing with textareas (i.e. multiple lines of text) then you can set the multiline option to allow newlines. For example:

field :description, maxlength: 500, multiline: true

Error messages will include the field label, which by default is set to the field name, capitalized, and with underscores replace by spaces. If you want to override this, set the label option. For example:

field :twitter, label: 'Twitter Username'

If you want to validate that the field value matches a specific pattern you can specify the pattern option. This is useful for validating things with well defined formats, like numbers. For example:

field :number, pattern: /\A[1-9]\d*\z/

field :card_security_code, maxlength: 5, pattern: /\A\d+\z/

If you want to validate that the field value belongs to a set of predefined values then you can specify the values option. This is useful for dealing with input from select boxes, where the values are known upfront. For example:

field :card_expiry_month, values: (1..12).map(&:to_s)

The values option is also useful for checkboxes. Specify the key_required option to handle the case where the checkbox is unchecked. For example:

field :accept_terms, values: %w(true), key_required: false

Sometimes you'll have a field with multiple values, such as a multiple select input, or a set of checkboxes. For this case you can specify the multiple option, for example:

field :colour, multiple: true, values: Colour.keys

Sometimes you'll only want the field to be defined if some condition is true. The condition may depend on the state of other form fields, or some external state accessible from the form object. You can do this by specifying either the defined_if or defined_unless options with a proc. Here's an example of using the defined_if option:

field :business_name, defined_if: ->{ }

In this example the business_name field will only be defined and validated for business accounts. The proc is evaluated in the context of the form object, so has full access to instance variables and methods defined on the object. Here's an example of using the defined_unless option:

field :same_address, values: %w(true), key_required: false

field :billing_address_line_one, defined_unless: ->{ same_address? }

def same_address?
  same_address == 'true'

In this example, the billing_address_line_one field will only be defined and validated if the same_address checkbox is checked.

Validation errors can be a frustrating experience for end users, so ideally we want to be liberal in what we accept, but at the same time ensuring that data is consistently formatted to make it easy for us to process. The scrub option can be used to specify methods for "cleaning" input data before validation. For example:

field :postcode, scrub: [:strip, :squeeze, :upcase]

The input for this field will have leading/trailing whitespace stripped, double (or more) spaces squeezed, and the result upcased automatically. Custom scrub methods can be defined by adding a symbol/proc entry to the Formeze.scrub_methods hash.

Multipart form data

For file fields you can specify the accept and maxsize options, for example:

class ExampleForm < Formeze::Form
  field :image, accept: 'image/jpg,image/png', maxsize: 1000

For this to work you need to make sure your application includes the mime-types gem, and that the form is submitted with the multipart/form-data mime type.

Custom validation

You may need additional validation logic beyond what the field options described above provide, such as validating the format of a field without using a regular expression, validating that two fields are equal etc. This can be accomplished using the validates class method. Pass the name of the field to be validated, and a block/proc that encapsulates the validation logic. For example:

class ExampleForm < Formeze::Form
  field :email

  validates :email, &EmailAddress.method(:valid?)

If the block/proc takes no arguments then it will be evaluated in the scope of the form instance, which gives you access to the values of other fields (and methods defined on the form). For example:

class ExampleForm < Formeze::Form
  field :password
  field :password_confirmation

  validates :password_confirmation do
    password_confirmation == password

Specify the if option with a proc to peform the validation conditionally. Similar to the defined_if and defined_unless field options, the proc is evaluated in the scope of the form instance. For example:

class ExampleForm < Formeze::Form
  field :business_name, defined_if: :business_account?
  field :vat_number, defined_if: :business_account?

  validates :vat_number, if: :business_account? do
    # ...

  def initialize(account)
    @account = account

  def business_account?

Specify the error option with a symbol to control which error the validation generates. The I18n integration described below can be used to specify the error message used, both for errors that are explicitly specified using this option, and the default "invalid" error. For example:

class ExampleForm < Formeze::Form
  field :email
  field :password
  field :password_confirmation

  validates :email, &EmailAddress.method(:valid?)

  validates :password_confirmation, error: :does_not_match do
    password_confirmation == password

The error for the email field validation would include the value of the formeze.errors.invalid I18n key, defaulting to "is invalid" if the I18n key does not exist. The error for the password_confirmation field validation would include the value of the formeze.errors.does_not_match I18n key.

I18n integration

Formeze integrates with I18n so that you can define custom error messages and field labels within your locales (useful both for localization, and when working with designers). For example, here is how you would change the "required" error message (which defaults to "is required"):

# config/locales/en.yml
      required: "cannot be blank"

And here is an example of how you would set a custom label for fields named "first_name" (for which the default label would be "First name"):

# config/locales/en.yml
      first_name: "First Name"

Labels defined in this way apply globally to all Formeze forms, but can be overridden using the label field option which will take precedence.