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Validation of parameters for Sinatra


 Project Readme


Validate parameters in a Sinatra app.


Install the gem and add to the application's Gemfile by executing:

$ bundle add sinatra-param-validator

If bundler is not being used to manage dependencies, install the gem by executing:

$ gem install sinatra-param-validator

Sample Usage

validator :user_id do
  param :id, Integer, required: true

get '/user/:id', validate: :user_id do
  # ...

validator :new_user do
  param :name, String, required: true
  param :age, Integer, required: true, min: 0 

post '/new-user', validate: :new_user do
  # ...

Parameter Types

The following parameter types are built-in, and values will be coerced to an object of that type.

  • Array
    • Accepts a comma-separated list of values, as well as an array
    • e.g. a,b,c
  • Boolean
    • false|f|no|n|0 or true|t|yes|y|1
  • Date
    • All formats accepted by Date.parse
  • Float
  • Hash
    • Accepts a comma-separated list of colon-separated key-value pairs
    • e.g. a:1,b:2,c:3
  • Integer
  • String
  • Time
    • All formats accepted by Time.parse

Types can be defined using class names or symbols:

param :name, String
param :tick_box, :boolean 

Parameter Validations

param :number, Integer, required: true, in: 0..100

All parameters have the following validations available:

  • default
    • Set a default value if this parameter was not provided
    • This can be a lambda or a proc if required
  • nillable
    • If this is set, all other validations are skipped if the value is nil
  • required
    • The parameter must be present and cannot be nil
  • transform
    • Run a method against the validated parameter, allowing it to be changed
    • Anything that responds to to_proc will work; procs, lambdas, symbols...
  • in
    • The value is in the given array / range
  • is
    • Match a specific value

Array, Hash and String have the following validations:

  • min_length / max_length

Date, Time, Float and Integer have the following validations:

  • min / max

Running as a helper

param is available as a helper in routes, and will raise Sinatra::ParamValidator::InvalidParameterError if validation fails. It will return the parsed parameter, after coercion, defaults and transformations have been applied.

get '/page' do
  param :a, String


Rules work on multiple parameters:

rule :all_or_none_of, :a, :b
  • all_or_none_of
  • any_of
    • At least one of the given fields must be present
  • one_of
    • Only one of the given fields can be present

Running as a helper

rule is available as a helper in routes, and will raise Sinatra::ParamValidator::InvalidParameterError if validation fails.

get '/page' do
  rule :any_of, :a, :b

Custom Messages

It is possible to return a custom error message when a validation fails:

param :number, Integer, required: true, message: 'The number is required'

It is also possible to run multiple validations against a single parameter. This can be useful if different failures require different messages.

param :number, Integer, required: true, message: 'The number is required'
param :number, Integer, min: 100, message: 'The number is not large enough'

Validation blocks

It is possible to run code after a validation succeeds, by passing a block to param:

param :number, Integer, required: true do
  # ...

rule :any_of, :a, :b do
  # ...

If you wish to indicate a validation failure within a block, raise Sinatra::ParameterValidator::InvalidParameterError with a message, and it will be passed through as an error for the parameter.

If you need to do further validation with your parameter, the validator can be passed to the block:

param :number, Integer, required: true do |validator|
  validator.param :digit, Integer, min: params[:number]

Validator Types

The default validator will raise Sinatra::ParamValidator::ValidationFailedError when validation fails.

There are two other provided validators, that handle failure differently:

  • url_param
    • will halt 403
  • form
    • if sinatra-flash is available, it will flash the errors and redirect back
    • will provide a JSON object with errors to an XHR request
    • will halt 400

These validators can be invoked with a different conditional on the route:

post '/new-user', validate_form: :new_user do
  # ...

get '/user/:id', validate_url_param: :user_id do
  # ...

Form Helpers

There are some general helpers for handling values after validation. These require the sinatra-flash gem.

  • form_values(hash)
    • Set the values that will be returned by form_value.
    • These will be overridden by any values flashed to the session in the :params hash
  • form_value(field)
    • Get the form value for the given field
  • form_error?(field = nil)
    • With a field: returns if that field had any validation errors
    • Without a field: returns if any field had validation errors
  • form_errors(field)
    • Get the list of validation errors for the given field
  • invalid_feedback(field, default = nil)
    • Get the invalid feedback (error message) for the given field, defaulting to the default parameter.


For a form to edit something, you can define the values that the form should use:

get '/edit/:thing' do
  thing = #...
  form_values thing

This is an example form input with bootstrap styling:

<form method="post" <%== 'class="was-validated"' if form_error? %>>
  <div class="mb-3">
    <label class="form-label" for="name">Name</label>
    <input type="text" name="name" id="name" 
           class="form-control <%= 'is-invalid' if form_error? :name %>"
           value="<%= form_value :name %>">
    <div class="invalid-feedback">
      <%== invalid_feedback :name, 'Please provide a name.' %>

When the form is submitted, validation may fail, and the page will redirect back to the edit form. The redirect will flash the submitted parameters. form_value will now prefer the flashed parameters over the original values for thing.

Validators with parameters

It is possible to define a validator with a parameter. To call the validator, you can use the vi helper to wrap a validator identifier with arguments:

validator :number do |min|
  param :id, Integer, min: min

post '/number', validate: vi(:new_user, 10) do
  # ...


After checking out the repo, run bundle install to install dependencies. Then, run rake spec to run the tests.

To install this gem onto your local machine, run bundle exec rake install. To release a new version, update the version number in version.rb, and then run bundle exec rake release, which will create a git tag for the version, push git commits and the created tag, and push the .gem file to


Bug reports and pull requests are welcome on GitHub at


The gem is available as open source under the terms of the MIT License.