Parameter Validation & Type Coercion for Sinatra

REST conventions take the guesswork out of designing and consuming web APIs. Simply GET, POST, PATCH, or DELETE resource endpoints, and you get what you'd expect.

However, when it comes to figuring out what parameters are expected... well, all bets are off.

This Sinatra extension takes a first step to solving this problem on the developer side

sinatra-param allows you to declare, validate, and transform endpoint parameters as you would in frameworks like ActiveModel or DataMapper.

Use sinatra-param in combination with Rack::PostBodyContentTypeParser and Rack::NestedParams to automatically parameterize JSON POST bodies and nested parameters.


You can install sinatra-param from the command line with the following:

$ gem install sinatra-param

Alternatively, you can specify sinatra-param as a dependency in your Gemfile and run $ bundle install:

gem "sinatra-param", require: "sinatra/param"


require 'sinatra/base'
require 'sinatra/param'
require 'json'

class App < Sinatra::Base
  helpers Sinatra::Param

  before do
    content_type :json

  # GET /search?q=example
  # GET /search?q=example&categories=news
  # GET /search?q=example&sort=created_at&order=ASC
  get '/search' do
    param :q,           String, required: true
    param :categories,  Array
    param :sort,        String, default: "title"
    param :order,       String, in: ["ASC", "DESC"], transform: :upcase, default: "ASC"
    param :price,       String, format: /[<\=>]\s*\$\d+/

    one_of :q, :categories


Parameter Types

By declaring parameter types, incoming parameters will automatically be transformed into an object of that type. For instance, if a param is Boolean, values of '1', 'true', 't', 'yes', and 'y' will be automatically transformed into true.

  • String
  • Integer
  • Float
  • Boolean ("1/0", "true/false", "t/f", "yes/no", "y/n")
  • Array ("1,2,3,4,5")
  • Hash (key1:value1,key2:value2)
  • Date, Time, & DateTime


Encapsulate business logic in a consistent way with validations. If a parameter does not satisfy a particular condition, a 400 error is returned with a message explaining the failure.

  • required
  • blank
  • is
  • in, within, range
  • min / max
  • min_length / max_length
  • format

Custom Error Messages

Passing a message option allows you to customize the message for any validation error that occurs.

param :spelling,
      format: /\b(?![a-z]*cie)[a-z]*(?:cei|ie)[a-z]*/i,
      message: "'i' before 'e', except after 'c'"

Defaults and Transformations

Passing a default option will provide a default value for a parameter if none is passed. A default can defined as either a default or as a Proc:

param :attribution, String, default: "©"
param :year, Integer, default: lambda { }

Use the transform option to take even more of the business logic of parameter I/O out of your code. Anything that responds to to_proc (including Proc and symbols) will do.

param :order, String, in: ["ASC", "DESC"], transform: :upcase, default: "ASC"
param :offset, Integer, min: 0, transform: lambda {|n| n - (n % 10)}

One Of

Using one_of, routes can specify two or more parameters to be mutually exclusive, and fail if more than one of those parameters is provided:

param :a, String
param :b, String
param :c, String

one_of :a, :b, :c

Any Of

Using any_of, a route can specify that at least one of two or more parameters are required, and fail if none of them are provided:

param :x, String
param :y, String

any_of :x, :y


By default, when a parameter precondition fails, Sinatra::Param will halt 400 with an error message:

  "message": "Parameter must be within [\"ASC\", \"DESC\"]",
  "errors": {
    "order": "Parameter must be within [\"ASC\", \"DESC\"]"

To change this, you can set :raise_sinatra_param_exceptions to true, and intercept Sinatra::Param::InvalidParameterError with a Sinatra error do...end block. (To make this work in development, set :show_exceptions to false and :raise_errors to true):

set :raise_sinatra_param_exceptions, true

error Sinatra::Param::InvalidParameterError do
    { error: "#{env['sinatra.error'].param} is invalid" }.to_json

Custom exception handling can also be enabled on an individual parameter basis, by passing the raise option:

param :order, String, in: ["ASC", "DESC"], raise: true

one_of :q, :categories, raise: true


Mattt (@mattt)


sinatra-param is released under an MIT license. See LICENSE for more information.