Request parameters validations, type coercion and filtering for Rails params

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Validates the request params outside your controller logic in order to get a clean nice code, and also working as code documentation for the operations. It ensure that all endpoints input data is right and well formed before it even hits your controller action.

This gem allows you to validate the presence, type, length, format, value and more, of your request parameters. It also coerces the params to the specified type and filter the hash to only those you expect to receive.

It is designed to work for any expected params structure, from a simple hash to a complex one with deeply nested data. It pretends to be a flexible library where you can change and customize several options.

It is intended for REST-like Ruby on Rails APIs.


Add this line to your application's Gemfile:

gem 'request_params_validation'

And then execute bundle install from your shell.

Or, if you want to install it manually, run:

gem install request_params_validation


To start using the gem without setting up any configuration is as simple as adding a before_action with the helper method validate_params! and define your expected request parameters for your resources actions.

The approach of this gem is to have, for each controller file, a definition file. This definitions files is where it should be all data related to the endpoints of your API. This works as code documentation and allows to keep controllers code clean, ensuring that params object will always have the parameters you suppose to receive.

The default path for the definitions files is app/definitions, and their names should be the same as their respective controller's name, but ending with the suffix _definition. They should also respect the folder structure of the controllers folder. Please see the following project structure to clarify the idea:

ā”œā”€ā”€ app
ā”‚   ā”œā”€ā”€ controllers
ā”‚   ā”‚   ā”œā”€ā”€ commerces
|   |   |   ā””ā”€ā”€ branches_controller.rb
|   |   |
|   |   ā”œā”€ā”€ transactions_controller.rb
ā”‚   ā”‚   ā””ā”€ā”€ users_controller.rb
|   |
ā”‚   ā”œā”€ā”€ definitions
ā”‚   ā”‚   ā”œā”€ā”€ commerces
|   |   |   ā””ā”€ā”€ branches_definition.rb
|   |   |
|   |   ā”œā”€ā”€ transactions_definition.rb
ā”‚   ā”‚   ā””ā”€ā”€ users_definition.rb
ā”‚   ā””ā”€ā”€ ...
ā””ā”€ā”€ ...

This gem comes with a set of configurable options allowing you to customize it to your needs. For example, you can change the default helper method validate_params! for whatever name you want. You can also change the default path folder for the definitions app/definitions and even the suffix _definition of the file names. Here you can see all globals configuration options


Add the before_action callback for all actions:

# app/controllers/application_controller.rb

class ApplicationController < ActionController::Base
  before_action :validate_params!

Imagine we have the following resource and we want to define the params for the action create and notify:

# app/controllers/users_controller.rb

class UsersController < ApplicationController
  def create
    params # will have only the defined parameters

  def notify
    params # will have only the defined parameters

  def another_action
    params # will have whatever the user sends

Then, we will need to create the definition for the users resource:

# app/definitions/users_definition.rb

RequestParamsValidation.define do
  action :create do
    request do
      required :user, type: :hash do
        required :first_name, type: :string
        required :last_name, type: :string
        required :emails, type: :array, elements: :email
        required :birth_date,
                 type: :datetime,
                 validate: lambda { |value| value <= 18.years.ago.to_date }

  action :notify do
    request do
      required :user_id, type: :integer
      required :message, type: :string, length: { min: 10, max: 250 }
      optional :by, inclusion: %w(email text_msg push), default: :email

The above definition is just a silly example, but is good enough to explain some important things.

The first thing to say is, as we already mentioned, that each controller file matches with a definition file with the same name and path of it, as you can see in the first line of the example above. Be aware that if the definition file doesn't exist for a controller, then the gem will not validate any param, unless you change this behavior with the global configuration option config.on_definition_not_found. Here you can see all globals configuration options.

As you might notice, the method RequestParamsValidation.define allow you to define a resource/controller. Notice that the resource you are defining is given by the current definition file path/name. After defining the resource, you can continue defining the actions for that resource with the action method. Then, for each action you can define the request using the request method, and there is where you will define the params validations for the current resource/action. You could think that the request step is not strictly necessary, because we could just defined the params validations inside de action block. However, it will have more sense in the future, when more extra options be added.

For defining required parameters we use the required method, otherwise we have the optional method. This two methods accept 2 arguments and a block. The first argument is the only one required, and is the name or key of the parameter. The second argument is an options hash for specifing the extra validations, and the block is for defining nested params.

In the following section we will see all the options validations in-depth look.

Validations & Options

None of the below options are required, so they can be omitted if you don't need to use them.


If a parameter is required, then you should use the required method on the definition of the param. Otherwise use the optional method. For default, required parameters don't accept blank values, if you would like to allow them for that parameter, you can use the option allow_blank

request do
  required :key_1
  required :key_2, allow_blank: true
  optional :key_3


The type option specified the type of the parameter. The supported types are:

  1. hash
  2. array
  3. string
  4. integer
  5. decimal
  6. boolean
  7. date
  8. datetime
  9. email

So if this option is present, the gem will validate that the value of the parameter matches with the specified type. And if it does, it will convert the value to the right type. This means that if a parameter should be an integer, a valid string integer like "100" will be converter to 100. The same applies to the other types.

If you want to add your own types, you can extend the supported types with the global configuration option extend.types. See here all globals configuration options.

request do
  required :key_1, type: :boolean
  required :key_2, type: :decimal
  # ...

Let's see each of the types now.

Hash type

When defining a hash parameter, you will need to pass a block for specifing the nested object. If no block is passed, the gem will only check that the value of the parameter be a valid hash object, without validating the content of it.

request do
  # Allows any keys and values for the hash
  required :key_1, type: :hash

  # Only allows the keys nested_key_1 and nested_key_2
  required :key_2, type: :hash do
    required :nested_key_1, type: :string
    required :nested_key_2, type: :integer

Array type

If you define an array parameter, the gem will only check the value to be a valid array, allowing the elements of the array to be anything. If you also want to validate the elements, you can use the option elements.

The value for this option can be a type or a hash. elements: :integer is equivalent to elements: { type: :integer }.

The second way is useful when you want to validate other things of the elements than just the type. The option elements accepts all validations options.

request do
  # Allows any value for the elements of the array
  required :key_1, type: :array

  # Only allows decimals with a value less than 1_000 for the elements of the array
  required :key_2, type: :array, elements: { type: :decimal, value: { max: 1_000 } }

  # Only allows objects with a required key 'nested_key' of type 'email' for the
  # elements of the array
  required :key_3, type: :array, elements: :hash do
    required :nested_key, type: :email

String type

Any value is a valid string.

Integer type

Accepts only valid integers like 5 or "5".

Decimal type

Accepts only valid decimals like 5 or "1.5" or 10.45. With decimals parameters you can use the option precision. Go here for more details about this option.

Boolean type

Accepts only valid boolean values. The default valid boolean values are:

[true, false, 'true', 'false']

If you need to add more values for the boolean type, for example ['yes', 'no', 1, 0, 't', 'f'], you can extend the true values and the false values independently, with the global configuration options extend.boolean_true_values and extend.boolean_false_values respectively. See here all globals configuration options.

Date type

Date type accepts only valid dates. This means that values like '04/10/1995' are valids, and will be converter to a Date object like Wed, 04 Oct 1995.

However, they are cases when you only want to accept a specific format for a date, like "%Y-%m-%e". In this cases you have two options.

  1. Use the global configuration option, so all date types must have the specified format through all the requests. See here all globals configuration options.

  2. Specify the option format: "%Y-%m-%e" locally.

You can perfectly use both approaches, but the second one will locally override the first one on that parameter validation.

Notice that if no format is specified, the date will be validated using the ruby Date.parse method.

request do
  required :key_1, type: :date
  required :key_2, type: :date, format: '%Y-%m-%e'

Datetime type

Same as date type but for datetime.

Email type

Accepts only valid emails like [email protected]. It's just a helper for a string type with an email regexp format.


The inclusion option is for validating that the param value is included in a given array.

The value for this option can be an enumerable or a hash. inclusion: %w(asc desc) is equivalent to inclusion: { in: %w(asc desc) }.

Besides from the in option, you can also use the message option for passing a custom error detail when the parameter is not valid.

request do
  required :key_1, type: :string, inclusion: %w(asc desc)
  required :key_2,
                  type: :string,
                  inclusion: { in: %w(s m l), message: 'Value is not a valid size' }


The length option is for validating the length of the param value.

The value for this option can be an integer or a hash. length: 5 is equivalent to length: { min: 5, max: 5 }.

Besides from the min and max options, you can also use the message option for passing a custom error detail when the parameter is not valid.

request do
  required :key_1, type: :string, length: 10
  required :key_2, type: :string, length: { min: 5, max: 12 }
  required :key_3, type: :array, elements: :email, length: { max: 3 }
  required :key_4, type: :string, length: { max: 25, message: '25 characters is the maximum allowed' }

Value Size

The value option is for validating the value size of numerics parameters.

The value for this option is a hash with the following options: min, max and message.

request do
  required :key_1, type: :integer, value: { min: 0 }
  required :key_2, type: :integer, value: { max: 1_000_000, message: 'Value too big!' }
  required :key_3, type: :decimal, value: { min: 0, max: 1 }


The format option allows to validate the format of the value with a regular expression.

The value for this option is a regexp, string or a hash. The string value is only valid when the type is a date or a datetime. Otherwise, you should use a regexp. The options for the hash are: regexp, strptime and message.

So, for date and datetime types, format: '%u%F' is equivalent to format: { strptime: '%u%F' }. For the other types, format: /^5[1-5]\d{14}$/ is equivalent to format: { regexp: /^5[1-5]\d{14}$/ }.

request do
  required :key_1, type: :string, format: /^5[1-5]\d{14}$/
  required :key_2, type: :string, format: { regexp: /^1.*/,
                                            message: 'Value should start with a 1' }

Custom Validation

You can add custom validations to the parameter with the option validate.

This option accepts a Proc as value or a hash. For example, validate: lambda { |value| value > } is equivalent to validate: { function: lambda { |value| value > } }. The hash value also accepts the message option.

request do
  required :key_1, type: :date, validate: { function: lambda { |value| value >= },
                                            message: 'The date can not be in the past' }


The precision option are for decimal types. This option does not execute any validation on the value of the parameter, but it will round the decimal when the value is converter to the specified type.

If you want to set a precision value to all decimal parameters, you can use the global configuration option format.decimal_precision. Keep in mind that if you set the precision option on a parameter, it will locally override the global configuration. See here for all globals configuration options.

This option accepts an integer as value.

request do
  required :key_1, type: :decimal, precision: 2

Default Values

When parameters are optional, with the default option you can set a default value when the parameter is not present.

The value for the option default could be anything, including a proc.

request do
  optional :key_1, type: :string, default: 'Jane'
  optional :key_2, type: :string, default: lambda {'%A') }


Transformations are functions that are called to the value of the parameter, after it has already been validated. The option for this is transform.

The transform option could be a symbol, or a proc. The proc will receive the value of the parameter as an argument, so keep in mind that the value will be already of the type specified in the definition. So, transform: :strip is equivalent to transform: lambda { |value| value.strip }.

request do
  optional :key_1, type: :string, transform: :strip
  optional :key_2,
           type: :string,
           format: /^\d{3}-\d{3}-\d{3}$/,
           transform: lambda { |value| value.gsub(/-/, '') }

Rename Parameters

You can rename parameters using the as option.

request do
  required :email_address, type: :email, as: :email

This means that in the request params you expect a valid email value in the key email_address, but in your controller you will access with the key email.

Dependent Parameters

If you want to receive and validate a parameter only if another one is given, you can use the is_given option.

request do
  optional :label, type: :string
  required :description, type: :string, if_given: :label
  required :card_type, inclusion: %w(credit_card debit_card)
  required :ccv, if_given: { card_type: lambda { |value| value == 'credit_card' } }

On the example above, the param description will be only validated if the param label is present. RequestParamsValidation will use the method blank? to check that. On the other hand, the param ccv will only be validated if the param type_card is equal to the string credit_card.

Notice that if the global option filter_params is set to true (default behaviour), then the dependent parameters will be filtered from the params object if they haven't beeen validated. This way we make sure to only receive those parameters that have been validated against our request definitions.

Be aware that if you rename a param, then you should use the new name in the if_given option.


RequestParamsValidation will start validating the presence of the parameters. Then, if the value is not present and the parameter has a default value, it will assign that value and not execute any further validation. Otherwise, it will validate the type, convert it to the right type and then continue with the others validations. So, all others validations will be executed with the parameter value already converter to the specified type, so keep in mind that at defining the validations.

Errors & Messages

For default, when a required parameter failed the presence validation, the exception RequestParamsValidation::MissingParameterError will be raised. If it failed for any of the others validations, the raised exception will be RequestParamsValidation::InvalidParameterValueError with a proper descriptive error message.

This two exceptions inherits from RequestParamsValidation::RequestParamError, so you can rescue the exceptions like this:

# app/controllers/application_controller.rb

class ApplicationController < ActionController::Base
  rescue_from RequestParamsValidation::RequestParamError do |exception|
    # do whatever you want

Both exceptions has getters methods to access data related to the failure. For example, the RequestParamsValidation::MissingParameterError exception has two public methods param_key and param_type for getting the name and type of the parameter which failed. And the RequestParamsValidation::InvalidParameterValueError exception has the two mentioned methods, plus the methods param_value and details. param_value returns the value of the parameter, and details give more information about the reason of the failure.

Errors messages

For the exception RequestParamsValidation::MissingParameterError, the error message is the following:

"The parameter '#{param_key}' is missing"

And for RequestParamsValidation::InvalidParameterValueError the message is:

"The value for the parameter '#{param_key}' is invalid"

Or, if details is present:

"The value for the parameter '#{param_key}' is invalid. #{details}"

The details is different depending on the reason of the failure, and whether the parameter is an element of an array or not. If you have specified the message option in the parameter definition, then the details will be that value, otherwise it will took a default value from the table below:

Failure Default Message
Missing parameter N/A
Invalid type - Value should be a valid %{param_type}
- All elements of the array should be a valid %{type}
If has date or datetime type with specified format:
    - with the format %{format} is added to the message
Invalid inclusion - Value should be in %{include_in}
- All elements values of the array should be in %{include_in}
Invalid length - Length should be greater or equal than %{min}
- Length should be less or equal than %{max}
- Length should be equal to %{min/max}
- Length should be between %{min} and %{max}
- All elements of the array should have a length ...
Invalid value size - Value should be greater or equal than %{min}
- Value should be less or equal than %{max}
- Value should be between %{min} and %{max}
- All elements of the array should have a value ...
Invalid format - Value format is invalid
- An element of the array has an invalid format
Invalid custom validation N/A

Custom Exceptions

However, if the above is not enough for your app, and you need to fully customize the exceptions and the messages, you can setup your own exceptions classes for each type of failure. They are globals configurations options that allow you to do that. See below to see them all.

Global Configurations

Global configurations help you to customize the gem to fulfill your needs. To change this configuration, you need to create an initializer and configure what you want to change:

# config/initializers/request_params_validation.rb

RequestParamsValidation.configure do |config|
  #... here goes the configuration

To see a complete initializer file of the configuration with all the options and their description, please see here.


This gem is strongly inspired in a Ruby framework named Angus developed by Moove It


  1. Fork it
  2. Create your feature branch (git checkout -b my-new-feature)
  3. Commit your changes (git commit -am 'Add some feature')
  4. Push to the branch (git push origin my-new-feature)
  5. Create a Pull Request


This software is released under the MIT license. See the MIT-LICENSE file for more info.


Request parameters validations, type coercion and filtering for Rails params

Request_params_validation Info

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šŸ˜Ž Author felipefava