Params Deserializers

Modeled after active_model_serializers, this gem allows you to create deserializer classes to modify incoming parameters in your Rails app.

Rails controllers often receive incoming parameters (via JSON payloads, for instance) that are not in the format Rails expects for things like saving models. With Params Deserializers, you can easily transform these incoming parameters into the format you want (example).

Create a subclass of ParamsDeserializer:

# app/deserializers/user_params_deserializer.rb

class UserParamsDeserializer < ParamsDeserializer
end

To deserialize a hash with your deserializer, instantiate your deserializer with the hash, and call deserialize on it:

new_hash = UserParamsDeserializer.new(old_hash).deserialize

Call the class method attributes to list incoming params:

class UserParamsDeserializer < ParamsDeserializer
  attributes :first_name, :last_name, :birthday
end

This class indicates that it expects first_name, last_name, and birthday keys on an incoming hash. Any other keys in the hash will be discarded.

Keys can also be listed individually using attribute:

class UserParamsDeserializer < ParamsDeserializer
  attribute :first_name
  attribute :last_name
  attribute :birthday
end

Call the class method strict to specify that only explicitly defined keys be allowed. An invalid key will raise a ParamsDeserializer::InvalidKeyError. If you want to use strict mode, but still allow a specific key that will not be automatically transformed in the output, you can use the class method ignore.

class UserParamsDeserializer < ParamsDeserializer
  strict true
  attribute :firstName
  ignore :lastName
end


# Incoming hash: { :firstName => "Grace" }
# Deserialized params: { :first_name => "Grace" }

# Incoming hash: { :firstName => "Grace", :lastName => "Hopper" ]
# Deserialized params: { :first_name => "Grace" }

# Incoming hash: { :firstName => "Grace", :lastName => "Hopper", :birthday => "12/9/1906" }
# Result: ParamsDeserializer::InvalidKeyError: Invalid keys in params: :birthday.

You can pass an options hash to attribute. If you include the :rename_to key in the options hash, your deserialized params will contain the renamed key instead of the original:

class UserParamsDeserializer < ParamsDeserializer
  attribute :firstName, rename_to: :first_name
  attribute :lastName, rename_to: :last_name
  attribute :birthday
end

# Incoming hash: { :firstName => "Grace", :lastName => "Hopper", :birthday => "12/9/1906" }
# Deserialized params: { :first_name => "Grace", :last_name => "Hopper", :birthday => "12/9/1906" }

You can apply snake_case, CamelCase, or lowerCamel cases to all your keys automatically using format_keys:

class UserParamsDeserializer < ParamsDeserializer
  attributes :firstName, :lastName, :birthday
  format_keys :snake_case # (or :camel_case or :lower_camel)
end

# Incoming hash: { :firstName => "Grace", :lastName => "Hopper", :birthday => "12/9/1906" }
# Deserialized params: { :first_name => "Grace", :last_name => "Hopper", :birthday => "12/9/1906" }

If you expect a root key containing an object with all the other keys, call the root class method on your deserializer. Otherwise, it will look for the attributes you've listed at the top level:

class UserParamsDeserializer < ParamsDeserializer
  root: :user
  attributes :first_name, :last_name
end

# Incoming hash: { :user => { :first_name => "Grace", :last_name => "Hopper" } }
# Deserialized params: { :user => { :first_name => "Grace", :last_name => "Hopper" } }

To discard the root key in your deserialized params, pass { discard: true } as the second argument to root:

class UserParamsDeserializer < ParamsDeserializer
  root: :user, discard: true
  attributes :first_name, :last_name
end

# Incoming hash: { :user => { :first_name => "Grace", :last_name => "Hopper" } }
# Deserialized params: { :first_name => "Grace", :last_name => "Hopper" }

By default, any key listed via attribute or attributes will be included in the deserialized params if the key is present in params. However, the attribute class method takes an options hash. If the hash includes a :present_if key with a lambda value, it will execute that lambda to determine whether that key should be present in the deserialized params:

class UserParamsDeserializer < ParamsDeserializer
  attribute :first_name
  attribute :last_name, present_if: -> { params[:last_name].length > 5 }
end

# Incoming hash: { :first_name => "Grace", :last_name => "Hop" }
# Deserialized params: { :first_name => "Grace" }

The lambda has access to the params hash, and can use it to determine whether the key should be present. In the example above, last_name is only included when its length is greater than 5.

If you define a method of the same name as a parameter, that method will be called to provide the parameter's value:

class UserParamsDeserializer < ParamsDeserializer
  attribute :birthday

  def birthday
    month, day, year = params[:birthday].split("/")
    Time.new(year.to_i, month.to_i, day.to_i)
  end
end

# Incoming hash: { :birthday => "12/9/1906" }
# Deserialized params: { :birthday => 1906-12-09 00:00:00 -0000 }

Note that the method name should equal to the renamed parameter name, if the parameter has been renamed. For example, if you had passed { rename_to: :birth_date } to attribute in the example above, the method you define would have to be birth_date instead of birthday.

You can deserialize incoming parameters in two ways:

If you include ParamsDeserializers in your controller, your controller will have access to a class method called deserialize_params_with. By default, deserialize_params_with will create an instance variable called deserialized_params. Think of this as the params instance variable, but transformed according to your deserializer.

# /app/controllers/users_controller.rb

class UsersController < ApplicationController
  include ParamsDeserializers
  deserialize_params_with UserParamsDeserializer

  def create
    @user = User.create(deserialized_params)
    render json: @user
  end
end

If you'd like a different instance variable name, such as better_params, pass an options hash to deserialize_params_with that contains an :as key:

  deserialize_params_with UserParamsDeserializer, as: :better_params

better_params will now be available to your controller action.

deserialize_params_with passes on any remaining keys in the options hash to before_filter, which means you can restrict deserialization to only certain controller actions:

  deserialize_params_with UserParamsDeserializer, as: :better_params, only: [:create, :update]

You can also simply instantiate your deserializer with params in your controller action, and then call deserialize on it to get the deserialized params:

# /app/controllers/users_controller.rb

class UsersController < ApplicationController
  def create
    deserialized_params = UserParamsDeserializer.new(params).deserialize
    @user = User.create(deserialized_params)
    render json: @user
  end
end

Create a subclass of ParamsDeserializer:

# app/deserializers/user_params_deserializer.rb

class UserParamsDeserializer < ParamsDeserializer
  root :user, discard: true

  format_keys :snake_case

  attributes :birthday,
             :firstName,
             :lastName

  attribute :heightInInches,
            rename_to: :height_in_feet,
            present_if: -> { params[:height_in_inches].is_a? Integer }

  def birthday
    month, day, year = params[:birthday].split("/")
    Time.new(year.to_i, month.to_i, day.to_i)
  end

  def height_in_feet
    params[:heightInInches] / 12.0
  end
end

Then, include it in your UsersController:

# app/controllers/users_controller.rb

class UsersController < ApplicationController
  include ParamsDeserializers
  deserialize_params_with UserParamsDeserializer, as: :new_params, only: :create

  def create
    @user = User.create(new_params)
    render json: @user
  end
end

Now, when an API user hits UsersController#create with the following JSON payload:

{
  "user": {
    "firstName": "Grace",
    "lastName": "Hopper",
    "birthday": "12/9/1906",
    "heightInInches": 66
  }
}

...UsersController#create will have access to an instance variable called new_params with the following contents:

{
  :first_name => "Grace",
  :last_name => "Hopper",
  :birthday => 1906-12-09 00:00:00 -0000,
  :height_in_feet => 5.5
}

• Clone this repo.
• bundle install
• bundle exec rspec (or bundle exec guard to watch for changes)

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