DelegateIfNil

A simple gem to add nil delegation to an associated model if an attribute is nil.

Usage

Add this to your model:

class SomeModel < ApplicationRecord
    extend DelegateIfNil
    belongs_to :owning_model

    nil_delegate :animal, to: :owning_model
end

animal will now delegate to owning_model if it's nil on an instance of SomeModel.

delegate_if_nil is available as an alias:

class SomeModel < ApplicationRecord
    extend DelegateIfNil
    belongs_to :owning_model

    nil_delegate :animal, to: :owning_model
    delegate_if_nil :animal, to: :owning_model # Exact same as the line above
    
end

You also get _source methods, which will tell you where the animal value comes from. If it's not set on either one, it will return "unset"

You'll get the following results:

om = OwningModel.create(animal: "Cat")

some_model = SomeModel.create(animal: nil, owning_model: om)
some_model.animal # "Cat"
some_model.animal_source # "owning_model"
some_model.animal_delegated? # true

some_model = SomeModel.create(animal: "Dog", owning_model: om)
some_model.animal # Dog
some_model.animal_source # "self"
some_model.animal_delegated? # false

om = OwningModel.create(animal: nil)
some_model = SomeModel.create(animal: nil, owning_model: om)
some_model.animal # nil
some_model.animal_source # "unset"
some_model.animal_delegated? # true

It also works for multiple attributes. EG:

class SomeModel < ApplicationRecord
    extend DelegateIfNil
    belongs_to :owning_model

    nil_delegate :animal, :phone_number, :name, to: :owning_model
end

It also resolves correctly for recursive delegations. IE: if owning_model delgates an attribute if nil, it will correctly report the source all the way down the chain, terminating either in self, unset, or the association name.

Example

This is an example based off a real-world project.

A User has many notification_channels. Each user and notification_channel has a notifications_enabled boolean flag.

With this setup, a user can set a default value for notifications_enabled, and each channel can override the default.

eg:

class User < ActiveRecord::Base
    # notifications_enabled is a boolean attribute on the user model
    has_many :notification_channels
end

class NotificationChannel < ActiveRecord::Base
    # notifications_enabled is a boolean attribute on the notification_channel model
    belongs_to :user
    nil_delegate :notifications_enabled, to: :user
end

Now, to see if a specific channel should be enabled, simple call notification_channel.notifications_enabled, and it will resolve the correct value for that channel.

Installation

Add this line to your application's Gemfile:

gem 'delegate_if_nil'

And then execute:

$ bundle

Or install it yourself as:

$ gem install delegate_if_nil

License

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