Mongoid field hash as Struct

Question

Is it possible to configure a mongoid field to deserialize as a Struct rather than a Hash ? (with defaults)

My use case : a company with a subscription plan stored as a hash in my model.

Previously as a hash

class Company
  include Mongoid::Document
  field :subscription, type: Hash, default: {
      ends_at: 0,
      quantity: 0,
      started_at: 0,
      cancelled: false,
    }

I wish I didn't have to write Company.first.subscription[:ends_at], I'd rather write Company.subscription.ends_at

I figured something like the following would work better

class Company
  include Mongoid::Document
  field :subscription_plan, type: Struct, default: Struct.new(
    :ends_at, :quantity, :started_at, :cancelled
  ) do
    def initialize(
      ends_at: nil, 
      quantity: 0,
      starts_at: nil,
      cancelled: false
    ); super end
  end
end

It would be even better if the plan could be defined in a class

class SubscriptionPlan < Struct.new(
  ends_at, :quantity, :starts_at, :cancelled
) do
  def initialize(
  ends_at: nil, 
  quantity: 0,
  starts_at: nil,
  cancelled: false
); super; end
end

class Company
  field :subscription_plan, type: SubscriptionPlan, default: SubscriptionPlan.new
end

How can I make it work ?

Answer 1

Take this with a grain of salt, as I've never used either MongoDB or Mongoid. Still, googling for "custom type" brought me to this documentation.

Here's an adapted version of the custom type example :

class SubscriptionPlan

  attr_reader :ends_at, :quantity, :started_at, :cancelled

  def initialize(ends_at = 0, quantity = 0, started_at = 0, cancelled = false)
    @ends_at = ends_at
    @quantity = quantity
    @started_at = started_at
    @cancelled = cancelled
  end

  # Converts an object of this instance into a database friendly value.
  def mongoize
    [ends_at, quantity, started_at, cancelled]
  end

  class << self

    # Get the object as it was stored in the database, and instantiate
    # this custom class from it.
    def demongoize(array)
      SubscriptionPlan.new(*array)
    end

    # Takes any possible object and converts it to how it would be
    # stored in the database.
    def mongoize(object)
      case object
      when SubscriptionPlan then object.mongoize
      when Hash then SubscriptionPlan.new(object.values_at(:ends_at, :quantity, :started_at, :cancelled)).mongoize
      else object
      end
    end

    # Converts the object that was supplied to a criteria and converts it
    # into a database friendly form.
    def evolve(object)
      case object
      when SubscriptionPlan then object.mongoize
      else object
      end
    end
  end
end

class Company
  include Mongoid::Document
  field :subscription, type: SubscriptionPlan, default: SubscriptionPlan.new
end

This should bring you closer to what you wanted to do.

Note that the default SubscriptionPlan will be shared by every company with a default. It might lead to some weird bugs if you modify the default plan in one company.

Answer 2

I realized I was just reimplementing a nested document without the ID. In the end I decided to switch to a normal embedded document for my subscription, since having an extra ID field isn't a problem and I get mongoid scopes as bonus. I can always add Mongoid::Attributes::Dynamic in case I want to support any key.

Nevertheless the question and other answer remains relevant for one who wants to create his own types.