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Stamp it out with ActiveRecord concerns.
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Stamp out domain-specific clones of a model object, even after the original has departed, with these lightweight ActiveRecord concerns.

Quick demo

If you want to write code like this, snapshotting an ActiveRecord object (and perhaps some associations) into a template:

@template = Template.create!(origin: @project, name: "New template from project")

hoping to use it like this, at some later time (perhaps long after the origin has been deleted):

template = Template.find(params[:template_id])
@new_project = template.stamp do |project| = "New project from template"

then you have (maybe) come to the right place.


Add this line to your application's Gemfile:

gem 'hokusai'

And then execute:

$ bundle

Or install it yourself as:

$ gem install hokusai


Include Hokusai::Container in an ActiveRecord model so that we can save templates in it. The columns hokusai_class and hokusai_template are required. You can add whatever data or metadata you wish, from a simple name to complex lifecycle or ownership attributes. In this example we just require a name:

class Template < ApplicationRecord
  include Hokusai::Container
  validates :name, presence: true
  before_validation :ensure_name

  def ensure_name = "New #{hokusai_class.constantize.model_name.singular} template"

Now create and run a suitable migration:

class CreateTemplates < ActiveRecord::Migration
  def change
    create_table :templates do |t|
      t.string :name, null: false
      t.string :hokusai_class, null: false
      t.text :hokusai_template, null: false

Then include Hokusai::Templatable in the models you want to make templates from. Declare a list of columns to persist, and which associations to include:

class Device < ApplicationRecord
  include Hokusai::Templatable

  template :name, :model, :location, :year, include: [:interfaces]
  has_many :interfaces

class Interface < ApplicationRecord
  include Hokusai::Templatable
  belongs_to :device

  template :name, :address, :enabled

You can then create a new template, and stamp out a copy:

device = Device.create!(name: "router-1", model: "CX-6790", location: "SFO", year: 2017)
  {name: "de0", address: "", enabled: false},
  {name: "lo0", address: "", enabled: true}

template = Template.create!(origin: device, name: 'SFO router template')

new_device = template.stamp do |device| = "router-2"

This example ends with a deep, domain-specific clone of the origin object. You can delete the origin and the template is still useful.

What can Hokusai stamp out?

Broadly speaking, this is intended for any ActiveRecord object that can be serialized to & from YAML.

The Hokusai::Templatable concern is provided. This handles ordinary has_many, has_one, and belongs_to associations via the include: option, in which case it will call as_template on the associated records. If you want a belongs_to reference id to carry across, serialize the _id column rather than including the association in the template.

Recursive serialization is not currently detected and will cause a "stack level too deep" error. For those you may need to implement as_template by hand.

Aggregates types may also need special treatment. You can override read_attribute_for_template in the model for these.

Advanced configuration

You're not constrained to using Hokusai::Templatable. Any model that implements #as_template and ::from_template will do, and the container doesn't impose constraints on their behaviour. The only expectation is that #as_template returns a data structure ready for serialization as YAML, and that ::from_template accepts the same structure as the first parameter. Beyond that you can do as you please with the data.


  • Generator for the container migration.
  • Support for serializing self-referential data structures.
  • Clearer tests.
  • Comprehensive tests for a wide range of complicated associations.
  • Remove dependency on ActiveRecord.
  • Support configurable column names and template assignment method name.


After checking out the repo, run bin/setup to install dependencies. Then, run rake test to run the tests. You can also run bin/console for an interactive prompt that will allow you to experiment.

To install this gem onto your local machine, run bundle exec rake install.


Bug reports and pull requests are welcome on GitHub at


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

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