FrozenRecord

Active Record-like interface for read only access to static data files of reasonable size.

Installation

Add this line to your application's Gemfile:

gem 'frozen_record'

And then execute:

$ bundle

Or install it yourself as:

$ gem install frozen_record

Models definition

Just like with Active Record, your models need to inherits from FrozenRecord::Base:

class Country < FrozenRecord::Base
end

But you also have to specify in which directory your data files are located. You can either do it globally

FrozenRecord::Base.base_path = '/path/to/some/directory'

Or per model:

class Country < FrozenRecord::Base
  self.base_path = '/path/to/some/directory'
end

FrozenRecord has two built-in backends, for JSON and YAML. Backends are classes that know how to load records from a static file.

The default backend is YAML and it expects a file that looks like this:

- id: 'se'
  name: 'Sweden'
  region: 'Europe'
  language: 'Swedish'
  population: 10420000
- id: 'de'
  name: 'Germany'
  region: 'Europe'
  language: 'German'
  population: 83200000

# …

You can also specify a custom backend:

class Country < FrozenRecord::Base
  self.backend = FrozenRecord::Backends::Json
end

Custom backends

A custom backend must implement the methods filename and load as follows:

module MyCustomBackend
  extend self

  def filename(model_name)
    # Returns the file name as a String
  end

  def load(file_path)
    # Reads file and returns records as an Array of Hash objects
  end
end

Query interface

FrozenRecord aim to replicate only modern Active Record querying interface, and only the non "string typed" ones.

# Supported query interfaces
Country.
  where(region: 'Europe').
  where.not(language: 'English').
  where(population: 10_000_000..).
  order(id: :desc).
  limit(10).
  offset(2).
  pluck(:name)

# Non-supported query interfaces
Country.
  where('region = "Europe" AND language != "English"').
  order('id DESC')

Scopes

Basic scope :symbol, lambda syntax is now supported in addition to class method syntax.

class Country
  scope :european, -> { where(continent: 'Europe' ) }

  def self.republics
    where(king: nil)
  end

  def self.part_of_nato
    where(nato: true)
  end
end

Country.european.republics.part_of_nato.order(id: :desc)

Supported query methods

• where
• where.not
• order
• limit
• offset

Supported finder methods

• find
• first
• last
• to_a
• exists?

Supported calculation methods

• count
• pluck
• ids
• minimum
• maximum
• sum
• average

Indexing

Querying is implemented as a simple linear search (O(n)). However if you are using Frozen Record with larger datasets, or are querying a collection repeatedly, you can define indices for faster access.

class Country < FrozenRecord::Base
  add_index :name, unique: true
  add_index :continent
end

Composite index keys are not supported.

The primary key isn't indexed by default.

Rich Types

The attribute method can be used to provide a custom class to convert an attribute to a richer type. The class must implement a load class method that takes the raw attribute value and returns the deserialized value (similar to ActiveRecord serialization).

class ContinentString < String
  class << self
    alias_method :load, :new
  end
end

Size = Struct.new(:length, :width, :depth) do
  def self.load(value) # value is lxwxd eg: "23x12x5"
    new(*value.split('x'))
  end
end

class Country < FrozenRecord::Base
  attribute :continent, ContinentString
  attribute :size, Size
end

Limitations

Frozen Record is not meant to operate on large unindexed datasets.

To ensure that it doesn't happen by accident, you can set FrozenRecord::Base.max_records_scan = 500 (or whatever limit makes sense to you), in your development and test environments. This setting will cause Frozen Record to raise an error if it has to scan more than max_records_scan records. This property can also be set on a per model basis.

Configuration

Reloading

By default the YAML files are parsed once and then cached in memory. But in development you might want changes to be reflected without having to restart your application.

For such cases you can set auto_reloading to true either globally or on a model basis:

FrozenRecord::Base.auto_reloading = true # Activate reloading for all models
Country.auto_reloading # Activate reloading for `Country` only

Testing

Testing your FrozenRecord-backed models with test fixtures is made easier with:

require 'frozen_record/test_helper'

# During test/spec setup
test_fixtures_base_path = 'alternate/fixture/path'
FrozenRecord::TestHelper.load_fixture(Country, test_fixtures_base_path)

# During test/spec teardown
FrozenRecord::TestHelper.unload_fixtures

Here's a Rails-specific example:

require 'test_helper'
require 'frozen_record/test_helper'

class CountryTest < ActiveSupport::TestCase
  setup do
    test_fixtures_base_path = Rails.root.join('test/support/fixtures')
    FrozenRecord::TestHelper.load_fixture(Country, test_fixtures_base_path)
  end

  teardown do
    FrozenRecord::TestHelper.unload_fixtures
  end

  test 'countries have a valid name' do
  # ...

Contributors

FrozenRecord is a from scratch reimplementation of a Shopify project from 2007 named YamlRecord. So thanks to:

• John Duff - @jduff
• Dennis O'Connor - @dennisoconnor
• Christopher Saunders - @csaunders
• Jonathan Rudenberg - @titanous
• Jesse Storimer - @jstorimer
• Cody Fauser - @codyfauser
• Tobias Lütke - @tobi