Icelab

A conversational introduction to rom-rb

By Tim Riley03 Jun 2016

We’ve spent a few weeks now exploring a next-generation web app architecture for Ruby, but we’re yet to cover one very important thing: database persistence.

How we choose to build our app’s persistence support will have a profound impact on its overall design.

We could follow the active record pattern and wind up with a select few models deeply intertwined with every aspect of our application’s functionality.

Or we could build a clean separation of responsibilities and make an app that remains easy to change. This is where rom-rb can help.

rom-rb is a flexible persistence and data mapping toolkit for Ruby. It separates queries and commands and offers powerful, workable abstractions at every point between our app and the data store. This mean there are a few concepts to learn, but the payoff is significant: a persistence layer that can be perfectly tailored to our app’s needs.

Let’s get started, then: follow the code and commentary below for an introduction to rom-rb and a playground for making your own experiments.

Let’s get started

This article is a bit of an experiment. Everything from here is written entirely in literate-style Ruby. For the best reading experience, read the code and conversation side-by-side. Otherwise, you can still follow along below.

# ## Install the rom-rb gems

# Over in the `Gemfile`, we're installing the rom-rb gems from their master
# branches, since we'll be looking at some major improvements that are slated
# for release later this month.
require "bundler/setup"

# Require the gems we need.
#
# One thing to note here is that rom-rb is a _multi-adapter_ toolkit. It
# supports many kinds of data sources (and even allows you to use them
# together!). For today, though we'll stick to SQL. Apart from the "rom-sql"
# adapter, the only other `require` we need is "rom-repository", which will
# act as our app's primary persistence interface.
require "rom-sql"
require "rom-repository"

# ## Define our app's persistence API
#
# In this playground, we'll be using rom-rb to work with _articles_ in our
# database.

# ### Relations
#
# The first thing we need to define is a **Relation**. Relations are the
# interface to a particular collection in our data source, which in SQL terms
# is either a table or a view.
module Relations
  # This relation is for the `articles` table.
  class Articles < ROM::Relation[:sql]
    # Define a canonical schema for this relation. This will be used when we
    # use commands to make changes to our data. It ensures that only
    # appropriate attributes are written through to the database table.
    schema(:articles) do
      attribute :id, Types::Serial
      attribute :title, Types::String
      attribute :published, Types::Bool
    end

    # Define some composable, reusable query methods to return filtered
    # results from our database table. We'll use them in a moment.
    def by_id(id)
      where(id: id)
    end

    def published
      where(published: true)
    end
  end
end

# ### Repositories
#
# Now, let's define a **Repository**. Repositories are the primary persistence
# interfaces in our app. Responsitories contribute a couple of important things
# to a well-designed app:
#
# 1. They hide low-level persistence details, ensuring the rest of our app
#    doesn't have any accidental or unnecessary coupling to the implementation
#    details for our data source.
# 2. They return objects that are appropriate for our app's domain. The data
#    for these objects may come from one or more relations, may be transformed
#    into a different shape, and may be returned as objects that are designed
#    to be passed around the other components in our app.
module Repositories
  # This simple repository uses articles as its main relation.
  class Articles < ROM::Repository[:articles]
    # Define a command to create new articles.
    commands :create

    # Define methods to return the article objects we want to use within our
    # app. Each of these can access the relation via `articles` and use its
    # query methods.
    #
    # Unlike the query methods inside the relations, these ones should not be
    # chainable. Their purpose is to return a set of articles for each
    # distinct use case within our app. This means that our repository API
    # (and therefore our persistence API in general) is a perfect reflection
    # of our app's persistence requirements.
    def [](id)
      articles.by_id(id).one!
    end

    def published
      articles.published.to_a
    end
  end
end

# ## Initialize rom-rb
#
# rom-rb is built to be non-intrusive. When we initialize it here, all our
# relations and commands are bundled into a single container that we can
# inject into our app.
#
# In any kind of framework, this setup will be taken care of for us, but
# because rom-rb is flexible, explicit setup for our playground is still nice
# and easy.

# Configure rom-rb to use an in-memory SQLite database via its SQL adapter,
# register our articls relation, then build and finalize the persistence
# container.
config = ROM::Configuration.new(:sql, "sqlite::memory")
config.register_relation Relations::Articles
container = ROM.container(config)

# ## Prepare our database
#
# Since this is a standalone playground, run a migration to give us a database
# table to work with.
container.gateways[:default].tap do |gateway|
  migration = gateway.migration do
    change do
      create_table :articles do
        primary_key :id
        string :title, null: false
        boolean :published, null: false, default: false
      end
    end
  end
  migration.apply gateway.connection, :up
end

# ## Let's play!
#
# Alright, everything's in place. Let's try some things!

# First, get a repo to use.
repo = Repositories::Articles.new(container)

# Let's see if we have any published articles
#
# Nope, nothing. We've just created this table, after all!
#
# ```ruby
# repo.published
# # => []
# ```
puts "Published articles?"
puts repo.published.inspect

# It's time to create an article then. We can do this via the repo too. rom-rb
# will give it back to us in a convenient struct for accessing the attributes
# of the new article.
#
# ```ruby
# repo.create(title: "Hello rom-rb")
# # => #<ROM::Struct[Article] id=1 title="Hello rom-rb" published=false>
# ```
first_article = repo.create(title: "Hello rom-rb")
puts "\nCreate an article:"
puts first_article.inspect

# Can we fetch this article in a list? Not yet, because it's not published.
#
# ```ruby
# repo.published
# # => []
# ```
puts "\nPublished articles?"
puts repo.published.inspect

# But we can find it by ID, because we've build our our repo's `#[]` method to
# finds article regardless of published status.
#
# ```ruby
# repo[first_article.id]
# # => #<ROM::Struct[Article] id=1 title="Hello rom-rb" published=false>
# ```
puts "\nFind first article by ID"
puts repo[first_article.id].inspect

# Let's create a published article now.
#
# ```ruby
# repo.create(title: "Hello world", published: true)
# # => #<ROM::Struct[Article] id=2 title="An alien or sutin" published=true>
# ```
published_article = repo.create(title: "An alien or sutin", published: true)
puts "\nCreate a published article:"
puts published_article.inspect

# This article should be in our list now!
#
# ```ruby
# repo.published
# # => [#<ROM::Struct[Article] id=2 title="An alien or sutin" published=true>]
# ```
puts "\nPublished articles?"
puts repo.published.inspect

Next steps

This is the end of our first little foray into the world of rom-rb. I hope you’ve enjoyed it! Next week we’ll look at a few more advanced features.

Until then, if you can clone this playground from GitHub and experiment:

git clone https://github.com/icelab/conversational-intro-to-rom-rb
cd conversational-intro-to-rom-rb
bundle
./intro.rb

Feel free to extend the playground, make your own changes and see how things work!

And if you want to learn more, check out the helpful documentation on the rom-rb website, and join the gitter chat if you have any questions!


Read part 1 of this series: My past and future Ruby

Part 2: Inactive records: the value objects your app deserves

Part 3: Functional command objects in Ruby

Part 4: Effective Ruby dependency injection at scale

Part 5: Better code with an inversion of control container

Part 6: A change-positive Ruby web application architecture

Part 7: Put HTTP in its place with Roda

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