Active Model Datastore

Makes the google-cloud-datastore gem compliant with active_model conventions and compatible with your Rails 5+ applications.

Why would you want to use Google's NoSQL Firestore in Datastore mode with Rails?

Use it when you want a Rails app backed by a fully managed, massively scalable NoSQL database, without provisioning database servers or manually sharding data. Datastore stores records as entities with flexible properties, so your models do not require a fixed database schema. It automatically handles scaling and replication, provides highly available and durable storage, and supports indexed queries and ACID transactions.

Setup

Generate your Rails app without ActiveRecord:

rails new my_app -O

You can remove the db/ directory as it won't be needed.

To install, add this line to your Gemfile and run bundle install:

gem 'activemodel-datastore'

Create a Google Cloud account here and create a project.

Follow the activation instructions to create a Firestore in Datastore mode database for the project.

The Google Cloud client libraries use Application Default Credentials (ADC). On Google Cloud, credentials are discovered automatically from the service account attached to the application. Grant that service account only the access it needs; roles/datastore.user provides read/write access to Datastore data.

Set your project id in an ENV variable named GCLOUD_PROJECT.

To locate your project ID:

Go to the Cloud Platform Console.
From the projects list, select the name of your project.
On the left, click Dashboard. The project name and ID are displayed in the Dashboard.

For applications outside Google Cloud, configure ADC for the hosting environment. If a service account key is required, the Ruby client supports GOOGLE_APPLICATION_CREDENTIALS with the path to its JSON file. Active Model Datastore also supports the following environment variables for platforms where the JSON must be stored directly in environment variables:

export SERVICE_ACCOUNT_PRIVATE_KEY='-----BEGIN PRIVATE KEY-----\nMIIFfb3...5dmFtABy\n-----END PRIVATE KEY-----\n'
export SERVICE_ACCOUNT_CLIENT_EMAIL='web-app@app-name.iam.gserviceaccount.com'

On Heroku the environment variables can be set under Settings > Config Vars.

Active Model Datastore will then handle the authentication for you, and the datastore instance can be accessed with CloudDatastore.dataset.

Datastore retries use Rails.logger automatically when Rails is available, which preserves tagged logging such as request IDs. Other applications fall back to standard output. To use another logger, configure it during application initialization:

ActiveModel::Datastore.logger = MyApplication.logger

There is an example Puma config file here.

Model Example

Let's start by implementing the model:

class User
  include ActiveModel::Datastore

  attr_accessor :email, :enabled, :name, :role, :state

  def entity_properties
    %w[email enabled name role]
  end
end

Data objects in Datastore are known as entities. Entities are of a kind. An entity has one or more named properties, each of which can have one or more values. Think of them like this:

'Kind' (which is your table and the name of your Rails model)
'Entity' (which is the record from the table)
'Property' (which is the attribute of the record)

The entity_properties method defines an Array of properties that belong to the entity in Datastore. Define the attributes of your model using attr_accessor. With this approach, Rails deals solely with ActiveModel objects. The objects are converted to/from entities automatically during save/query operations. You can still use virtual attributes on the model (such as the :state attribute above) by simply excluding it from entity_properties. In this example state is available to the model but won't be persisted with the entity in Datastore.

Validations work as you would expect:

class User
  include ActiveModel::Datastore

  attr_accessor :email, :enabled, :name, :role, :state

  validates :email, format: { with: /\A([^@\s]+)@((?:[-a-z0-9]+\.)+[a-z]{2,})\z/i }
  validates :name, presence: true, length: { maximum: 30 }

  def entity_properties
    %w[email enabled name role]
  end
end

Callbacks work as you would expect. We have also added the ability to set default values through default_property_value and type cast the format of values through format_property_value:

class User
  include ActiveModel::Datastore

  attr_accessor :email, :enabled, :name, :role, :state

  before_validation :set_default_values
  after_validation :format_values
  
  before_save { puts '** something can happen before save **'}
  after_save { puts '** something can happen after save **'}

  validates :email, format: { with: /\A([^@\s]+)@((?:[-a-z0-9]+\.)+[a-z]{2,})\z/i }
  validates :name, presence: true, length: { maximum: 30 }
  validates :role, presence: true

  def entity_properties
    %w[email enabled name role]
  end

  def set_default_values
    default_property_value :enabled, true
    default_property_value :role, 1
  end

  def format_values
    format_property_value :role, :integer
  end
end

Controller Example

Now on to the controller! A scaffold generated controller works out of the box:

class UsersController < ApplicationController
  before_action :set_user, only: [:show, :edit, :update, :destroy]

  def index
    @users = User.all
  end

  def show
  end

  def new
    @user = User.new
  end

  def edit
  end

  def create
    @user = User.new(user_params)
    respond_to do |format|
      if @user.save
        format.html { redirect_to @user, notice: 'User was successfully created.' }
      else
        format.html { render :new }
      end
    end
  end

  def update
    respond_to do |format|
      if @user.update(user_params)
        format.html { redirect_to @user, notice: 'User was successfully updated.' }
      else
        format.html { render :edit }
      end
    end
  end

  def destroy
    @user.destroy
    respond_to do |format|
      format.html { redirect_to users_url, notice: 'User was successfully destroyed.' }
    end
  end

  private

  def set_user
    @user = User.find(params[:id])
  end

  def user_params
    params.require(:user).permit(:email, :name)
  end
end

Retrieving Entities

Each entity in Datastore has a key that uniquely identifies it. The key consists of the following components:

the kind of the entity, which is User in these examples
an identifier for the individual entity, which can be either a a key name string or an integer numeric ID
an optional ancestor path locating the entity within the Datastore hierarchy

all(options = {})

Queries entities using the provided options. When a limit option is provided queries up to the limit and returns results with a cursor.

users = User.all(options = {})

parent_key = CloudDatastore.dataset.key('Parent', 12345)
users = User.all(ancestor: parent_key)

users = User.all(ancestor: parent_key, where: ['name', '=', 'Bryce'])

users = User.all(where: [['name', '=', 'Ian'], ['enabled', '=', true]])

users, cursor = User.all(limit: 7)

# @param [Hash] options The options to construct the query with.
#
# @option options [Google::Cloud::Datastore::Key] :ancestor Filter for inherited results.
# @option options [String] :cursor Sets the cursor to start the results at.
# @option options [Integer] :limit Sets a limit to the number of results to be returned.
# @option options [String] :order Sort the results by property name.
# @option options [String] :desc_order Sort the results by descending property name.
# @option options [Array] :select Retrieve only select properties from the matched entities.
# @option options [Array] :distinct_on Group results by a list of properties.
# @option options [Array] :where Adds a property filter of arrays in the format[name, operator, value].

find(*ids, parent: nil)

Find entity by id - this can either be a specific id (1), a list of ids (1, 5, 6), or an array of ids ([5, 6, 10]). The parent key is optional. This method is a lookup by key and results will be strongly consistent.

user = User.find(1)

parent_key = CloudDatastore.dataset.key('Parent', 12345)
user = User.find(1, parent: parent_key)

users = User.find(1, 2, 3)

find_by(args)

Queries for the first entity matching the specified condition.

user = User.find_by(name: 'Joe')

user = User.find_by(name: 'Bryce', ancestor: parent_key)

Google documents how Datastore queries work, including their restrictions.

Datastore Consistency and Concurrency

Firestore in Datastore mode is strongly consistent by default. Queries and key lookups reflect completed writes, including queries that do not use an ancestor. The legacy Cloud Datastore behavior where a newly created entity might not immediately appear in a global query does not apply.

Datastore supports three concurrency modes that determine how concurrent transactions interact:

PESSIMISTIC uses reader/writer locks and is the default for new databases.
OPTIMISTIC allows concurrent transactions, but only the first conflicting transaction to commit succeeds.
OPTIMISTIC_WITH_ENTITY_GROUPS preserves legacy Cloud Datastore entity-group transaction semantics. Transactions are limited to 25 entity groups, writes to an entity group are limited to one per second, and queries within transactions must be ancestor queries.

Use the following command to inspect a database's concurrency mode:

gcloud firestore databases describe --project=PROJECT_ID --database=DATABASE_ID

Entity groups are hierarchies formed by a root entity and its children. They are useful for modeling related entities and are required by the transactional restrictions of OPTIMISTIC_WITH_ENTITY_GROUPS. To create an entity group, specify an ancestor path as part of the child entity's key.

Before using the save method, assign the parent_key_id attribute an ID. Let's say that 12345 represents the ID of the company that the users belong to. The key of the user entity will now look like this:

@key=#<Google::Cloud::Datastore::Key @kind="User", @id=1, @parent=#<Google::Cloud::Datastore::Key @kind="ParentUser", @id=12345>>

All of the User entities will now belong to an entity group named ParentUser and can be queried by the Company ID. When we query for the users we will provide User.parent_key(12345) as the ancestor option. The entity group relationship cannot be changed after creating the entity because an entity's key cannot be modified after it has been saved.

The Users controller would now look like this:

class UsersController < ApplicationController
  before_action :set_user, only: [:show, :edit, :update, :destroy]

  def index
    @users = User.all(ancestor: User.parent_key(12345))
  end

  def show
  end

  def new
    @user = User.new
  end

  def edit
  end

  def create
    @user = User.new(user_params)
    @user.parent_key_id = 12345
    respond_to do |format|
      if @user.save
        format.html { redirect_to @user, notice: 'User was successfully created.' }
      else
        format.html { render :new }
      end
    end
  end

  def update
    respond_to do |format|
      if @user.update(user_params)
        format.html { redirect_to @user, notice: 'User was successfully updated.' }
      else
        format.html { render :edit }
      end
    end
  end

  def destroy
    @user.destroy
    respond_to do |format|
      format.html { redirect_to users_url, notice: 'User was successfully destroyed.' }
    end
  end

  private

  def set_user
    @user = User.find(params[:id], parent: User.parent_key(12345))
  end

  def user_params
    params.require(:user).permit(:email, :name)
  end
end

See the Datastore documentation for details about transactions, isolation, and concurrency modes.

Datastore Indexes

Every Datastore query computes its results using one or more indexes. The indexes contain entity keys in a sequence specified by the index's properties and, optionally, the entity's ancestors.

There are two types of indexes, built-in and composite.

Built-in

By default, Datastore automatically predefines an index for each property of each entity kind. These single property indexes are suitable for simple types of queries.

Composite

Composite indexes include multiple property values per indexed entity. Composite indexes support complex queries and are defined in an index.yaml file.

Composite indexes are required for queries of the following form:

queries with ancestor and inequality filters
queries with one or more inequality filters on a property and one or more equality filters on other properties
queries with a sort order on keys in descending order
queries with multiple sort orders
queries with one or more filters and one or more sort orders

NOTE: Inequality filters are LESS_THAN, LESS_THAN_OR_EQUAL, GREATER_THAN, GREATER_THAN_OR_EQUAL.

See Google's Datastore index documentation for more information.

Firestore Emulator

Install the Google Cloud CLI and the Firestore emulator:

gcloud components install cloud-firestore-emulator

The emulator requires Java 21 or later. Add the emulator executable to your shell's PATH because the test helpers invoke it directly:

export PATH="$HOME/google-cloud-sdk/platform/cloud-firestore-emulator:$PATH"

The Firestore emulator runs in memory by default, so there is no datastore directory to create. Start it in Datastore mode on the development port with:

cloud_firestore_emulator start --database-mode=datastore-mode --port=8180

Set DATASTORE_EMULATOR_HOST=localhost:8180 so the Ruby client connects to the emulator. The gem sets this automatically for Rails development and uses port 8181 for Rails tests.

Example Rails App

There is an example Rails 8.1 app in the test directory here.

Start the Firestore emulator in Datastore mode in one terminal:

cloud_firestore_emulator start --database-mode=datastore-mode --port=8180

Then start the example application in another terminal:

bundle install
rails server

Navigate to http://localhost:3000.

CarrierWave File Uploads

Active Model Datastore has built in support for CarrierWave which is a simple and extremely flexible way to upload files from Rails applications. You can use different stores, including filesystem and cloud storage such as Google Cloud Storage or AWS.

Simply require active_model/datastore/carrier_wave_uploader and extend your model with the CarrierWaveUploader (after including ActiveModel::Datastore). Follow the CarrierWave instructions for generating an uploader.

In this example it will be something like:

rails generate uploader ProfileImage

Define an attribute on the model for your file(s). You can then mount the uploaders using mount_uploader (single file) or mount_uploaders (array of files). Don't forget to add the new attribute to entity_properties and whitelist the attribute in the controller if using strong parameters.

require 'active_model/datastore/carrier_wave_uploader'

class User
  include ActiveModel::Datastore
  extend CarrierWaveUploader

  attr_accessor :email, :enabled, :name, :profile_image, :role
  
  mount_uploader :profile_image, ProfileImageUploader

  def entity_properties
    %w[email enabled name profile_image role]
  end
end

You will want to add something like this to your Rails form:

<%= form.file_field :profile_image %>

Track Changes

TODO: document the change tracking implementation.

Nested Forms

Adds support for nested attributes to ActiveModel. Heavily inspired by Rails ActiveRecord::NestedAttributes.

Nested attributes allow you to save attributes on associated records along with the parent. It's used in conjunction with fields_for to build the nested form elements.

See Rails ActionView::Helpers::FormHelper::fields_for for more info.

NOTE: Unlike ActiveRecord, the way that the relationship is modeled between the parent and child is not enforced. With NoSQL the relationship could be defined by any attribute, or with denormalization exist within the same entity. This library provides a way for the objects to be associated yet saved to the datastore in any way that you choose.

You enable nested attributes by defining an :attr_accessor on the parent with the pluralized name of the child model.

Nesting also requires that a <association_name>_attributes= writer method is defined in your parent model. If an object with an association is instantiated with a params hash, and that hash has a key for the association, Rails will call the <association_name>_attributes= method on that object. Within the writer method call assign_nested_attributes, passing in the association name and attributes.

Let's say we have a parent Recipe with Ingredient children.

Start by defining within the Recipe model:

an attr_accessor of :ingredients
a writer method named ingredients_attributes=
the validates_associated method can be used to validate the nested objects

Example:

class Recipe
  attr_accessor :ingredients
  validates :ingredients, presence: true
  validates_associated :ingredients

  def ingredients_attributes=(attributes)
    assign_nested_attributes(:ingredients, attributes)
  end
end

You may also set a :reject_if proc to silently ignore any new record hashes if they fail to pass your criteria. For example:

class Recipe
 def ingredients_attributes=(attributes)
   reject_proc = proc { |attributes| attributes['name'].blank? }
   assign_nested_attributes(:ingredients, attributes, reject_if: reject_proc)
 end
end

Alternatively, :reject_if also accepts a symbol naming a method:

class Recipe
  def ingredients_attributes=(attributes)
    assign_nested_attributes(:ingredients, attributes, reject_if: :reject_recipes)
  end

  def reject_recipes(attributes)
    attributes['name'].blank?
  end
end

Within the parent model valid? will validate the parent and associated children and nested_models will return the child objects. If the nested form submitted params contained a truthy _destroy key, the appropriate nested models will have marked_for_destruction set to true.

Datastore Gotchas

Ordering of query results is undefined when no sort order is specified.

When a query does not specify a sort order, the results are returned in the order they are retrieved. As the Datastore implementation evolves (or if a project's indexes change), this order may change. Therefore, if your application requires its query results in a particular order, be sure to specify that sort order explicitly in the query.

Name		Name	Last commit message	Last commit date
.github/workflows		.github/workflows
lib		lib
test		test
tmp		tmp
.gitignore		.gitignore
.rubocop.yml		.rubocop.yml
.ruby-gemset		.ruby-gemset
.ruby-version		.ruby-version
CHANGELOG.md		CHANGELOG.md
Gemfile		Gemfile
LICENSE.txt		LICENSE.txt
README.md		README.md
RELEASING.md		RELEASING.md
Rakefile		Rakefile
activemodel-datastore.gemspec		activemodel-datastore.gemspec

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Active Model Datastore

Table of contents

Setup

Model Example

Controller Example

Retrieving Entities

all(options = {})

find(*ids, parent: nil)

find_by(args)

Datastore Consistency and Concurrency

Datastore Indexes

Built-in

Composite

Firestore Emulator

Example Rails App

CarrierWave File Uploads

Track Changes

Nested Forms

Datastore Gotchas

Ordering of query results is undefined when no sort order is specified.

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Active Model Datastore

Table of contents

Setup

Model Example

Controller Example

Retrieving Entities

all(options = {})

find(*ids, parent: nil)

find_by(args)

Datastore Consistency and Concurrency

Datastore Indexes

Built-in

Composite

Firestore Emulator

Example Rails App

CarrierWave File Uploads

Track Changes

Nested Forms

Datastore Gotchas

Ordering of query results is undefined when no sort order is specified.

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