Neo4j.rb is a graph database for JRuby.

You can think of Neo4j as a high-performance graph engine with all the features of a mature and robust database. The programmer works with an object-oriented, flexible network structure rather than with strict and static tables — yet enjoys all the benefits of a fully transactional, enterprise-strength database. This JRuby gem uses the mature Neo4j Java library.

It has been tested with Neo4j version 1.8.2 and 1.9.M03 (see here) and JRuby 1.7.4 (see Travis).

Notice, you do not need to install the Neo4j server since this gem comes included with the database. However, if you still want to use the Neo4j server (e.g. the admin UI) you can connect the embedded database with the Neo4j server using a Neo4j HA Cluster.

• Github Wiki
• Blog
• YARD
• Specs - There are 2023 RSpecs (478/neo4j-core, 425/neo4j-wrapper and 1120/this gem - 2012 April)
• Docs from Neo Technology

• Neo4j.rb with HA Cluster Screencast
• The Kvitter Rails 3.2x App (kvitter = tweets in Swedish)
• Simple Rails 3.0 App

Major benefits of Neo4j.rb:

• Domain Modeling - use the language of a graph (nodes/relationship/properties) to express your domain!
  • Schema Less and Efficient storage of Semi Structured Information
  • No O/R mismatch - very natural to map a graph to an Object Oriented language like Ruby.
• Performance
• Embedded Database - no database tier, easier to install, test, deploy and configure. It is run in the same process as your application.
• Express Queries as Traversals
  • Fast deep traversal instead of slow SQL queries that span many table joins.
  • Very natural to express graph related problem with traversals (recommendation engine, find shortest parth etc..)
• Seamless integration with Ruby on Rails.
• ACID Transaction with rollbacks support.

The Ruby libraries for Neo4j are divided into 4 separate gems.

• Layer 3: neo4j provides all the niceties you'd expect from a Rails ORM. (You are here..) An implementation of the Rails Active Model and a subset of the Active Record API, see Neo4j::Rails::Model and Neo4j::Rails::Relationship.
• Layer 2: neo4j-wrapper provides Ruby wrappers for objects and query results. A binding API to Ruby objects, see Neo4j::NodeMixin and Neo4j::RelationshipMixin.
• Layer 1: neo4j-core is a JRuby compatibility layer over the standard Java Neo4j API. For interacting with the basic building blocks of the graph database (node, properties and relationship), see Neo4j::Node and Neo4j::Relationship.
• Additionally, neo4j-cypher provides a DSL for the Cypher Query Language. The DSL lets you create Cypher queries in an easy to understand way instead of hand-crafting strings.

Notice that you can always access the lower layers if you want to do something more advanced. You can even access the Java API directly.

The neo4j gem depends on the neo4j-wrapper, neo4j-core, and neo4j-cypher gem. You can use neo4j-wrapper directly if you do not need the Rails functionality.

The documentation for all projects is combined in the neo4j Wiki.

Additionally, the neo4j-community, neo4j-advanced, neo4j-enterprise gems provide the necessary jars to embed the Neo4j server. Due to licensing concerns, only neo4j-community is required by default.

Major components of the Neo4j gem include:

• Neo4j::Rails::Model
• Neo4j::Rails::Relationship
• Neo4j::Rails::Observer
• Neo4j::Rails::HaConsole::Railitie
• Neo4j::Rails::Versioning
• Neo4j::Rails::Compositions::ClassMethods
• Neo4j::Rails::AcceptId

Example of creating an Neo4j Application from scratch:

Make sure you are using JRuby!

gem install rails
rails new myapp -m http://andreasronge.github.com/neo4j/rails.rb -O
cd myapp
bundle
rails generate scaffold User name:string email:string
rails s
open a webbrowser: http://localhost:3000/users

The -O flag above means that it will skip active record. For more information, read the Scaffolds & Generators Wiki.

Example of using Neo4j with Rails 3 (ActiveModel)

class User < Neo4j::Rails::Model
  attr_accessor :password
  attr_accessible :email, :password, :password_confirmation, :pending_account

  after_save   :encrypt_password

  email_regex = /\A[\w+\-.]+@[a-z\d\-.]+\.[a-z]+\z/i

  # add an exact lucene index on the email property
  property :email, index: :exact

  has_one(:avatar).to(Avator)

  validates :email, presence: true, format: { :with => email_regex }
  validates :email, uniqueness: true, unless: :pending_account?
  accepts_nested_attributes_for :avatar, allow_destroy: true

end

u = User.new(name: 'kalle', age: 42, email: "[email protected]")
u.save

The neo4j-wrapper gem provides both the Neo4j::NodeMixin and Neo4j::RelationshipMixin. These can be mixed into normal Ruby classes to provide a persistence mechanism for Neo4j.

Example of mapping a Ruby class to a Node and delaring Properties and Relationships and Lucene index.

class Person
  include Neo4j::NodeMixin
  property :name, index: :exact
  property :city

  has_n :friends
  has_one :address
end

# NOTE: we *must* perform write operations in a transaction!
Neo4j::Transaction.run do
  andreas = Person.new (:name => 'andreas')
  andreas.friends << Person.new (:name => 'peter')
  andreas.friends.each {|person| puts "name #{person.name}" }
  Person.find("name: andreas").first.name # => 'andreas'
end

The neo4j-core gem provides a thin layer around the Java API.

• Neo4j::Node The Java Neo4j Node
• Neo4j::Relationship The Java Relationship
• Neo4j The Database
• Neo4j::Algo Included algorithms, like shortest_path

Example of creating a Neo4j::Node

require 'neo4j-core'

Neo4j::Transaction.run do
  node = Neo4j::Node.new(:name => 'andreas')
  node.outgoing(:friends) << Neo4j::Node.new(:name => 'peter')
  node.outgoing(:friends).each {|node| puts "name #{node[:name]}"}
end

Yes, check Neo4j.rb Ha Cluster or Screencast. Notice, you don't need to install the Neo4j Server, but it could be a useful tool to visualize the graph.

• GitHub
• Issue Tracking
• Twitter
• Mailing list, [email protected]
• #neo4j on IRC

Development configuration

You can configure Neo4j through the Neo4j::Config object.

Neo4j::Config[:storage_path] = "/var/neo4j"

Configuring Neo4j from Rails

When using Neo4j.rb from Rails you can use the normal Rails config/application.rb to set Neo4j configuration.

config.neo4j.storage_path = "#{config.root}/db/neo4j"

• Have you found a bug, need help or have a patch ?
• Just clone neo4j.rb and send me a pull request or email me.
• Do you need help - send me an email (andreas.ronge at gmail dot com).

• Neo4j.rb - MIT, see the LICENSE.
• Lucene - Apache, see the Lucene Documentation.
• Neo4j - Dual free software/commercial license, see Lisencing Guide.

Notice: there are different license for the neo4j-community, neo4j-advanced, and neo4j-enterprise jar gems. Only the neo4j-community gem is by default required.