A Graph Database for JRuby. It uses the java lib neo4j as storage and lucene for quering/indexing
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Neo4j.rb is a graph database for JRuby.

It provides:

  • Mapping of ruby objects to nodes in networks rather than in tables.

  • Dynamic, and schema-free - no need to declare nodes/properties/relationships in advance.

  • Storage of ruby object to a file system.

  • Fast traversal of relationships between nodes in a huge node space.

  • Transaction with rollbacks support.

  • Indexing and querying of ruby objects.

  • Can be used instead of ActiveRecord in Ruby on Rails or Merb

  • Can be accessible as REST resources.

It uses two powerful and mature java libraries:


  • There are over 300 RSpecs.

  • Has been tested with a simple rails application, used Neo4j.rb instead of ActiveRecord

  • Has been load tested (loaded 18000 nodes and done queries/traversal in several threads.)

Project information

Presentation Materials and other URLs


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). Please also check/add issues at lighthouse, neo4j.lighthouseapp.com



This page contains the following information:

  • Installation guide

  • Three Minute Tutorial

  • Ten Minute Tutorial

  • Lucene API Documentation

  • Neo4j API Documentation

  • Extension: REST (see Neo4j::RestMixin)

  • Extension: find_path

  • Ruby on Rails with Neo4j.rb

There are also some complete examples in the example folder

  • admin - an uncomplete admin web gui for the Neo4j.rb/REST interface

  • railway - an example of a railway network application

  • imdb - an example of a neo4j database consisting of movies, role and actors nodes/relationships (over 18000 nodes).

  • rest - an example how to expose neo4j nodes as REST resources

  • Ruby on Rails - see github.com/andreasronge/neo4j-rails-example/tree/master


To install it:

gem install neo4j

To install from the latest source:

git clone git://github.com/andreasronge/neo4j.git
cd neo4j
rake gem:install

This has been verified to work on JRuby 1.3.0

Running all RSpecs

To check that neo4j.rb is working:

cd neo4j # the folder containing the Rakefile
rake     # you may have to type jruby -S rake depending how you installed JRuby

Three Minute Tutorial

Neo node space consists of three basic elements: nodes, relationships that connect nodes and properties attached to both nodes and relationships. All relationships have a type, for example if the node space represents a social network, a relationship type could be KNOWS. If a relationship of the type KNOWS connects two nodes, that probably represents two people that know each other. A lot of the semantics, the meaning, of a node space is encoded in the relationship types of the application.

Creating Nodes

Example of creating a neo4j node:

require "rubygems"
require 'neo4j'

Neo4j::Transaction.run do
  node = Neo4j::Node.new


Almost all Neo4j operation must be wrapped in a transaction as shown above. In the following examples we assume that the operations are inside an Neo4j transaction. Neo4j.rb supports auto commits without wrapping Neo4j operations in Neo4j::Transactions. To enable that require the file 'neo4j/auto_tx' instead of 'neo4j'


require 'neo4j/auto_tx'

# no need to wrap operations in Neo4j::Transaction.run do .. end

This feature should be used for testing and development purpose.

Setting Properties

Example of setting properties

node = Neo4j::Node.new
node[:name] = 'foo'
node[:age]  = 123
node[:hungry] = false
node[4] = 3.14

Getting Properties

Example of getting properties

node[:name] # => 'foo'

Creating Relationships

Example of creating a relationship

node1 = Neo4j::Node.new
node2 = Neo4j::Node.new
node1.relationships.outgoing(:friends) << node2  # connects node1 to node2 with relationship type 'friends'

Accessing Relationships

Example of getting relationships

node1.relationships.empty? # => false

# The relationships method returns an enumeration of relationship objects.
# The nodes method on the relationships returns the nodes instead.
node1.relationships.nodes.include?(node2) # => true

node1.relationships.first # => the first relationship this node1 has which is between node1 and node2 of any type
node1.relationships.nodes.first # => node2  first node of any relationship type
node1.relationships.incoming(:friends).nodes.first # => node1  first node of relationship type 'friends'
node2.relationships.incoming(:friends).first # => a relationship object between node1 and node2

Properties on Relationships

Example of setting properties on relationships

rel = node1.relationships(:friends).first
rel[:since] = 1982
node1.relationships.first[:since] # => 1982 (there is only one relationship defined on node1 in this example)

The Node and NodeMixin

Here is the complete implemenation of the Neo4j::Node class

module Neo4j
  class Node
    include Neo4j::NodeMixin

In the next tutorial you will learn how to use this Neo4j::NodeMixin in your own domain model classes.

Ten Minute Tutorial

Creating a Model

The following example specifies how to map a Neo4j node to a Ruby Person instance.

require "rubygems"
require "neo4j"

class Person
  include Neo4j::NodeMixin

  # define Neo4j properties
  property :name, :salary

  # define an one way relationship to any other node
  has_n :friends

  # adds a lucene index on the following properties
  index :name, :salary
  index 'friends.age' # index each friend age as well

Neo properties and relationships are declared using the 'property' and 'has_n'/'has_one' NodeMixin class method. Adding new types of properties and relationships can also be done without declaring those propertie/relationships by using the operator '[]' on Neo4j::NodeMixin and the

'<<' on the Neo4j::Relationships::RelationshipTraverser.

By using the NodeMixin and by declaring properties and indices, all instances of the Person class can now be stored in the Neo4j node space and be retrieved/queried by traversing the node space or performing Lucene queries.

A lucene index will be updated when the name or salary property changes. The salary of all friends are also indexed which means we can query for people who has friends with a certain salary.

Creating a node

Creating a Person node instance

person = Person.new

Setting properties

Setting a property:

person.name = 'kalle'
person.salary  = 10000

If a transaction is not specified then the operation will automatically be wrapped in a transaction.

Dynamic Properties

Notice that it is not required to specify which attributes should be available on a node. Any attributes can be set using the [] operator. Declared properties set an expectation, not an requirement. It can be used for documenting your model objects and catching typos.


person['an_undefined_property'] = 'hello'

So, why declare properties in the class at all? By declaring a property in the class, you get the sexy dot notation. But also, if you declare a Lucene index on the declared property and update the value, then the Lucene index will automatically be updated. The property declaration is required before declaring an index on the property.

Dynamic Relationships

Like dynamic properties, relationships do not have to be defined using has_n or has_one for a class. A relationship can be added at any time on any node.


person.relationships.outgoing(:best_friends) << other_node
person.relationship(:best_friend).end_node # => other_node (if there is only one relationship of type 'best_friend' on person)

Finding Nodes and Queries

There are three ways of finding/quering nodes in neo4j:

1. by traversing the graph
2. by using lucene queries
3. using the unique neo4j id (Neo4j::NodeMixin#neo_node_id).

When doing a traversal one start from a node and traverse one or more relationships (one or more levels deep). This start node can be either the reference node which is always found (Neo4j.ref_node) or by finding a start node from a lucene query.

Lucene Queries

There are different ways to write lucene queries. Using a hash:

Person.find (:name => 'kalle', :age => 20..30)  # find people with name kalle and age between 20 and 30

or using the lucene query language:

Person.find("name:kalle AND salary:[10000 TO 30000]")

The Lucene query language supports wildcard, grouping, boolean, fuzzy queries, etc… For more information see: lucene.apache.org/java/2_4_0/queryparsersyntax.html

Sorting, example

Person.find(:name => 'kalle').sort_by(:salary)
Person.find(:name => 'kalle').sort_by(Desc[:salary], Asc[:country])
Person.find(:name => 'kalle').sort_by(Desc[:salary, :country])

Search Results

The query is not performed until the search result is requested. Example of using the search result.

res = Person.find(:name => 'kalle')
res.size  # => 10
res.each {|x| puts x.name}
res[0].name = 'sune'

Creating a Relationships

Adding a relationship between two nodes:

person2 = Person.new
person.friends << person2

The person.friends returns an object that has a number of useful methods (it also includes the Enumerable mixin). Example

person.friends.empty? # => false
person.friends.first  # => person2
person.friends.include?(person2) # => true

Deleting a Relationship

To delete the relationship between person and person2:


If a node is deleted then all its relationship will also be deleted Deleting a node is performed by using the delete method:


Node Traversals

The has_one and has_many methods create a convenient method for traversals and managing relationships to other nodes. Example:

Person.has_n :friends # generates the friends instance method
# all instances of Person now has a friends method so that we can do the following
person.friends.each {|n| ...  }

Traversing using a filter

person.friends{ salary == 10000 }.each {|n| ...}

Traversing with a specific depth (depth 1 is default)

person.friends{ salary == 10000}.depth(3).each { ... }

There is also a more powerful method for traversing several relationships at the same time - Neo4j::NodeMixin#traverse, see below.

Example on Relationships

In the first example the friends relationship can have relationships to any other node of any class. In the next example we specify that the 'acted_in' relationship should use the Ruby classes Actor, Role and Movie. This is done by using the has_n class method:

class Role
  include Neo4j::RelationshipMixin
  # notice that neo4j relationships can also have properties
  property :name

class Actor
  include Neo4j::NodeMixin

  # The following line defines the acted_in relationship
  # using the following classes:
  # Actor[Node] --(Role[Relationship])--> Movie[Node]

class Movie
  include Neo4j::NodeMixin
  property :title
  property :year

  # defines a method for traversing incoming acted_in relationships from Actor
  has_n(:actors).from(Actor, :acted_in)

Creating a new Actor-Role-Movie relationship can be done like this:

keanu_reeves = Actor.new
matrix       = Movie.new
keanu_reeves.acted_in << matrix

or you can also specify this relationship on the incoming node (since we provided that information in the has_n methods).

keanu_reeves = Actor.new
matrix       = Movie.new
matrix.actors << keanu_reeves

Example of accessing the Role relationship object between an Actor and a Movie

keanu_reeves.relationships.outgoing(:acted_in)[matrix].name = 'neo'

More information about neo4j can be found after the Lucene section below.

The Lucene Module

You can use this module without using the Neo4j module.

Lucene provides:

  • Flexible Queries - Phrases, Wildcards, Compound boolean expressions etc…

  • Field-specific Queries eg. title, artist, album

  • Sorting

  • Ranked Searching

Lucene Document

In lucene everything is a Document. A document can represent anything textual: Word Document, DVD (the textual metadata only), or a Neo4j.rb node. A document is like a record or row in a relationship database.

The following example shows how a document can be created by using the ''<<'' operator on the Lucene::Index class and found using the Lucene::Index#find method.

Example of how to write a document and find it:

require 'lucene'

include Lucene

# the var/myindex parameter is either a path where to store the index or
# just a key if index is kept in memory (see below)
index = Index.new('var/myindex')  

# add one document (a document is like a record or row in a relationship database)
index << {:id=>'1', :name=>'foo'}

# write to the index file

# find a document with name foo
# hits is a ruby Enumeration of documents
hits = index.find{name == 'foo'}

# show the id of the first document (document 0) found
# (the document contains all stored fields - see below)
hits[0][:id]   # => '1'

Notice that you have to call the commit method in order to update the index on the disk/RAM. By performing several update and delete operations before a commit will be much faster then performing commit after each operation.

Keep indexing on disk

By default Neo4j::Lucene keeps indexes in memory. That means that when the application restarts the index will be gone and you have to reindex everything again.

To keep indexes in memory:

Lucene::Config[:store_on_file] = true
Lucene::Config[:storage_path] => '/home/neo/lucene-db'

When creating a new index the location of the index will be the Lucene::Config + index path Example:

Lucene::Config[:store_on_file] = true
Lucene::Config[:storage_path] => '/home/neo/lucene-db'
index = Index.new('/foo/lucene')

The example above will store the index at /home/neo/lucene-db/foo/lucene

Indexing several values with the same key

Let say a person can have several phone numbers. How do we index that ?

index << {:id=>'1', :name=>'adam', :phone => ['987-654', '1234-5678']}

Id field

All Documents must have one id field. If one is not specified it is :id of type String. A different id can be specified using the field_infos id_field property on the index:

index = Index.new('some/path/to/the/index')
index.field_infos.id_field = :my_id

To change the type of the my_id from String to a different type see below.

Conversion of types

Lucene.rb can handle type conversion for you. (The java lucene library stores all the fields as Strings) For example if you want the id field to be a fixnum

require 'lucene'
include Lucene

index = Index.new('var/myindex')  # store the index at dir: var/myindex
index.field_infos[:id][:type] = Fixnum    

index << {:id=>1, :name=>'foo'} # notice 1 is not a string now


# find that document, hits is a ruby Enumeration of documents
hits = index.find(:name => 'foo') 

# show the id of the first document (document 0) found
# (the document contains all stored fields - see below)
doc[0][:id]   # => 1

If the field_info type parameter is not set then it has a default value of String.

Storage of fields

By default only the id field will be stored. That means that in the example above the :name field will not be included in the document.


doc = index.find('name' => 'foo') 
doc[:id]   # => 1     
doc[:name] # => nil

Use the field info :store=true if you want a field to be stored in the index (otherwise it will only be searchable).


require 'lucene'
include Lucene

index = Index.new('var/myindex')  # store the index at dir: var/myindex
index.field_infos[:id][:type] = Fixnum    
index.field_infos[:name][:store] = true # store this field

index << {:id=>1, :name=>'foo'} # notice 1 is not a string now


# find that document, hits is a ruby Enumeration of documents
hits = index.find('name' => 'foo') 

# let say hits only contains one document so we can use doc[0] for that one
# that document contains all stored fields (see below)
doc[0][:id]   # => 1     
doc[0][:name] # => 'foo'

Setting field infos

As shown above you can set field infos like this

index.field_infos[:id][:type] = Fixnum

Or you can set several properties like this:

index.field_infos[:id] = {:type => Fixnum, :store => true}

Simple Queries

Lucene.rb support search in several fields: Example

# finds all document having both name 'foo' and age 42
hits = index.find('name' => 'foo', :age=>42)

Range queries

# finds all document having both name 'foo' and age between 3 and 30
hits = index.find('name' => 'foo', :age=>3..30)

Lucene Queries

If the query is string then the string is a lucene query.

hits = index.find('name:foo')

For more information see: lucene.apache.org/java/2_4_0/queryparsersyntax.html

Advanced Queries (DSL)

The queries above can also be written in a lucene.rb DSL:

hits = index.find { (name == 'andreas') & (foo == 'bar')}

Expression with OR (|) is supported, example

# find all documents with name 'andreas' or age between 30 and 40
 hits = index.find { (name == 'andreas') | (age == 30..40)}


Sorting is specified by the 'sort_by' parameter Example

hits = index.find(:name => 'foo', :sort_by=>:category)

To sort by several fields:

hits = index.find(:name => 'foo', :sort_by=>[:category, :country])

Example sort order

hits = index.find(:name => 'foo', :sort_by=>[Desc[:category, :country], Asc[:city]])


The Lucene::Index is thread safe. It guarantees that an index is not updated from two thread at the same time.

Lucene Transactions

Use the Lucene::Transaction in order to do atomic commits. By using a transaction you do not need to call the Index.commit method.


Transaction.run do |t|
  index = Index.new('var/index/foo')        
  index << { id=>42, :name=>'andreas'}
  t.failure  # rollback

result = index.find('name' => 'andreas')
result.size.should == 0

You can find which documents are uncommited by using the uncommited index property.


index = Index.new('var/index/foo')        
index.uncommited #=> [document1, document2]

Notice that even if it looks like a new Index instance object was created the index.uncommited may return an not empty array. This is because Index.new is a singleton - a new instance object is not created.

The Neo4j Module

The Neo4j module is used to map Ruby objects to nodes and relationships in a network. It supports two different ways of retrieval/quering:

  • Neo4j traversal, Neo4j::NodeMixin#traverse (or Neo4j::NodeMixin#has_n)

  • Lucene indexes, Neo4j::NodeMixin#find

Start and Stop of the Neo4j

Unlike the Java Neo4j implementation it is not neccessarly to start Neo4j. It will automatically be started when needed. It also uses a hook to automatically shutdown Neo4j. Shutdown of neo4j can also be done using the stop method, example:


Neo4j Configuration

Before using Neo4j the location where the database is stored on disk should be configured. The neo4j configuration is kept in the Neo4j::Config class:

Neo4j::Config[:storage_path] = '/home/neo/neodb'

Lucene Integration

Neo4j.rb uses the Lucene module. That means that the Neo4j::NodeMixin has method for both traversal and lucene queries/indexing.

Lucene Configuration

By default lucene indexes are kept in memory. Keeping index in memory will increase the performance of lucene operations (such as updating the index).

Example to configure Lucene to store indexes on disk instead

Lucene::Config[:store_on_file] = true
Lucene::Config[:storage_path] => '/home/neo/lucene-db'

Lucene Index in Memory

If index is stored in memory then one needs to reindex all nodes when the application starts up again.

MyNode.update_index # will traverse all MyNode instances and (re)create the lucene index in memory.


Neo4j::NodeMixin is a mixin that lets instances to be stored as a node in the neo node space on disk. A node can have properties and relationships to other nodes.

Example of how declare a class that has this behaviour:

class MyNode 
   include Neo4j::NodeMixin

Create a Node

node = MyNode.new

Delete a Node

The Neo4j::NodeMixin mixin defines a delete method that will delete the node and all its relationships.


node = MyNode.new

The node in the example above will be removed from the neo database on the filesystem and the lucene index

Node Properties

In order to use properties they have to be declared first

class MyNode
   include Neo4j::NodeMixin
   property :foo, :bar

These properties (foo and bar) will be stored in the Neo database. You can set those properties:

# create a node with two properties in one transaction
node = MyNode.new { |n|
   n.foo = 123  
   n.bar = 3.14

# access those properties
puts node.foo

You can also set a property like this:

f = SomeNode.new
f.foo = 123

Neo4j.rb supports properties to by of type String, Fixnum, Float and true/false

Property Types and Marshalling

If you want to set a property of a different type then String, Fixnum, Float or true/false you have to specify its type.

Example, to set a property to any type

class MyNode
  include Neo4j::NodeMixin
  property :foo, :type => Object

node = MyNode.new
node.foo = [1,"3", 3.14]

Neo4j.load(node.neo_node_id).foo.class # => Array

Property of type Date and DateTime

Example of using Date queries:

class MyNode
  include Neo4j::NodeMixin
  property :since, :type => Date
  index :since, :type => Date

node.since = Date.new 2008,05,06
MyNode.find("born:[20080427 TO 20100203]")[0].since # => Date 2008,05,06

Example of using DateTime queries:

class MyNode
  include Neo4j::NodeMixin
  property :since, :type => DateTime
  index :since, :type => DateTime

node.since = DateTime.civil 2008,04,27,15,25,59
MyNode.find("since:[200804271504 TO 201002031534]")[0].since # => DateTime ...

Only UTC timezone is allowed.

Finding all nodes

To find all nodes of a specific type use the all method.


require 'neo4j/extensions/reindexer'

class Car
  include Neo4j::Node
  property :wheels

class Volvo < Car

v = Volvo.new
c = Car.new

Car.all   # will return all relationships from the index node to car objects
Volvo.all # will return the same as Car.all

To return nodes (just like the relationships method)

Car.all.nodes    # => [c,v]
Volvo.all.nodes  # => [c,v]

The reindexer extension that is used in the example above will for each created node create a relationship from the index node (Neo4j.ref_node.relationships.outgoing(:index_node)) to that new node. The all method use these relationships in order to return nodes of a certain class. The update_index method also uses this all method in order to update the index for all nodes of a specific class.

Relationship has_n and has_one

Neo relationships are asymmetrical. That means that if A has a relationship to B then it may not be true that B has a relationship to A.

Relationships can be declared by using the 'has_n' or 'has_one' Neo4j::NodeMixin class methods.


The has_n Neo4j::NodeMixin class method creates a new instance method that can be used for both traversing and adding new objects to a specific relationship type.

For example, let say that Person can have a relationship to any other node class with the type 'friends':

class Person 
   include Neo::Node
   has_n :knows  # will generate a knows method for outgoing relationships

The generated knows method will allow you to add new relationships, example:

me = Person.new
neo = Person.new
me.knows << neo  # me knows neo but neo does not know me

You can add any object to the 'knows' relationship as long as it includes the Neo4j::NodeMixin, example:

person = Person.new
car = Volvo.new # Volvo is a class that includes the Neo4j::NodeMixin
person.knows << car

If you want to express that the relationship should point to a specific class use the 'to' method on the has_n method.

class Person
   include Neo::Node

It is also possible to generate methods for incoming relationships by using the 'from' method on the has_n method.


class Person
   include Neo::Node
   has_n :knows  # will generate a knows method for outgoing relationships
   has_n(:known_by).from(:knows)  #  will generate a known_by method for incomming knows relationship

By doing this you can add a relationships on either the incoming or outgoing node. The from method can also take an additional class parameter if it has incoming nodes from a different node class (see the Actor-Role-Movie example at the top of this document).

Example of adding a 'knows' relationship from the other node:

me = Person.new 
neo = Person.new
neo.known_by << me # me knows neo but neo does not know me

The known_by method creates a 'knows' relationship between the me and neo nodes. This is the same as doing:

me.knows << neo # me knows neo but neo does not know me

Relationship has_one

Example of has_one: A person can have at most one Address

class Person
  include Neo4j::NodeMixin

class Address
  include Neo4j::NodeMixin
  property :city, :road
  has_n(:people).from(Person, :address)

In the example above we have Neo4j.rb will generate the following methods

  • in Person, the method ''address='' and ''address''

  • in Address, the traversal method ''people'' for traversing incomming relationships from the Person node.

Example of usage:

p = Person.new
p.address = Address.new
p.address.city = 'malmoe'

Or from the incoming ''address'' relationship

a = Address.new {|n| n.city = 'malmoe'}
a.people << Person.new

Relationship has_list

The has_n relationship will not maintain the order of when items are inserted to the relationship. If order should be preserved then use the has_list class method instead.


class Company
  has_list :employees

company = Company.new
company.employees << employee1 << employee2

# prints first employee2 and then employee1
company.employees.each {|employee| puts employee.name}

Traversing Relationships

Each type of relationship has a method that returns an Enumerable object that enables you to traverse that type of relationship.

For example the Person example above declares one relationship of type friends. You can traverse all Person's friend (depth 1 is default)

f.friends.each { |n| puts n }

It is also possible to traverse a relationship of an arbitrary depth. Example finding all friends and friends friends.

f.friends.depth(2).each { ...}

Traversing to the end of the graph

f.friends.depth(:all).each { ...}

Filtering Nodes

If you want to find one node in a relationship you can use a filter. Example, let say we want to find a friend with name 'andreas'

n1 = Person.new
n2 = Person.new {|n| n.name = 'andreas'}
n3 = Person.new
n1.friends << n2 << n3
n1.friends{ name == 'andreas' }.to_a # => [n2]

The block { name == 'andreas' } will be evaluated on each node in the relationship. If the evaluation returns true the node will be included in the filter search result.

Traversing Nodes

The Neo4j::NodeMixin#traverse method is a more powerful method compared to the generated has_n and has_one methods. Unlike those generated method it can traverse several relationship types at the same time. The types of relationships being traversed must therefore always be specified in the incoming, outgoing or both method. Those three methods can take one or more relationship types parameters if more then one type of relationship should be traversed.

Traversing Nodes of Arbitrary Depth

The depth method allows you to specify how deep the traverse should be. If not specified only one level traverse is done.


me.traverse.incoming(:friends).depth(4).each {} # => people with a friend relationship to me

Traversing Nodes With Several Relationship Types

It is possible to traverse sevaral relationship types at the same type. The incoming, both and outgoing methods takes list of arguments.

Example, given the following holiday trip domain:

# A location contains a hierarchy of other locations
# Example region (asia) contains countries which contains  cities etc...
class Location
  include Neo4j::NodeMixin
  has_n :contains
  has_n :trips
  property :name
  index :name

# A Trip can be specific for one global area, such as "see all of sweden" or
# local such as a 'city tour of malmoe'
class Trip
  include Neo4j::NodeMixin
  property :name

# create all nodes
# ...

# setup the relationship between all nodes
@europe.contains << @sweden << @denmark
@sweden.contains << @malmoe << @stockholm

@sweden.trips << @sweden_trip
@malmoe.trips << @malmoe_trip
@malmoe.trips << @city_tour
@stockholm.trips << @city_tour # the same city tour is available both in malmoe and stockholm

Then we can traverse both the contains and the trips relationship types Example:

@sweden.traverse.outgoing(:contains, :trips).to_a # => [@malmoe, @stockholm, @sweden_trip]

It is also possible to traverse both incoming and outgoing relationships, example:

@sweden.traverse.outgoing(:contains, :trips).incoming(:contains).to_a # => [@malmoe, @stockholm, @sweden_trip, @europe]

Traversing Nodes With a Filter

It's possible to filter which nodes should be returned from the traverser by using the filter function. This filter function will be evaluated differently depending on if it takes one argument or no arguments, see below.

Filtering: Using Evaluation in the Context of the Current Node

If the provided filter function does not take any parameter it will be evaluted in the context of the current node being traversed. That means that one can writer filter functions like this:

@sweden.traverse.outgoing(:contains, :trips).filter { name == 'sweden' }

Filtering: Using the TraversalPostion

If the filter method takes one parameter then it will be given an object of type TraversalPosition which contains information about current node, how many nodes has been returned, depth etc.

The information contained in the TraversalPostion can be used in order to decide if the node should included in the traversal search result. If the provided block returns true then the node will be included in the search result.

The filter function will not be evaluated in the context of the current node when this parameter is provided.

The TraversalPosition is a wrapper of java interface TraversalPosition, see api.neo4j.org/current/org/neo4j/api/core/TraversalPosition.html

For example if we only want to return the Trip objects in the example above:

# notice how the tp (TraversalPosition) parameter is used in order to only
# return nodes included in a 'trips' relationship.
traverser = @sweden.traverse.outgoing(:contains, :trips).filter do |tp|
  tp.last_relationship_traversed.relationship_type == :trips

traverser.to_a # => [@sweden_trip]


A relationship between two nodes can have properties just like a node.


p1 = Person.new
p2 = Person.new

relationship = p1.friends.new(p2)

# set a property 'since' on this relationship bewteen p1 and p2
relationship.since = 1992

If a Relationship class has not been specified for a relationship then any properties can be set on the relationship. It has a default relationship class: Neo4j::DynamicRelation

If you instead want to use your own class for a relationship use the Neo4j::NodeMixin#has_n.relationship method, example:

class Role
  # This class can be used as the relationship between two nodes
  # since it includes the following mixin
  include Neo4j::RelationMixin
  property :name

class Actor
  include Neo4j::NodeMixin
  # use the Role class above in the relationship between Actor and Movie

Finding Relationships

The Neo4j::NodeMixin#relationships method can be used to find incoming or outgoing relationship objects. Example of listing all types of outgoing (default) relationship objects (of depth one) from the me node.

me.relationships.each {|rel| ... }

If we instead want to list the nodes that those relationships points to then the nodes method can be used.

me.relationships.nodes.each {|rel| ... }

Listing all incoming relationship obejcts of any relationship type:

me.relationships.incoming.each { ... }

Listing both incoming and outgoing relationship object of a specific type:

me.relationships.both(:friends) { }

Finding one outgoing relationship of a specific type and node (you)

me.relationships.outgoing(:friends)[you] # => [#<Neo4j::RelationshipMixin:0x134ae32]

Finding Relationships Example

Example, given we have the two nodes with a relationship between them:

n1 = Person.new
n2 = Person.new

n1.friends << n2

Then we can find all incoming and outgoing relationships like this:

n1.relationships.to_a # => [#<Neo4j::RelationshipMixin:0x134ae32]

A Neo4j::RelationshipMixin object represents a relationship between two nodes.

n1.relationships[0].start_node # => n1
n1.relationships[0].end_node # => n2

A RelationshipMixin contains the relationship type which connects it connects two nodes with, example:

n1.relationships[0].relationship_type # => :friends

Relationships can also have properties just like a node (NodeMixin).

Finding outgoing and incoming relationships

If we are only interested in all incoming nodes, we can do

n2.relationships.incoming # => [#<Neo4j::RelationshipMixin:0x134ae32]

Or outgoing:

n1.relationships.outgoing # => [#<Neo4j::RelationshipMixin:0x134aea2]

To find a specific relationship use the [] operator:

n1.relationships.outgoing[n2] = #<Neo4j::RelationshipMixin:0x134aea2

Or which is better performance wise (since only friends relationships are being traversed):

n1.relationships.outgoing(:friends)[n2] = #<Neo4j::RelationshipMixin:0x134aea2

Deleting a relationship

Use the Neo4j::RelationshipMixin#delete method. For example, to delete the relationship between n1 and n2 from the example above:


Finding nodes in a relationship

If you do not want those relationship object but instead want the nodes you can use the 'nodes' method in the Neo4j::RelationshipMixin object.

For example:

n2.relationships.incoming.nodes # => [n1]

Finding outgoing/incoming nodes of a specific relationship type

Let say we want to find who has my phone number and who consider me as a friend

# who has my phone numbers
me.relationships.incoming(:phone_numbers).nodes # => people with my phone numbers

# who consider me as a friend
me.relationships.incoming(:friends).nodes # => people with a friend relationship to me

Remember that relationships are not symmetrical. Notice there is also a otherway of finding nodes, see the Neo4j::NodeMixin#traverse method below.


All operations that work with the node space (even read operations) must be wrapped in a transaction. For example all get, set and find operations will start a new transaction if none is already not runnig (for that thread).

If you want to perform a set of operation in a single transaction, use the Neo4j::Transaction.run method:


Neo4j::Transaction.run {
  node1.foo = "value"
  node2.bar = "hi"

There is also a auto commit feature available which is enabled by requiring 'neo4j/auto_tx' instead of 'neo4j', see the three minutes tutorial above.


Neo4j support rollbacks on transaction. Example: Example:

include 'neo4j'

node = MyNode.new

Neo4j::Transaction.run { |t|
   node.foo = "hej"
   # something failed so we signal for a failure 
   t.failure # will cause a rollback, node.foo will not be updated

You can also run it without a block, like this:

transaction = Neo4j::Transaction.new
# do something


Properties and relationships which should be indexed by lucene can be specified by the index class method. For example to index the proeprties foo and bar

class SomeNode
   include Neo4j::NodeMixin
   property :foo, :bar
   index :foo, :bar

Everytime a node of type SomeNode (or a subclass) is create, deleted or updated the lucene index of will be updated.


Sometimes it's neccessarly to change the index of a class after alot of node instances already have been created. To delete an index use the class method 'remove_index' To update an index use the class method 'update_index' which will update all already created nodes in the neo database.


require 'neo4j'
require 'neo4j/extensions/reindexer' # needed for the update_index method
class Person
  include Neo4j
  property :name, :age, :phone
  index :name, :age

p1 = Person.new {|n| n.name = 'andreas'; n.phone = 123}
Person.find (:name => 'andreas') # => [p1]
Person.find (:phone => 123) # => []

# change index and reindex all person nodes already created in the neo database.
Person.remove_index :name
Person.index :phone  # add an index on phone

Person.find (:name => 'andreas') # => []
Person.find (:phone => 123) # => [p1]

In order to use the update_index method you must include the reindexer neo4j.rb extension. This extensions will keep a relationship to each created node so that it later can recreate the index by traversing those relationships.

Updating Lucene Index

The lucene index will be updated after the transaction commits. It is not possible to query for something that has been created inside the same transaction as where the query is performed.

Quering (using lucene)

You can declare properties to be indexed by lucene by the index method:


class Person 
  include Neo4j::NodeMixin
  property :name, :age
  index :name, :age

node = Person.new
node.name = 'foo'
node.age  = 42

Person.find(:name => 'foo', :age => 42) # => [node]

The query parameter (like property on a Neo4j::NodeMixin) can be of type String, Fixnum, Float, boolean or Range. The query above can also be written in a lucene query DSL:

Person.find{(name =='foo') & (age => 42)} # => [node]

Or lucene query language:

Person.find("name:foo AND age:42")

For more information see: lucene.apache.org/java/2_4_0/queryparsersyntax.html or the lucene module above.

Indexing and Property Types

In order to use range querie on numbers the property types must be converted. This is done by using the :type optional parameter:

class Person
  include Neo4j::NodeMixin
  property :name, :age
  index :age, :type => Fixnum

By using :type => Fixnum the age will be padded with '0's (lucene only support string comparsion).

Example, if the :type => Fixnum was not specified then

p = Person.new {|n| n.age = 100 }
Person.find(:age => 0..8) # => [p]

Indexing and Quering Relationships

The Neo4j::NodeMixin#index method can be used to index relationships to other classes.

Example, let say we have to classes, Customer and Orders:

class Customer
  include Neo4j::NodeMixin

  property :name

  # specifies outgoing relationships to Order

  # create an index on customer-->order#total_cost
  index "orders.total_cost"

class Order
  include Neo4j::NodeMixin

  property :total_cost

  # specifies one incoming relationship from Customer
  has_one(:customer).from(Customer, :orders)

  # create an index on the order<--customer#name relationship
  index "customer.name"

Notice that we can index both incoming and outgoing relationships.

Let's create a customer and one order for that customer

Neo4j::Transaction.run do
  cust = Customer.new
  order = Order.new
  cust.name = "kalle"
  order.total_cost = "1000"

  cust.orders << order 

Now we can find both Orders with a total cost between 500 and 2000 and Customers with name 'kalle' using lucene


customers = Customer.find('orders.total_cost' => 500..2000, 'name' => 'kalle')

Or also possible from the other way:

orders = Order.find('total_cost' => 500..2000, 'customer.name' => 'kalle')

Full text search

Neo4j supports full text search by setting the tokenized property to true on an index. (see JavaDoc for org.apache.lucene.document.Field.Index.ANALYZED).

class Comment
  include Neo4j::NodeMixin

  property :comment
  index comment, :tokenized => true


The neo module will automatically unmarshalling nodes to the correct ruby class. It does this by reading the classname property and loading that ruby class with that node.

class Person 
  include Neo::Node

  def hello

f1 = Person.new {}

# load the class again
f2 = Neo4j.load(foo.neo_node_id)

# f2 will now be new instance of Person, but will be == f1
f1 == f2 # => true

Reference node

There is one node that can always be find - the reference node, Neo4j::ReferenceNode. Example:


This node can have a relationship to the index node (Neo4j::IndexNode), which has relationships to all created nodes. You can add relationships from this node to your nodes.

Performance Issues

It is recommended to wrap several Neo4j operations including read operations in a singel transaction if possible for better performance. Updating a lucene index can be slow. A solution to this is to keep the index in memory instead of on disk.

I'm currently looking at how to scale neo4j.rb by a simple master-slave cluster by using REST, see the REST extension below.

Extensions: Replication

There is an experimental extension that makes it possible to replicate an neo4j database to another machine. For example how to use it see the test/replication/test_master.rb and test_slave.rb It has only been tested to work with a very simple node space.

Extension: REST

There is an REST extension to Neo4j.rb. It requires the following gems

* Sinatra >= 0.9.4
* Rack >= 1.0
* json-jruby >= 1.1.6

For RSpec testing it also needs:

* rack-test

For more information see the examples/rest/example.rb or the examples/admin or Neo4j::RestMixin.

Extension: find_path

Extension which finds the shortest path (in terms of number of links) between two nodes. Use something like this:

require 'neo4j/extensions/find_path'
# => [node1, node42, node1234, node256, node2]

This extension is still rather experimental. The algorithm is based on the one used in the Neo4j Java IMDB example. For more information see Neo4j::Relationships::NodeTraverser#path_to or the RSpec find_path_spec.rb.

Ruby on Rails with Neo4j.rb

Neo4j.rb does work nicely with R&R. There are two ways to use neo4j.rb with rails - embedded or accessing it via REST.

Embedded Rails

A complete example of embedding neo4j with rails can be found github.com/andreasronge/neo4j-rails-example/tree/master (please fork and improve it).

Config rails

Config rails to use Neo4j.rb instead of ActiveRecord, edit movies/config/environment.rb environment.rb:

config.frameworks -= [ :active_record ] #, :active_resource, :action_mailer ]
config.gem "neo4j", :version => "0.3.1"  # or the latest one

If you need to reindex all nodes or use the Neo4j::NodeMixin#all method you must require the reindexer neo4j.rb extension. Add a require in the environment.rb file:

require 'neo4j/extensions/reindexer'


Create a new file for each neo4j node or relationship class Example for an Actor class create the file: app/models/actor.rb

# filename app/models/actor.rb
class Actor
  include Neo4j::NodeMixin
  property :name, :phone, :salary
  index :name

Create RESTful routes

Edit the config/routes.rb file Example:

ActionController::Routing::Routes.draw do |map|
   map.resources :actors do |actor|
    actor.resources :acted_in
    actor.resource :movies, :controller => 'acted_in'

Create Controllers

Since all neo4j operations must be wrapped in a transaction add a around filter for all operations Example:


class ActedInController < ApplicationController
  around_filter :neo_tx

  def index
    @actor = Neo4j.load(params[:actor_id])
    @movies = @actor.acted_in.nodes

  def create
    @actor = Neo4j.load(params[:actor_id])
    @movie = Movie.new
    @actor.acted_in << @movie
    flash[:notice] = 'Movie was successfully created.'

  def update
    @actor = Neo4j.load(params[:actor_id])
    @movie = Movie.new
    @actor.acted_in.new @movie
    flash[:notice] = 'Movie was successfully updated.'

  def show
    @movie = Neo4j.load(params[:id])

  def new
    @actor = Neo4j.load(params[:actor_id])
    @movie = Movie.value_object.new

  def edit
    @movie = Neo4j.load(params[:id])


  def neo_tx

Add views

Add the following views in app/views/actors index.html.erb:

<h1>Listing actors</h1>


  <% for actor in @actors %>
      <td><%=h actor.name %></td>
      <td><%= link_to 'Edit', edit_actor_path(actor) %></td>
      <td><%= link_to 'Show', actor %></td>
      <td><%= link_to 'Destroy', actor, :confirm => 'Are you sure?', :method => :delete %></td>
  <% end %>

<br />

<%= link_to 'New actor', new_actor_path %>


<h1>New Actor</h1>

<% form_for(@actor) do |f| %>
    <%= f.label :name %><br />
    <%= f.text_field :name %>
    <%= f.label :phone %><br />
    <%= f.text_field :phone %>
    <%= f.label :salary%><br />
    <%= f.text_field :salary %>
    <%= f.submit "Update" %>

<% end %>

<%= link_to 'Back', actors_path %>