ActiveRel is a module in the neo4j gem which wraps relationships. ActiveRel objects share most of their behavior with ActiveNode objects. ActiveRel is purely optional and offers advanced functionality for complex relationships.

When to Use?

It is not always necessary to use ActiveRel models but if you have the need for validation, callback, or working with properties on unpersisted relationships, it is the solution.

Note that in Neo4j it isn’t possible to access relationships except by first accessing a node. Thus ActiveRel doesn’t implement a uuid property like ActiveNode.

... Documentation notes

Separation of relationship logic instead of shoehorning it into Node models

Validations, callbacks, custom methods, etc.

Centralize relationship type, no longer need to use :type or :origin options in models


ActiveRel model definitions have four requirements:

  • include Neo4j::ActiveRel
  • call from_class with a valid model constant or :any
  • call to_class with a valid model constant or :any
  • call type with a Symbol or String to define the Neo4j relationship type

See the note on from/to at the end of this page for additional information.

# app/models/enrolled_in.rb
class EnrolledIn
  include Neo4j::ActiveRel
  before_save :do_this

  from_class Student
  to_class    Lesson
  type 'enrolled_in'

  property :since, type: Integer
  property :grade, type: Integer
  property :notes

  validates_presence_of :since

  def do_this
    #a callback

See also

There is also a screencast available reviewing ActiveRel:

Relationship Creation

From an ActiveRel Model

Once setup, ActiveRel models follow the same rules as ActiveNode in regard to properties. Declare them to create setter/getter methods. You can also set created_at or updated_at for automatic timestamps.

ActiveRel instances require related nodes before they can be saved. Set these using the from_node and to_node methods.

rel =
rel.from_node = student
rel.to_node = lesson

You can pass these as parameters when calling new or create if you so choose.

rel = student, to_node: lesson)
rel = EnrolledIn.create(from_node: student, to_node: lesson)

From a has_many or has_one association

Pass the :rel_class option in a declared association with the constant of an ActiveRel model. When that relationship is created, it will add a hidden _classname property with that model’s name. The association will use the type declared in the ActiveRel model and it will raise an error if it is included in more than one place.

class Student
  include Neo4j::ActiveNode
  has_many :out, :lessons, rel_class: :EnrolledIn

Query and Loading existing relationships

Like nodes, you can load relationships a few different ways.

:each_rel, :each_with_rel, or :pluck methods

Any of these methods can return relationship objects.

Student.first.lessons.each_rel { |r| }
Student.first.lessons.each_with_rel { |node, rel| }

These are available as both class or instance methods. Because both each_rel and each_with_rel return enumerables when a block is skipped, you can take advantage of the full suite of enumerable methods:{ |n, r| r.grade > 85 }

Be aware that select would be performed in Ruby after a Cypher query is performed. The example above performs a Cypher query that matches all students with relationships of type enrolled_in to Lesson.first, then it would call select on that.

The :where method

Because you cannot search for a relationship the way you search for a node, ActiveRel’s where method searches for the relationship relative to the labels found in the from_class and to_class models. Therefore:

EnrolledIn.where(since: 2002)
# Generates the Cypher:
# "MATCH (node1:`Student`)-[rel1:`enrolled_in`]->(node2:`Lesson`) WHERE rel1.since = 2002 RETURN rel1"

If your from_class is :any, the same query looks like this:

"MATCH (node1)-[rel1:`enrolled_in`]->(node2:`Lesson`) WHERE rel1.since = 2002 RETURN rel1"

And if to_class is also :any, you end up with:

"MATCH (node1)-[rel1:`enrolled_in`]->(node2) WHERE rel1.since = 2002 RETURN rel1"

As a result, this combined with the inability to index relationship properties can result in extremely inefficient queries.

Advanced Usage

Separation of Relationship Logic

ActiveRel really shines when you have multiple associations that share a relationship type. You can use an ActiveRel model to separate the relationship logic and just let the node models be concerned with the labels of related objects.

class User
  include Neo4j::ActiveNode
  property :managed_stats, type: Integer #store the number of managed objects to improve performance

  has_many :out, :managed_lessons,  model_class: Lesson,  rel_class: :ManagedRel
  has_many :out, :managed_teachers, model_class: Teacher, rel_class: :ManagedRel
  has_many :out, :managed_events,   model_class: Event,   rel_class: :ManagedRel
  has_many :out, :managed_objects,  model_class: false,   rel_class: :ManagedRel

  def update_stats
    managed_stats += 1

class ManagedRel
  include Neo4j::ActiveRel
  after_create :update_user_stats
  validate :manageable_object
  from_class User
  to_class :any
  type 'manages'

  def update_user_stats

  def manageable_object
    errors.add(:to_node) unless to_node.respond_to?(:managed_by)

# elsewhere
rel = user, to_node: any_node)
  # validation passed, to_node is a manageable object
  # something is wrong

Additional methods

:type instance method, _:type class method: return the relationship type of the model

:_from_class and :_to_class class methods: return the expected classes declared in the model

Regarding: from and to

:from_node, :to_node, :from_class, and :to_class all have aliases using start and end: :start_class, :end_class, :start_node, :end_node, :start_node=, :end_node=. This maintains consistency with elements of the Neo4j::Core API while offering what may be more natural options for Rails users.