AshNeo4j

Ash DataLayer for Neo4j, configurable using a simple DSL

Installation

Add to the deps:

def deps do
  [
    {:ash_neo4j, "~> 0.2.5"},
  ]
end

Tutorial

To get started you need a running instance of Livebook

Usage

Configure AshNeo4j.DataLayer as data_layer: within use Ash.Resource options:

  use Ash.Resource,
    data_layer: AshNeo4j.DataLayer

Configuration

Each Ash.Resource requires configuration of its AshNeo4j.DataLayer. An example Comment resource is given below, it can belong to a Post resource.

defmodule Comment.Resource do
  use Ash.Resource,
    data_layer: AshNeo4j.DataLayer

  neo4j do
    label :Comment
    relate [{:post, :BELONGS_TO, :outgoing}]
    translate id: :uuid
  end

  actions do
    default_accept :*
    defaults [:create, :read, :update, :destroy]
  end

  attributes do
    uuid_primary_key :id
    attribute :title, :string, public?: true
  end

  relationships do
    belongs_to :post, Post, public?: true
  end
end

Label

The DSL may be used to label the Ash Resource's underlying graph node. If omitted the Ash Resource's short module name will be used.

  neo4j do
    label :Comment
  end

Relate

The DSL must be used to direct each relationship.

  neo4j do
    relate [{:post, :BELONGS_TO, :outgoing}]
  end

Guard

The DSL may be used to guard destroy actions.

  neo4j do
    relate [{:WRITTEN_BY, :outgoing, :Post}]
  end

Guard is useful where the resource has no explicit relationships, but other resources expect the resource to exist while they are related. Guard can also be used where the underlying node has other edges which should prevent resource destruction.

Translate

The DSL may be used to translate the Ash Resource's attributes to node properties.

  neo4j do
    translate id: :uuid
  end

The :id attribute must be translated as it cannot be used as a Neo4j property name. Attributes with underscores are translated to camelCase Neo4j properties so don't need to be explicitly listed in translate.

Skip

The DSL may be used to skip storing attributes as node properties. This is typically used for foreign keys, which not required with relate.

  neo4j do
    skip [:other_id]
  end

Verifiers

The DSL is verified against misconfiguration and violation of accepted neo4j conventions providing compile time errors:

• label: neo4j label must be PascalCase
• neo4j: neo4j property names must be camelCase
• relate: relationship_name must match the name of a relationship
• relate: edge label must be upper case and may have an underscore
• translate: :id attribute must be translated

Installing Neo4j and Configuring Boltx

ash_neo4j uses neo4j which must be installed and running.

Your Ash application needs to configure, start and supervise boltx, see boltx documentation. Make sure to configure any required authorisation.

I've used Neo4j community edition 4.4 (bolt 4.4) and 5.28 (boltx limits to bolt 5.4) and any version in between should work. To connect to Neo4j 4.4 using boltx I needed to also set the environment variable BOLT_VERSIONS="4.4" to steer [bolt protocol handshake] (https://neo4j.com/docs/bolt/current/bolt/handshake). I've raised negotiate range on boltx to improve version negotiation so that this won't be necessary.

Elixir, Ash and Neo4j Types

We've made some decisions around how Ash/Elixir types are used to persist attributes as Neo4j properties. Where possible we've used 'native' Neo4j types, where this is not possible we've simply quoted to strings. Ash Array support is limited by Neo4j to lists of simple types which must be homogenous.

Ash :date, :datetime, :time and :naive_datetime are second precision, whereas :utc_datetime_usec and :time_usec are microsecond precision. Note that :time_usec requires ash ~> 3.5.10 (to be released).

Structs and String.Chars

Structs (including Ash embedded resources) are supported and stored in their string representation, this requires String.Chars to be implemented using the representation common for Elixir structs. This is straightforward whether or not you own the module. Here is an example for a simple embedded resource:

defmodule Money do
  use Ash.Resource,
    data_layer: :embedded

  attributes do
    attribute :amount, :integer
    attribute :currency, :atom
  end

  defimpl String.Chars do
    def to_string(v) do
      "%AshNeo4j.Test.Resource.Money{amount: #{v.amount}, currency: :#{v.currency}}"
    end
  end
end

Elixir nil and Neo4j Null

Generally attributes with nil value are not persisted, rather they are simply not created or removed on update to nil. However values of nil within string quoted 'Elixir' types (keyword, tuple, map and struct) are persisted.

Limitations and Future Work

Ash Neo4j has initial support for Ash create, update, read, destroy actions. Calculations are supported but not evaluated in Neo4j itself. Aggregates are not yet supported. The DSL is likely to evolve further and this may break back compatibility. Collaboration on ash_neo4j welcome via github, please use discussions and/or raise issues as you encounter them.

Acknowledgements

Thanks to the Ash Core for ash 🚀, including ash_csv which was an exemplar.

Thanks to Sagastume for boltx which was based on bolt_sips by Florin Patrascu.

Thanks to the Neo4j Core for neo4j and pioneering work on graph databases.

Links

Diffo.devNeo4j Deployment Centre.