TL;DR: How do you practically use the embedded schemas when implementing single table inheritance in a Phoenix application?

There are several topics on STI here and I’m pretty sure I’ve read them all, so please bare with me on this one

My users’ domain requires strict management of entities and their relationships. I’m therefore looking into modeling the entities part on the “entity engine” of Apache Open For Business (OfBiz).

OfBiz is an enterprise resource planning (ERP) software written in Java. Crucially, it offers a comprehensive, general, off the shelf, data model to build on. I believe that the “entity engine” part of that may, at least, offer a reasonable point of departure for my users’ needs.

Note that OfBiz’s “entity engine” includes XML-based schema definitions, and much more, that is of no interest to my use case.

Basically, there is a finite set of models and fields one could reasonably imagine when it comes to entities:

1. natural persons and
2. legal persons.

So, I’m creating an Entity schema tied to a corresponding table, together with embedded schemas NaturalPerson and LegalPerson following the Darren Wilson presentation at ElixirConf 2017 (see resources, references below).

The considered domain

What is particularly relevant to my application is that the domain of the industry (consultancy) centers around the concept of a Customer where:

1. A Customer has one or more Entity:s,
2. Each Entity may be either a NaturalPerson (a human) or, by definition, a LegalPerson (a corporation, trust etc.) and
3. An Entity belongs to one or more Customers.

It’s inconceivable there would ever be any other types of entities than natural or legal persons.

No, I refuse to consider adding a third kind of entity anticipating the singularity!

A common use case for my users is to have “customers” in constellations such as:

• A natural person and their fully owned corporation
• Two natural persons
• Two natural persons and their jointly owned corporation
• A corporation and its subsidiaries

A new “customer” must be set up when, at some later point, constellations change. For instance, a customer doing business primarily through their fully owned corporation may sell their business but continue doing business personally.

Schemas

This is the data model.

Entity

defmodule MyApp.EntityEngine.Entity do
  use Ecto.Schema
  import Ecto.Changeset

  @primary_key {:id, :binary_id, autogenerate: true}
  @foreign_key_type :binary_id
  schema "entities" do
    field :name1, :string
    field :name2, :string
    field :name3, :string
    field :start_date, :date
    field :end_date, :date
    field :entity_type, Ecto.Enum, values: [:natural, :legal]

    timestamps()
  end

  @doc false
  def changeset(entity, attrs) do
    entity
    |> cast(attrs, [:name1, :name2, :name3, :start_date, :end_date, :entity_type])
    |> validate_required([:name1, :entity_type])
  end
end

NaturalPerson

defmodule MyAppp.EntityEngine.NaturalPerson do
  use Ecto.Schema
  import Ecto.Changeset

  alias MyApp.EntityEngine.Entity

  @primary_key false

  embedded_schema do
    field :first_name, :string
    field :middle_name, :string
    field :last_name, :string
    field :birth_date, :date
    field :death_date, :date

    timestamps()
  end

  @doc false
  def changeset(natural_person, attrs \\ %{}) do
    natural_person
    |> cast(attrs, [:first_name, :middle_name, :last_name, :birth_date, :death_date])
    |> validate_required([:first_name, :last_name])
  end

  def to_entity(%__MODULE__{} = natural_person) do
    %Entity{
      name1: natural_person.first_name,
      name2: natural_person.middle_name,
      name3: natural_person.last_name,
      start_date: natural_person.birth_date,
      end_date: natural_person.death_date,
      entity_type: :natural
    }
  end
end

LegalPerson

defmodule MyApp.EntityEngine.LegalPerson do
  use Ecto.Schema
  import Ecto.Changeset

  alias MyApp.EntityEngine.Entity

  @primary_key false

  embedded_schema do
    field :name, :string
    field :established_date, :date
    field :closed_date, :date

    timestamps()
  end

  @doc false
  def changeset(legal_person, attrs \\ %{}) do
    legal_person
    |> cast(attrs, [:name, :established_date, :closed_date])
    |> validate_required([:name])
  end

  def to_entity(%__MODULE__{}, legal_person) do
    %Entity{
      name1: legal_person.name,
      start_date: legal_person.established_date,
      end_date: legal_person.closed_date,
      entity_type: :legal
    }
  end
end

Practical use?

I’ve tried to approach this by adapting the generated Phoenix LiveView code – with some rather great frustrations along the way.

• How would you approach using these schemas when creating lists, detail views and forms?

Creating a new NaturalPerson I’ve set up functions in my context like this:

  def create_natural_person(attrs \\ %{}) do
    %NaturalPerson{}
    |> NaturalPerson.changeset(attrs)
    |> Ecto.Changeset.apply_changes()
    |> NaturalPerson.to_entity()
    |> Repo.insert()
  end

The whole idea is of course to have separate form components for natural and legal persons. You would have separate changesets for these obviously. When updating an entity, you will be faced with an Entity from the database and would therefore need to “cast” this to the appropriate changeset for the embedded schema. I guess that one could run in to issues there? Then that has to go back into the database through the Entity schema again.

I guess one would set up separate routes for natural and legal persons with their own forms in live components using their respective changesets and casting from and to the Entity changeset?

Maybe you can tell that I’m stuck between the generators and just doing it myself in the best way possible.

Some pointers would be amazingly helpful here.

I feel stupid asking these questions

Hey, I’m recovering from a terrible condition that left me handicapped in many ways. I simply don’t have the energy to take things in and excel like I used to. I’m only now starting to be able to handle “stuff” and it’s actually going a lot better.

Now – this is what I’m afraid of when I post, but I know that this community is sooo amazing that my worries are unfounded Really, it’s different here!

I came for the BEAM but stayed for the community.

Darren Wilson’s ElixirConf 2017, regarding embedded_schema

As promised and for reference:

• Darren’s slides (corrected) as a PDF file, see page 121 and forward