Polymorphic embeds for Ecto

polymorphic_embed brings support for polymorphic/dynamic embedded schemas in Ecto.

Ecto's embeds_one and embeds_many macros require a specific schema module to be specified. This library removes this restriction by dynamically determining which schema to use, based on data to be stored (from a form or API) and retrieved (from the data source).

Usage

Enable polymorphism

Let's say we want a schema Reminder representing a reminder for an event, that can be sent either by email or SMS.

We create the Email and SMS embedded schemas containing the fields that are specific for each of those communication channels.

The Reminder schema can then contain a :channel field that will either hold an Email or SMS struct, by setting its type to the custom type PolymorphicEmbed that this library provides.

Find the schema code and explanations below.

defmodule MyApp.Reminder do
  use Ecto.Schema
  import Ecto.Changeset
  import PolymorphicEmbed, only: [cast_polymorphic_embed: 2]

  schema "reminders" do
    field :date, :utc_datetime
    field :text, :string

    field :channel, PolymorphicEmbed,
      types: [
        sms: MyApp.Channel.SMS,
        email: [module: MyApp.Channel.Email, identify_by_fields: [:address, :confirmed]]
      ],
      on_replace: :update
  end

  def changeset(struct, values) do
    struct
    |> cast(values, [:date, :text])
    |> cast_polymorphic_embed(:channel)
    |> validate_required(:date)
  end
end

defmodule MyApp.Channel.Email do
  use Ecto.Schema
  import Ecto.Changeset

  @primary_key false

  embedded_schema do
    field :address, :string
    field :confirmed, :boolean
  end

  def changeset(email, params) do
    email
    |> cast(params, ~w(address confirmed)a)
    |> validate_required(:address)
    |> validate_length(:address, min: 4)
  end
end

defmodule MyApp.Channel.SMS do
  use Ecto.Schema

  @primary_key false

  embedded_schema do
    field :number, :string
  end
end

The :types option for the PolymorphicEmbed custom type contains a keyword list mapping an atom representing the type (in this example :email and :sms) with the corresponding embedded schema module.

There are two strategies to detect the right embedded schema to use:

[sms: MyApp.Channel.SMS]

When receiving parameters to be casted (e.g. from a form), we expect a "__type__" (or :__type__) parameter containing the type of channel ("email" or "sms").

[email: [
  module: MyApp.Channel.Email,
  identify_by_fields: [:address, :confirmed]]]

Here we specify how the type can be determined based on the presence of given fields. In this example, if the data contains :address and :confirmed parameters (or their string version), the type is :email. A "__type__" parameter is then no longer required.

Note that you may still include a __type__ parameter that will take precedence over this strategy (this could still be useful if you need to store incomplete data, which might not allow identifying the type).

List of polymorphic embeds

Lists of polymorphic embeds are also supported:

field :contexts, {:array, PolymorphicEmbed},
  types: [
    location: MyApp.Context.Location,
    age: MyApp.Context.Age,
    device: MyApp.Context.Device
  ],
  on_replace: :delete

Options

• :types - discussed above.
• :on_type_not_found - specify whether to raise or add a changeset error if the embed's type cannot be inferred. Possible values are :raise and :changeset_error. By default, a changeset error "is invalid" is added.
• :on_replace - mandatory option that can only be set to :update for a single embed and :delete for a list of embeds (we force a value as the default value of this option for embeds_one and embeds_many is :raise).

Displaying form inputs and errors in Phoenix templates

The library comes with a form helper in order to build form inputs for polymorphic embeds and display changeset errors.

In the entrypoint defining your web interface (lib/your_app_web.ex file), add the following import:

def view do
  quote do
    # imports and stuff
    import PolymorphicEmbed.HTML.Form
  end
end

This provides you with a polymorphic_embed_inputs_for/4 function.

Here is an example form using the imported function:

<%= inputs_for f, :reminders, fn reminder_form -> %>
  <%= polymorphic_embed_inputs_for reminder_form, :channel, :sms, fn sms_form -> %>
    <div class="sms-inputs">
      <label>Number<label>
      <%= text_input sms_form, :number %>
      <div class="error">
        <%= error_tag sms_form, :number %>
      </div>
    </div>
  <% end %>
<% end %>

polymorphic_embed_inputs_for/4 also renders a hidden input for the "__type__" field.

Get the type of a polymorphic embed

Sometimes you need to serialize the polymorphic embed and, once in the front-end, need to distinguish them. get_polymorphic_type/3 returns the type of the polymorphic embed:

PolymorphicEmbed.get_polymorphic_type(Reminder, :channel, SMS) == :sms

Features

• Detect which types to use for the data being cast-ed, based on fields present in the data (no need for a type field in the data)
• Run changeset validations when a changeset/2 function is present (when absent, the library will introspect the fields to cast)
• Support for nested polymorphic embeds
• Support for nested embeds_one/embeds_many embeds
• Display form inputs for polymorphic embeds in Phoenix templates
• Tests to ensure code quality

Installation

Add polymorphic_embed for Elixir as a dependency in your mix.exs file:

def deps do
  [
    {:polymorphic_embed, "~> 1.1.0"}
  ]
end

HexDocs

HexDocs documentation can be found at https://hexdocs.pm/polymorphic_embed.