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).

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

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

    polymorphic_embeds_one :channel,
      types: [
        sms: MyApp.Channel.SMS,
        email: MyApp.Channel.Email
      ],
      on_type_not_found: :raise,
      on_replace: :update
  end

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

In your migration file, you may use the type :map for both polymorphic_embeds_one/2 and polymorphic_embeds_many/2 fields.

add(:channel, :map)

It is not recommended to use {:array, :map} for a list of embeds.

cast_polymorphic_embed/3 must be called to cast the polymorphic embed's parameters.

• :required – if the embed is a required field.

• :with – allows you to specify a custom changeset. Either pass an MFA or a function:

changeset
|> cast_polymorphic_embed(:channel,
  with: [
    sms: {SMS, :custom_changeset, ["hello"]},
    email: &Email.custom_changeset/2
  ]
)

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).

Lists of polymorphic embeds are also supported:

polymorphic_embeds_many :contexts,
  types: [
    location: MyApp.Context.Location,
    age: MyApp.Context.Age,
    device: MyApp.Context.Device
  ],
  on_type_not_found: :raise,
  on_replace: :delete

• :types – discussed above.

• :type_field – specify a custom type field. Defaults to :__type__.

• :on_type_not_found – specify what to do if the embed's type cannot be inferred. Possible values are

  • :raise: raise an error
  • :changeset_error: add a changeset error
  • :nilify: replace the data by nil; only for single (non-list) embeds
  • :ignore: ignore the data; only for lists of embeds

  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).

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 the polymorphic_embed_inputs_for/3 and polymorphic_embed_inputs_for/4 functions.

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 %>

When using polymorphic_embed_inputs_for/4, you have to manually specify the type. When the embed is nil, empty fields will be displayed.

polymorphic_embed_inputs_for/3 doesn't require the type to be specified. When the embed is nil, no fields are displayed.

They both render a hidden input for the "__type__" field.

You may use polymorphic_embed_inputs_for/2 when working with LiveView.

<.form
  let={f}
  for={@changeset}
  id="reminder-form"
  phx-change="validate"
  phx-submit="save"
>
  <%= for channel_form <- polymorphic_embed_inputs_for f, :channel do %>
    <%= hidden_inputs_for(channel_form) %>

    <%= case get_polymorphic_type(channel_form, Reminder, :channel) do %>
      <% :sms -> %>
        <%= label channel_form, :number %>
        <%= text_input channel_form, :number %>

      <% :email -> %>
        <%= label channel_form, :email %>
        <%= text_input channel_form, :email %>
  <% end %>
</.form>

Using this function, you have to render the necessary hidden inputs manually as shown above.

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

The function Ecto.changeset.traverse_errors/2 won't include the errors of polymorphic embeds. You may instead use PolymorphicEmbed.traverse_errors/2 when working with polymorphic embeds.

• 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

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

def deps do
  [
    {:polymorphic_embed, "~> 3.0.5"}
  ]
end

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