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.

defmoduleMyApp.ReminderdouseEcto.SchemaimportEcto.ChangesetimportPolymorphicEmbed,only: [cast_polymorphic_embed: 3]schema"reminders"dofield:date,:utc_datetimefield:text,:stringfield:channel,PolymorphicEmbed,types: [sms: MyApp.Channel.SMS,email: [module: MyApp.Channel.Email,identify_by_fields: [:address,:confirmed]]],on_type_not_found: :raise,on_replace: :updateenddefchangeset(struct,values)dostruct|>cast(values,[:date,:text])|>cast_polymorphic_embed(:channel,required: true)|>validate_required(:date)endend

defmoduleMyApp.Channel.EmaildouseEcto.SchemaimportEcto.Changeset@primary_keyfalseembedded_schemadofield:address,:stringfield:confirmed,:booleanenddefchangeset(email,params)doemail|>cast(params,~w(address confirmed)a)|>validate_required(:address)|>validate_length(:address,min: 4)endend

defmoduleMyApp.Channel.SMSdouseEcto.Schema@primary_keyfalseembedded_schemadofield:number,:stringendend

cast_polymorphic_embed/3

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

Options

• :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])

PolymorphicEmbed Ecto type

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_type_not_found: :raise,on_replace: :delete

Options

• :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).

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:

defviewdoquotedo# imports and stuffimportPolymorphicEmbed.HTML.Formendend

This provides you with a polymorphic_embed_inputs_for/4 function.

Here is an example form using the imported function:

<%=inputs_forf,:reminders,fnreminder_form -> %>
<%=polymorphic_embed_inputs_forreminder_form,:channel,:sms,fnsms_form -> %>
<divclass="sms-inputs"><label>Number<label>
<%=text_inputsms_form,:number%>
<divclass="error">
<%=error_tagsms_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

traverse_errors/2

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.

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:

defdepsdo[{:polymorphic_embed,"~> 1.8.0"}]end

HexDocs

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