View Source 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
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, [:address, :confirmed])
|> validate_required(:address)
|> validate_length(:address, min: 4)
end
end
defmodule MyApp.Channel.SMS do
use Ecto.Schema
import Ecto.Changeset
@primary_key false
embedded_schema do
field :number, :string
end
def changeset(sms, params) do
sms
|> cast(params, [:number])
|> validate_required(:number)
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
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.changeset |> cast_polymorphic_embed(:channel, with: [ sms: &SMS.custom_changeset/2, email: &Email.custom_changeset/2 ] )
:drop_param
– see sorting-and-deleting-from-many-collections.:sort_param
– see sorting-and-deleting-from-many-collections.:default_type_on_sort_create
– in some cases,sort
creates a new entry; this option specifies which type to use by default for the entry.
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:
1.
[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"
).
2.
[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:
polymorphic_embeds_many :contexts,
types: [
location: MyApp.Context.Location,
age: MyApp.Context.Age,
device: MyApp.Context.Device
],
on_replace: :delete,
on_type_not_found: :raise,
nilify_unlisted_types_on_load: [:deprecated_type]
Options
:types
– discussed above.:type_field_name
– specify a custom field name for the field holding the type. Defaults to:__type__
.:use_parent_field_for_type
– fetch the type from a specified field in the parent schema (the schema holding the embed).: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 bynil
; 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 forembeds_one
andembeds_many
is:raise
).:retain_unlisted_types_on_load
: allow unconfigured types to be loaded without raising an error. Useful for handling deprecated structs still present in the database.:nilify_unlisted_types_on_load
: same as:retain_unlisted_types_on_load
, but nilify the struct on load.
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, change view
to the one your component use:
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.
Displaying form inputs and errors in LiveView
You may use PolymorphicEmbed.HTML.Component.polymorphic_embed_inputs_for/1
when working with LiveView, which functions similarly to Phoenix.Component.inputs_for/1
.
<.form
:let={f}
for={@changeset}
id="reminder-form"
phx-change="validate"
phx-submit="save"
>
<.polymorphic_embed_inputs_for field={f[:channel]} :let={channel_form}>
<%= case source_module(channel_form) do %>
<% SMS -> %>
<.input field={channel_form[:number]} label="Number" />
<% Email -> %>
<.input field={channel_form[:address]} label="Email Address" />
<% end %>
</.polymorphic_embed_inputs_for>
</.form>
Get the type and module of a polymorphic embed
Sometimes you need to serialize the polymorphic embed and, once in the front-end, need to distinguish them.
PolymorphicEmbed.get_polymorphic_type/3
returns the type of the polymorphic embed:
PolymorphicEmbed.get_polymorphic_type(Reminder, :channel, SMS) == :sms
To get the module for a specific type, use:
PolymorphicEmbed.get_polymorphic_module(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.
Installation
Add polymorphic_embed
for Elixir as a dependency in your mix.exs
file:
def deps do
[
{:polymorphic_embed, "~> 5.0"}
]
end
HexDocs
HexDocs documentation can be found at https://hexdocs.pm/polymorphic_embed.