# 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.
```elixir
defmodule MyApp.Reminder do
use Ecto.Schema
import Ecto.Changeset
import PolymorphicEmbed, only: [cast_polymorphic_embed: 3]
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_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
```
```elixir
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
```
```elixir
defmodule MyApp.Channel.SMS do
use Ecto.Schema
@primary_key false
embedded_schema do
field :number, :string
end
end
```
### `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.
### 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.
```elixir
[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.
```elixir
[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 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:
```elixir
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:
```elixir
<%= 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:
```elixir
def deps do
[
{:polymorphic_embed, "~> 1.3.3"}
]
end
```
## HexDocs
HexDocs documentation can be found at [https://hexdocs.pm/polymorphic_embed](https://hexdocs.pm/polymorphic_embed).
