# 🧬 Polyjuice
A custom type that maps polymorphic data to different Ecto schemas based on a type field.
`Polyjuice` allows you to store different types of data in a single field, where each
type is validated and cast to its own embedded schema.
## Example
```elixir
defmodule Activity do
use Ecto.Schema
import Ecto.Changeset
schema "activities" do
field :title, :string
field :event, Polyjuice, schemas: %{
activated: ActivatedEvent,
cancelled: CancelledEvent
}
end
def changeset(activity, attrs) do
activity
|> cast(attrs, [:title])
|> Polyjuice.cast_embed(:event)
|> validate_required([:title, :event])
end
end
defmodule ActivatedEvent do
use Ecto.Schema
import Ecto.Changeset
@primary_key false
embedded_schema do
field(:type, :string, default: "activated")
field(:user_id, :integer)
field(:activated_at, :utc_datetime)
end
def changeset(activated, attrs) do
activated
|> cast(attrs, [:type, :user_id])
|> validate_required([:user_id, :activated_at])
|> validate_number(:user_id, greater_than: 0)
end
end
defmodule CancelledEvent do
use Ecto.Schema
import Ecto.Changeset
@primary_key false
embedded_schema do
field(:type, :string, default: "cancelled")
field(:user_id, :integer)
field(:reason, :string)
end
def changeset(cancelled, attrs) do
cancelled
|> cast(attrs, [:type, :user_id, :reason])
|> validate_required([:user_id, :reason])
|> validate_number(:user_id, greater_than: 0)
|> validate_inclusion(:reason, [
"user_request",
"system_timeout",
"payment_failed",
"fraud_detected"
])
end
end
```
## Usage
```elixir
%Activity{
title: "User Action",
event: %ActivatedEvent{
type: "activated",
user_id: 123,
activated_at: DateTime.utc_now()
}
}
|> Repo.insert()
```
## Caveat(s)
1) Map-based input & `Ecto.Changeset.cast/3`
Polyjuice does not currently support map/json-based input, only structs are allowed, thus the following won't work:
```elixir
%Activity{}
|> Activity.changeset(%{
title: "User Action",
event: %{
"type" => "activated",
"user_id" => 123,
"activated_at" => DateTime.utc_now()
}
})
|> Repo.insert()
```
2) Schema evolution
Be very careful about reading & writing back into database, currently this can be a lossy conversion.
For example, if your event has an extra field, when you read it back out, it'll not contain that field, and when you try to update this, it'll lose the extra field.
We are currently using it as an append-only database table with polymorphic embed, but we're open to ideas to make it more robust.
## Installation
If [available in Hex](https://hex.pm/docs/publish), the package can be installed
by adding `polyjuice` to your list of dependencies in `mix.exs`:
```elixir
def deps do
[
{:polyjuice, "~> 0.1.0"}
]
end
```
