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# Define Polymorphic Relationships
```elixir
Mix.install([{:ash, "~> 3.0"}], consolidate_protocols: false)
```
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```
:ok
```
## Introduction
Something that comes up in more complex domains is the idea of "polymorphic relationships". For this example, we will use the concept of a `BankAccount`, which can be either a `SavingsAccount` or a `CheckingAccount`. All accounts have an `account_number` and `transactions`, but checkings & savings accounts might have their own specific information. For example, a `SavingsAccount` has an `interest_rate`, and a `checkings_account` has many `DebitCard`s.
Ash does not support polymorphic relationships defined _as relationships_, but you can accomplish similar functionality via [calculations](documentation/topics/resources/calculations.md) with the type `Ash.Type.Union`.
For this tutorial, we will have a dedicated resource called `BankAccount`. I suggest taking that approach, as many things down the road will be simplified. With that said, you don't necessarily need to do that when there is no commonalities between the types. Instead of setting up the polymorphism on the `BankAccount` resource, you would define relationships to `SavingsAccount` and `CheckingAccount` directly.
This tutorial is not attempting to illustrate good design of accounting systems. We make many concessions for the sake of the simplicity of our example.
## Defining our Resources
```elixir
defmodule BankAccount do
use Ash.Resource,
domain: Finance,
data_layer: Ash.DataLayer.Ets
actions do
defaults [:read, :destroy, create: [:account_number, :type]]
end
attributes do
uuid_primary_key :id
attribute :account_number, :integer, allow_nil?: false
attribute :type, :atom, constraints: [one_of: [:checking, :savings]]
end
# calculations do
# calculate :implementation, AccountImplementation, GetAccountImplementation do
# allow_nil? false
# end
# end
relationships do
has_one :checking_account, CheckingAccount
has_one :savings_account, SavingsAccount
end
end
defmodule CheckingAccount do
use Ash.Resource,
domain: Finance,
data_layer: Ash.DataLayer.Ets
actions do
defaults [:read, :destroy, create: [:bank_account_id]]
end
attributes do
uuid_primary_key :id
end
identities do
identity :unique_bank_account, [:bank_account_id], pre_check?: true
end
relationships do
belongs_to :bank_account, BankAccount do
allow_nil? false
end
end
end
defmodule SavingsAccount do
use Ash.Resource,
domain: Finance,
data_layer: Ash.DataLayer.Ets
actions do
defaults [:read, :destroy, create: [:bank_account_id]]
end
attributes do
uuid_primary_key :id
end
identities do
identity :unique_bank_account, [:bank_account_id], pre_check?: true
end
relationships do
belongs_to :bank_account, BankAccount do
allow_nil? false
end
end
end
defmodule Finance do
use Ash.Domain,
validate_config_inclusion?: false
resources do
resource BankAccount
resource SavingsAccount
resource CheckingAccount
end
end
```
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```
{:module, Finance, <<70, 79, 82, 49, 0, 0, 44, ...>>,
[
Ash.Domain.Dsl.Resources.Resource,
Ash.Domain.Dsl.Resources.Options,
%{opts: [], entities: [%Ash.Domain.Dsl.ResourceReference{...}, ...]}
]}
```
We haven't implemented the polymorphic part yet, but lets create a few of the above resources to show how they relate. Below we create a `BankAccount` for checkings, and a `BankAccount` for savings, and connect them to their "specific" types, i.e `CheckingAccount` and `SavingsAccount`.
We load the data, you can see that one `BankAccount` has a `:checking_account` but no `:savings_account`. For the other, the opposite is the case.
```elixir
bank_account1 = Ash.create!(BankAccount, %{account_number: 1, type: :checking})
bank_account2 = Ash.create!(BankAccount, %{account_number: 2, type: :savings})
checking_account = Ash.create!(CheckingAccount, %{bank_account_id: bank_account1.id})
savings_account = Ash.create!(SavingsAccount, %{bank_account_id: bank_account2.id})
[bank_account1, bank_account2] |> Ash.load!([:checking_account, :savings_account])
```
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```
[
#BankAccount<
implementation: #Ash.NotLoaded<:calculation, field: :implementation>,
savings_account: nil,
checking_account: #CheckingAccount<
bank_account: #Ash.NotLoaded<:relationship, field: :bank_account>,
__meta__: #Ecto.Schema.Metadata<:loaded>,
id: "60f585f6-f352-410e-8c09-09ae49448851",
bank_account_id: "21d27a6c-49b8-4984-a1b7-f2ef030626af",
aggregates: %{},
calculations: %{},
...
>,
__meta__: #Ecto.Schema.Metadata<:loaded>,
id: "21d27a6c-49b8-4984-a1b7-f2ef030626af",
account_number: 1,
type: :checking,
aggregates: %{},
calculations: %{},
...
>,
#BankAccount<
implementation: #Ash.NotLoaded<:calculation, field: :implementation>,
savings_account: #SavingsAccount<
bank_account: #Ash.NotLoaded<:relationship, field: :bank_account>,
__meta__: #Ecto.Schema.Metadata<:loaded>,
id: "d2bcc1cc-d709-418d-a9ff-0d21fd7d667b",
bank_account_id: "4d8dc988-3e09-4efb-a643-d822013abfba",
aggregates: %{},
calculations: %{},
...
>,
checking_account: nil,
__meta__: #Ecto.Schema.Metadata<:loaded>,
id: "4d8dc988-3e09-4efb-a643-d822013abfba",
account_number: 2,
type: :savings,
aggregates: %{},
calculations: %{},
...
>
]
```
## Defining our union type
Below we define an `Ash.Type.NewType`. This allows defining a new type that is the combination of an existing type and custom constraints.
```elixir
defmodule AccountImplementation do
use Ash.Type.NewType, subtype_of: :union, constraints: [
checking: [
type: :struct,
constraints: [instance_of: CheckingAccount]
],
savings: [
type: :struct,
constraints: [instance_of: SavingsAccount]
]
]
end
```
<!-- livebook:{"output":true} -->
```
{:module, AccountImplementation, <<70, 79, 82, 49, 0, 0, 44, ...>>, :ok}
```
## Defining the calculation
Next, we'll define a calculation resolves to the specific type of any given account.
```elixir
defmodule GetAccountImplementation do
use Ash.Resource.Calculation
def load(_, _, _) do
[:checking_account, :savings_account]
end
def calculate(records, _, _) do
Enum.map(records, &(&1.checking_account || &1.savings_account))
end
end
```
<!-- livebook:{"output":true} -->
```
{:module, GetAccountImplementation, <<70, 79, 82, 49, 0, 0, 13, ...>>, {:calculate, 3}}
```
## Adding the calculation to our resource
Finally, we'll add the calculation to our `BankAccount` resource!
For those following along with the LiveBook, go back up and uncomment the commented out calculation.
Now we can load `:implementation` and see that, for one account, it resolves to a `CheckingAccount` and for the other it resolves to a `SavingsAccount`.
```elixir
bank_account1 = Ash.create!(BankAccount, %{account_number: 1, type: :checking})
bank_account2 = Ash.create!(BankAccount, %{account_number: 2, type: :savings})
checking_account = Ash.create!(CheckingAccount, %{bank_account_id: bank_account1.id})
savings_account = Ash.create!(SavingsAccount, %{bank_account_id: bank_account2.id})
[bank_account1, bank_account2] |> Ash.load!([:implementation])
```
<!-- livebook:{"output":true} -->
```
[
#BankAccount<
implementation: #CheckingAccount<
bank_account: #Ash.NotLoaded<:relationship, field: :bank_account>,
__meta__: #Ecto.Schema.Metadata<:loaded>,
id: "1d1a7b6c-dd08-4b8c-9769-2c1155a41a40",
bank_account_id: "ce370381-303e-49dc-9950-a05b5052e7f8",
aggregates: %{},
calculations: %{},
...
>,
savings_account: #Ash.NotLoaded<:relationship, field: :savings_account>,
checking_account: #Ash.NotLoaded<:relationship, field: :checking_account>,
__meta__: #Ecto.Schema.Metadata<:loaded>,
id: "ce370381-303e-49dc-9950-a05b5052e7f8",
account_number: 1,
type: :checking,
aggregates: %{},
calculations: %{},
...
>,
#BankAccount<
implementation: #SavingsAccount<
bank_account: #Ash.NotLoaded<:relationship, field: :bank_account>,
__meta__: #Ecto.Schema.Metadata<:loaded>,
id: "e537116d-c5b8-414a-a350-fa9b2afe0a4e",
bank_account_id: "c4739158-2a1f-4de8-bccb-ee1d1ee9e602",
aggregates: %{},
calculations: %{},
...
>,
savings_account: #Ash.NotLoaded<:relationship, field: :savings_account>,
checking_account: #Ash.NotLoaded<:relationship, field: :checking_account>,
__meta__: #Ecto.Schema.Metadata<:loaded>,
id: "c4739158-2a1f-4de8-bccb-ee1d1ee9e602",
account_number: 2,
type: :savings,
aggregates: %{},
calculations: %{},
...
>
]
```
## Taking it further
One of the best things about using `Ash.Type.Union` is how it is *integrated*. Every extension (provided by the Ash team) supports working with unions. For example:
* [Working with Unions in AshPhoenix.Form](https://hexdocs.pm/ash_phoenix/union-forms.html)
* [AshGraphql & AshJsonApi Unions](https://hexdocs.pm/ash_graphql/use-unions-with-graphql.html)
You can also synthesize filterable fields with calculations. For example, if you wanted to allow filtering `BankAccount` by `:interest_rate`, and that field only existed on `SavingsAccount`, you might have a calculation like this on `BankAccount`:
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```elixir
calculate :interest_rate, :decimal, expr(
if type == :savings_account do
savings_account.interest_rate
else
0
end
)
```
This would allow usage like the following:
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```elixir
BankAccount
|> Ash.Query.filter(interest_rate > 0.01)
|> Ash.read!()
```
