add ash_tutorial

ash_tutorial/relationships.livemd

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+# Ash: 6 - Relationships
+
+```elixir
+Application.put_env(:ash, :validate_domain_resource_inclusion?, false)
+Application.put_env(:ash, :validate_domain_config_inclusion?, false)
+Mix.install([{:ash, "~> 3.0"}], consolidate_protocols: false)
+```
+
+## Relationships
+
+<div class="flex items-center w-full flex-start justify-between rounded-xl p-4" style="background-color: #f0f5f9; color: #61758a;">
+<div class="flex">
+<i class="ri-arrow-left-fill"></i>
+<a class="flex ml-2" style="color: #61758a;" href="customizing_actions.livemd">Customizing Actions</a>
+</div>
+<div class="flex">
+<i class="ri-home-fill"></i>
+<a class="flex ml-2" style="color: #61758a;" href="overview.livemd">Home</a>
+</div>
+<div class="flex">
+<a class="flex mr-2" style="color: #61758a;" href="code_interfaces.livemd">Code Interfaces</a>
+<i class="ri-arrow-right-fill"></i>
+</div>
+</div>
+
+### In this tutorial you will create a relationship between a Ticket and Representative resource
+
+Create a `relationships do .. end` block in the Ticket resource.
+
+Inside the `relationships` block, define a `belongs_to` representative like so:
+
+`belongs_to :representative, Tutorial.Support.Representative`
+
+<details class="rounded-lg my-4" style="background-color: #96ef86; color: #040604;">
+  <summary class="cursor-pointer font-bold p-4"></i>Show Solution</summary>
+  <div class="p-4">
+
+  ```elixir
+  defmodule Tutorial.Support.Ticket do
+
+  # ...
+
+    relationships do
+      # belongs_to means that the destination attribute is unique, meaning only one related record could exist.
+      # We assume that the destination attribute is `representative_id` based
+      # on the name of this relationship and that the source attribute is `representative_id`.
+      # We create `representative_id` automatically.
+      belongs_to :representative, Tutorial.Support.Representative
+    end
+
+  # ...
+  ```
+
+  </div>
+</details>
+
+### Enter your solution
+
+```elixir
+defmodule Tutorial.Support.Ticket do
+  use Ash.Resource,
+    domain: Tutorial.Support,
+    data_layer: Ash.DataLayer.Ets
+
+  actions do
+    defaults [:read]
+
+    create :create do
+      accept [:subject, :description, :status]
+    end
+  end
+
+  attributes do
+    uuid_primary_key :id
+    attribute :subject, :string, allow_nil?: false
+    attribute :description, :string
+
+    attribute :status, :atom do
+      constraints one_of: [:open, :closed]
+      default :open
+      allow_nil? false
+    end
+
+    create_timestamp :created_at
+    update_timestamp :updated_at
+  end
+
+  # <-- Add the relationship here
+end
+
+defmodule Tutorial.Support.Representative do
+  use Ash.Resource,
+    domain: Tutorial.Support,
+    data_layer: Ash.DataLayer.Ets
+
+  actions do
+    defaults [:read]
+
+    create :create do
+      accept [:name]
+    end
+  end
+
+  attributes do
+    uuid_primary_key(:id)
+    attribute(:name, :string)
+  end
+end
+```
+
+```elixir
+defmodule Tutorial.Support do
+  use Ash.Domain
+
+  resources do
+    resource Tutorial.Support.Ticket
+    resource Tutorial.Support.Representative
+  end
+end
+```
+
+## Creating a Ticket record with a representative Relationship
+
+First, create the representative Joe.
+
+```elixir
+# Creates a Representative
+joe =
+  Tutorial.Support.Representative
+  |> Ash.Changeset.for_create(:create, %{name: "Joe Armstrong"})
+  |> Ash.create!()
+```
+
+Next up, create a ticket and assign the representative Joe to the ticket.
+
+```elixir
+# Creates a Ticket with the representative
+Tutorial.Support.Ticket
+|> Ash.Changeset.for_create(:create, %{subject: "My spoon is too big!", representative_id: joe.id})
+|> Ash.create!()
+# `load!/2` loads in the representative. Try removing this line to see what changes
+|> Ash.load!([:representative])
+```
+
+As you can see it didn't quite work. The `representative_id` is not accepted by the create action on `Tutorial.Support.Ticket`. If you add `:representative_id` to that list,
+and try again you should now see that the representative was correctly assigned.
+
+<!-- livebook:{"break_markdown":true} -->
+
+<div class="flex items-center w-full flex-start justify-between rounded-xl p-4" style="background-color: #f0f5f9; color: #61758a;">
+<div class="flex">
+<i class="ri-arrow-left-fill"></i>
+<a class="flex ml-2" style="color: #61758a;" href="customizing_actions.livemd">Customizing Actions</a>
+</div>
+<div class="flex">
+<i class="ri-home-fill"></i>
+<a class="flex ml-2" style="color: #61758a;" href="overview.livemd">Home</a>
+</div>
+<div class="flex">
+<a class="flex mr-2" style="color: #61758a;" href="code_interfaces.livemd">Code Interfaces</a>
+<i class="ri-arrow-right-fill"></i>
+</div>
+</div>