Adding likes to tweets

lib/mixer/posts/tweet.ex

@@ -1,4 +1,7 @@
 defmodule Mixer.Posts.Tweet do
+  import Ash.Expr
+  require Ash.Query
+
   use Ash.Resource,
     otp_app: :mixer,
     domain: Mixer.Posts,
@@ -25,7 +28,7 @@ defmodule Mixer.Posts.Tweet do
   end
 
   actions do
-    defaults [:read, :destroy, update: :*]
+    defaults [:read, :destroy]
 
     create :create do
       upsert? true
@@ -50,6 +53,62 @@ defmodule Mixer.Posts.Tweet do
         end
       end
     end
+
+    update :update do
+      accept [:content]
+    end
+
+    update :like do
+      accept []
+      require_atomic? false
+
+      change fn changeset, context ->
+        Ash.Changeset.after_action(changeset, fn _changeset, tweet ->
+          case ensure_like(tweet, context.actor) do
+            {:created, _like} ->
+              increment_likes(tweet, context.actor)
+
+            {:noop, _like} ->
+              {:ok, tweet}
+
+            {:error, error} ->
+              {:error, error}
+          end
+        end)
+      end
+    end
+
+    update :unlike do
+      accept []
+      require_atomic? false
+
+      change fn changeset, context ->
+        Ash.Changeset.after_action(changeset, fn _changeset, tweet ->
+          case remove_like(tweet, context.actor) do
+            {:deleted, _like} ->
+              decrement_likes(tweet, context.actor)
+
+            {:noop, _like} ->
+              {:ok, tweet}
+
+            {:error, error} ->
+              {:error, error}
+          end
+        end)
+      end
+    end
+
+    update :increment_likes do
+      accept []
+      require_atomic? false
+      change atomic_update(:likes, expr(likes + 1))
+    end
+
+    update :decrement_likes do
+      accept []
+      require_atomic? false
+      change atomic_update(:likes, expr(likes - 1))
+    end
   end
 
   attributes do
@@ -83,6 +142,15 @@ defmodule Mixer.Posts.Tweet do
     has_many :media, Mixer.Posts.Media do
       public? true
     end
+
+    has_many :tweet_likes, Mixer.Posts.TweetLike
+  end
+
+  aggregates do
+    exists :liked_by_me, :tweet_likes do
+      public? true
+      filter expr(user_id == ^actor(:id))
+    end
   end
 
   policies do
@@ -94,8 +162,93 @@ defmodule Mixer.Posts.Tweet do
       authorize_if actor_present()
     end
 
-    policy action_type([:destroy, :update]) do
+    policy action(:update) do
       authorize_if relates_to_actor_via(:user)
     end
+
+    policy action(:destroy) do
+      authorize_if relates_to_actor_via(:user)
+    end
+
+    policy action(:like) do
+      authorize_if actor_present()
+    end
+
+    policy action(:unlike) do
+      authorize_if actor_present()
+    end
+  end
+
+  defp ensure_like(_tweet, nil), do: {:error, Ash.Error.Forbidden.exception([])}
+
+  defp ensure_like(tweet, actor) do
+    case get_like(tweet.id, actor.id) do
+      {:ok, nil} ->
+        case create_like(tweet.id, actor) do
+          {:ok, like} ->
+            {:created, like}
+
+          {:error, error} ->
+            case get_like(tweet.id, actor.id) do
+              {:ok, nil} ->
+                {:error, error}
+
+              {:ok, like} ->
+                {:noop, like}
+
+              {:error, error} ->
+                {:error, error}
+            end
+        end
+
+      {:ok, like} ->
+        {:noop, like}
+
+      {:error, error} ->
+        {:error, error}
+    end
+  end
+
+  defp remove_like(_tweet, nil), do: {:error, Ash.Error.Forbidden.exception([])}
+
+  defp remove_like(tweet, actor) do
+    case get_like(tweet.id, actor.id) do
+      {:ok, nil} ->
+        {:noop, nil}
+
+      {:ok, like} ->
+        case Ash.destroy(like, actor: actor) do
+          :ok -> {:deleted, like}
+          {:ok, _destroyed_like} -> {:deleted, like}
+          {:error, error} -> {:error, error}
+        end
+
+      {:error, error} ->
+        {:error, error}
+    end
+  end
+
+  defp create_like(tweet_id, actor) do
+    Mixer.Posts.TweetLike
+    |> Ash.Changeset.for_create(:create, %{tweet_id: tweet_id}, actor: actor)
+    |> Ash.create()
+  end
+
+  defp get_like(tweet_id, user_id) do
+    Mixer.Posts.TweetLike
+    |> Ash.Query.filter(expr(tweet_id == ^tweet_id and user_id == ^user_id))
+    |> Ash.read_one(authorize?: false)
+  end
+
+  defp increment_likes(tweet, actor) do
+    tweet
+    |> Ash.Changeset.for_update(:increment_likes, %{}, actor: actor)
+    |> Ash.update(authorize?: false)
+  end
+
+  defp decrement_likes(tweet, actor) do
+    tweet
+    |> Ash.Changeset.for_update(:decrement_likes, %{}, actor: actor)
+    |> Ash.update(authorize?: false)
   end
 end

lib/mixer/posts/tweet_like.ex

@@ -0,0 +1,65 @@
+defmodule Mixer.Posts.TweetLike do
+  use Ash.Resource,
+    otp_app: :mixer,
+    domain: Mixer.Posts,
+    data_layer: AshPostgres.DataLayer,
+    authorizers: [Ash.Policy.Authorizer]
+
+  postgres do
+    table "tweet_likes"
+    repo Mixer.Repo
+  end
+
+  actions do
+    defaults [:read, :destroy]
+
+    create :create do
+      accept [:tweet_id]
+      change relate_actor(:user)
+    end
+  end
+
+  attributes do
+    uuid_primary_key :id
+
+    attribute :tweet_id, :uuid do
+      allow_nil? false
+    end
+
+    attribute :user_id, :uuid do
+      allow_nil? false
+    end
+  end
+
+  relationships do
+    belongs_to :tweet, Mixer.Posts.Tweet do
+      attribute_type :uuid
+      attribute_writable? true
+      allow_nil? false
+    end
+
+    belongs_to :user, Mixer.Accounts.User do
+      attribute_type :uuid
+      attribute_writable? true
+      allow_nil? false
+    end
+  end
+
+  identities do
+    identity :unique_user_tweet, [:tweet_id, :user_id]
+  end
+
+  policies do
+    policy action_type(:read) do
+      authorize_if always()
+    end
+
+    policy action(:create) do
+      authorize_if actor_present()
+    end
+
+    policy action_type(:destroy) do
+      authorize_if relates_to_actor_via(:user)
+    end
+  end
+end