test: add tweet creation + comment tests (13 tests)

Covers: create, blank content validation, guest restriction, read,
owner edit, non-owner edit forbidden, owner delete, non-owner delete
forbidden, reply creation, comment_count aggregate, tweet owner
deletes comment, third party forbidden, guest comment forbidden.

Key learnings:
- Tweet :destroy is not primary; use Ash.Changeset.for_destroy(:destroy)
- relate_actor fails with Invalid (not Forbidden) when no actor
- Ash.get returns NotFound error on miss; pass not_found_error?: false for nil

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

test/mixer/posts/tweet_test.exs

@@ -0,0 +1,220 @@
+defmodule Mixer.Posts.TweetTest do
+  use Mixer.DataCase, async: true
+
+  require Ash.Query
+
+  alias Mixer.Accounts.User
+  alias Mixer.Posts.Tweet
+
+  describe "tweet creation" do
+    test "a user can create a tweet" do
+      user = user_fixture("poster@example.com", "poster")
+
+      assert {:ok, tweet} =
+               Tweet
+               |> Ash.Changeset.for_create(:create, %{content: "hello world"}, actor: user)
+               |> Ash.create()
+
+      assert tweet.content == "hello world"
+      assert tweet.user_id == user.id
+      assert tweet.state == :posted
+      assert tweet.likes == 0
+    end
+
+    test "tweet content cannot be blank" do
+      user = user_fixture("blank@example.com", "blankuser")
+
+      assert {:error, error} =
+               Tweet
+               |> Ash.Changeset.for_create(:create, %{content: nil}, actor: user)
+               |> Ash.create()
+
+      assert Exception.message(error) =~ "content"
+    end
+
+    test "guests cannot create tweets" do
+      assert {:error, _error} =
+               Tweet
+               |> Ash.Changeset.for_create(:create, %{content: "spam"})
+               |> Ash.create()
+    end
+
+    test "all users can read tweets" do
+      user = user_fixture("readable@example.com", "readable")
+
+      Tweet
+      |> Ash.Changeset.for_create(:create, %{content: "public post"}, actor: user)
+      |> Ash.create!()
+
+      tweets = Tweet |> Ash.read!(authorize?: false)
+      assert length(tweets) >= 1
+    end
+  end
+
+  describe "tweet update" do
+    test "owner can edit their tweet" do
+      user = user_fixture("editor@example.com", "editor")
+      tweet = tweet_fixture(user, "original content")
+
+      assert {:ok, updated} =
+               tweet
+               |> Ash.Changeset.for_update(:update, %{content: "edited content"}, actor: user)
+               |> Ash.update()
+
+      assert updated.content == "edited content"
+    end
+
+    test "non-owner cannot edit a tweet" do
+      owner = user_fixture("owner@example.com", "tweetowner")
+      other = user_fixture("other@example.com", "otheruser")
+      tweet = tweet_fixture(owner, "owner's post")
+
+      assert {:error, error} =
+               tweet
+               |> Ash.Changeset.for_update(:update, %{content: "hacked"}, actor: other)
+               |> Ash.update()
+
+      assert Exception.message(error) =~ "forbidden"
+    end
+  end
+
+  describe "tweet deletion" do
+    test "owner can delete their tweet" do
+      user = user_fixture("deleter@example.com", "deleter")
+      tweet = tweet_fixture(user, "to be deleted")
+
+      assert :ok =
+               tweet
+               |> Ash.Changeset.for_destroy(:destroy, %{}, actor: user)
+               |> Ash.destroy()
+
+      assert {:ok, nil} = Tweet |> Ash.get(tweet.id, authorize?: false, not_found_error?: false)
+    end
+
+    test "non-owner cannot delete a tweet" do
+      owner = user_fixture("owner2@example.com", "owner2")
+      other = user_fixture("other2@example.com", "other2")
+      tweet = tweet_fixture(owner, "protected post")
+
+      assert {:error, error} =
+               tweet
+               |> Ash.Changeset.for_destroy(:destroy, %{}, actor: other)
+               |> Ash.destroy()
+
+      assert Exception.message(error) =~ "forbidden"
+    end
+  end
+
+  describe "comments (replies)" do
+    test "a user can reply to a tweet" do
+      author = user_fixture("author@example.com", "author")
+      replier = user_fixture("replier@example.com", "replier")
+      parent = tweet_fixture(author, "parent post")
+
+      assert {:ok, comment} =
+               Tweet
+               |> Ash.Changeset.for_create(
+                 :create,
+                 %{content: "great post!", parent_tweet_id: parent.id},
+                 actor: replier
+               )
+               |> Ash.create()
+
+      assert comment.parent_tweet_id == parent.id
+      assert comment.user_id == replier.id
+    end
+
+    test "comment_count reflects number of replies" do
+      author = user_fixture("countauthor@example.com", "countauthor")
+      replier = user_fixture("countreplier@example.com", "countreplier")
+      parent = tweet_fixture(author, "tweet with replies")
+
+      Tweet
+      |> Ash.Changeset.for_create(:create, %{content: "reply 1", parent_tweet_id: parent.id}, actor: replier)
+      |> Ash.create!()
+
+      Tweet
+      |> Ash.Changeset.for_create(:create, %{content: "reply 2", parent_tweet_id: parent.id}, actor: replier)
+      |> Ash.create!()
+
+      loaded = Tweet |> Ash.get!(parent.id, load: [:comment_count], authorize?: false)
+      assert loaded.comment_count == 2
+    end
+
+    test "tweet owner can delete a comment on their tweet" do
+      author = user_fixture("tweetowner3@example.com", "tweetowner3")
+      replier = user_fixture("commenter@example.com", "commenter")
+      parent = tweet_fixture(author, "parent tweet")
+
+      comment =
+        Tweet
+        |> Ash.Changeset.for_create(
+          :create,
+          %{content: "a comment", parent_tweet_id: parent.id},
+          actor: replier
+        )
+        |> Ash.create!()
+
+      # Tweet owner (author) can delete someone else's comment on their post
+      assert :ok =
+               comment
+               |> Ash.Changeset.for_destroy(:destroy, %{}, actor: author)
+               |> Ash.destroy()
+    end
+
+    test "a third party cannot delete a comment they don't own" do
+      author = user_fixture("tweetowner4@example.com", "tweetowner4")
+      replier = user_fixture("commenter2@example.com", "commenter2")
+      bystander = user_fixture("bystander@example.com", "bystander")
+      parent = tweet_fixture(author, "parent tweet 2")
+
+      comment =
+        Tweet
+        |> Ash.Changeset.for_create(
+          :create,
+          %{content: "a comment", parent_tweet_id: parent.id},
+          actor: replier
+        )
+        |> Ash.create!()
+
+      assert {:error, error} =
+               comment
+               |> Ash.Changeset.for_destroy(:destroy, %{}, actor: bystander)
+               |> Ash.destroy()
+
+      assert Exception.message(error) =~ "forbidden"
+    end
+
+    test "guests cannot post comments" do
+      author = user_fixture("tweetowner5@example.com", "tweetowner5")
+      parent = tweet_fixture(author, "parent post 3")
+
+      assert {:error, _error} =
+               Tweet
+               |> Ash.Changeset.for_create(
+                 :create,
+                 %{content: "spam comment", parent_tweet_id: parent.id}
+               )
+               |> Ash.create()
+    end
+  end
+
+  # ── helpers ───────────────────────────────────────────────────────────────
+
+  defp user_fixture(email, username) do
+    User
+    |> Ash.Changeset.for_create(:register_with_password, %{
+      email: email,
+      password: "password1234",
+      password_confirmation: "password1234",
+      username: username
+    })
+    |> Ash.create!(authorize?: false)
+  end
+
+  defp tweet_fixture(user, content) do
+    Tweet
+    |> Ash.Changeset.for_create(:create, %{content: content}, actor: user)
+    |> Ash.create!()
+  end
+end