Adding likes to tweets

test/mixer/posts/tweet_like_test.exs

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+defmodule Mixer.Posts.TweetLikeTest do
+  use Mixer.DataCase, async: true
+
+  import Ash.Expr
+
+  alias Mixer.Accounts.User
+  alias Mixer.Posts.Tweet
+  alias Mixer.Posts.TweetLike
+
+  describe "tweet likes" do
+    test "a user can like a tweet once and liked_by_me reflects the actor" do
+      user = user_fixture("liker@example.com")
+      tweet = tweet_fixture(user, "first post")
+
+      assert {:ok, liked_tweet} =
+               tweet
+               |> Ash.Changeset.for_update(:like, %{}, actor: user)
+               |> Ash.update()
+
+      assert liked_tweet.likes == 1
+      assert count_likes(tweet.id) == 1
+
+      tweet_for_actor =
+        Tweet
+        |> Ash.get!(tweet.id, actor: user, load: [:liked_by_me], authorize?: false)
+
+      refute Ash.ForbiddenField.forbidden?(tweet_for_actor.liked_by_me)
+      assert tweet_for_actor.liked_by_me
+
+      tweet_without_actor =
+        Tweet
+        |> Ash.get!(tweet.id, load: [:liked_by_me], authorize?: false)
+
+      refute Ash.ForbiddenField.forbidden?(tweet_without_actor.liked_by_me)
+      refute tweet_without_actor.liked_by_me
+    end
+
+    test "liking the same tweet twice does not create duplicate rows or inflate the counter" do
+      user = user_fixture("duplicate@example.com")
+      tweet = tweet_fixture(user, "duplicate like test")
+
+      assert {:ok, _tweet} =
+               tweet
+               |> Ash.Changeset.for_update(:like, %{}, actor: user)
+               |> Ash.update()
+
+      assert {:ok, liked_again} =
+               tweet
+               |> Ash.Changeset.for_update(:like, %{}, actor: user)
+               |> Ash.update()
+
+      assert liked_again.likes == 1
+      assert count_likes(tweet.id) == 1
+    end
+
+    test "unliking removes the relation and decrements the counter without going negative" do
+      user = user_fixture("unlike@example.com")
+      tweet = tweet_fixture(user, "unlike test")
+
+      tweet
+      |> Ash.Changeset.for_update(:like, %{}, actor: user)
+      |> Ash.update!()
+
+      assert {:ok, unliked_tweet} =
+               tweet
+               |> Ash.Changeset.for_update(:unlike, %{}, actor: user)
+               |> Ash.update()
+
+      assert unliked_tweet.likes == 0
+      assert count_likes(tweet.id) == 0
+
+      assert {:ok, still_unliked} =
+               tweet
+               |> Ash.Changeset.for_update(:unlike, %{}, actor: user)
+               |> Ash.update()
+
+      assert still_unliked.likes == 0
+      assert count_likes(tweet.id) == 0
+    end
+
+    test "guests cannot like tweets" do
+      owner = user_fixture("owner@example.com")
+      tweet = tweet_fixture(owner, "guest like test")
+
+      assert {:error, error} =
+               tweet
+               |> Ash.Changeset.for_update(:like, %{})
+               |> Ash.update()
+
+      assert Exception.message(error) =~ "forbidden"
+      assert count_likes(tweet.id) == 0
+    end
+  end
+
+  defp user_fixture(email) do
+    User
+    |> Ash.Changeset.for_create(:register_with_password, %{
+      email: email,
+      password: "password1234",
+      password_confirmation: "password1234"
+    })
+    |> Ash.create!()
+  end
+
+  defp tweet_fixture(user, content) do
+    Tweet
+    |> Ash.Changeset.for_create(:create, %{content: content}, actor: user)
+    |> Ash.create!()
+  end
+
+  defp count_likes(tweet_id) do
+    TweetLike
+    |> Ash.Query.filter(expr(tweet_id == ^tweet_id))
+    |> Ash.read!(authorize?: false)
+    |> length()
+  end
+end