fix a bug and add more tests

app/models/assistant/slug.rb

@@ -13,8 +13,8 @@ def clear_conflicting_deleted_assistant_slug
     return if slug.blank?
     return if !slug_changed?
 
-    conflicting_assistant = user.assistants_including_deleted.where.not(id: id, deleted_at: nil).find_by(slug: slug)
-    conflicting_assistant&.update_column(:slug, nil)
+    conflicting_assistant = user.assistants_including_deleted.where.not(deleted_at: nil).find_by(slug: slug)
+    conflicting_assistant&.update_column(:slug, nil) if conflicting_assistant != self
   end
 
   def set_default_slug

test/models/assistant/slug_test.rb

@@ -77,4 +77,22 @@ class Assistant::SlugTest < ActiveSupport::TestCase
     assistant.update!(name: "New Name")
     assert_equal original_slug, assistant.reload.slug
   end
+
+  test "fails to create a new assistant when slug collides with an existing assistant" do
+    existing = assistants(:samantha)
+    new_assistant = existing.user.assistants.new(
+      name: "Different Name",
+      slug: existing.slug,
+      language_model: existing.language_model
+    )
+    assert_not new_assistant.valid?
+    assert_includes new_assistant.errors[:slug], "has already been taken"
+  end
+
+  test "fails to update an assistant's slug when it collides with an existing assistant" do
+    existing = assistants(:samantha)
+    other = assistants(:keith_gpt4)
+    assert_not other.update(slug: existing.slug)
+    assert_includes other.errors[:slug], "has already been taken"
+  end
 end