Add Enforce Locals Passed To A Partial as a Rails TIL

README.md

@@ -10,7 +10,7 @@ pairing with smart people at Hashrocket.
 
 For a steady stream of TILs, [sign up for my newsletter](https://crafty-builder-6996.ck.page/e169c61186).
 
-_1599 TILs and counting..._
+_1600 TILs and counting..._
 
 See some of the other learning resources I work on:
 - [Ruby Operator Lookup](https://www.visualmode.dev/ruby-operators)
@@ -1004,6 +1004,7 @@ See some of the other learning resources I work on:
 - [Different Ways To Add A Foreign Key Reference](rails/different-ways-to-add-a-foreign-key-reference.md)
 - [Disambiguate Where In A Joined Relation](rails/disambiguate-where-in-a-joined-relation.md)
 - [Empty find_by Returns First Record](rails/empty-find-by-returns-first-record.md)
+- [Enforce Locals Passed To A Partial](rails/enforce-locals-passed-to-a-partial.md)
 - [Ensure A Rake Task Cannot Write Data](rails/ensure-a-rake-task-cannot-write-data.md)
 - [Ensure Migrations Use The Latest Schema](rails/ensure-migrations-use-the-latest-schema.md)
 - [Ensure Record Saved With after_commit Callback](rails/ensure-record-saved-with-after-commit-callback.md)

rails/enforce-locals-passed-to-a-partial.md

@@ -0,0 +1,42 @@
+# Enforce Locals Passed To A Partial
+
+I have a big form partial (`_form.html.erb`) that is rendered by several
+different _new_ and _edit_ views. It's hard to tell at a glance, but the
+partial requires that `blogmark` and `cancel_path` are included as locals when
+it is rendered.
+
+As of [Rails 7.1](https://github.com/rails/rails/pull/45602), we now have a
+built-in way to enforce locals passed to a partial. We can add a magic comment
+at the top of the partial that specifies the locals:
+
+```ruby
+<%# locals: (blogmark:, cancel_path:) -%>
+<%= form_with(model: blogmark, local: true, class: "w-full max-w-3xl mb-8") do |form| %>
+  <% ... %>
+<% end  %>
+```
+
+This particular ERB magic comment declares that
+[`blogmark`](https://still.visualmode.dev/blogmarks) and `cancel_path` are
+required locals.
+
+So, what happens if I have a `new.html.erb` view that looks like this:
+
+```ruby
+<h1>New Blogmark</h1>
+
+<%= render 'blogmarks/form', blogmark: @blogmark %>
+```
+
+When I try to view the page, an error is raised:
+
+```
+Showing /Some/path/app/views/blogmarks/_form.html.erb where line # raised:
+
+missing local: :cancel_path for app/views/blogmarks/_form.html.erb
+```
+
+Updating the `render` statement to include a `cancel_path` local fixes the
+issue.
+
+[source](https://gorails.com/episodes/template-locals-in-rails-7-1)