Add Provide Fake Form Helper To Controllers as a Rails TIL

rails/provide-fake-form-helper-to-controllers.md

@@ -0,0 +1,40 @@
+# Provide Fake Form Helper To Controllers
+
+I'm rendering a partial from a turbo stream. The partial is meant to be
+rendered within a Rails form object because it contains an input element that
+needs to reference the form object. The problem is that from the controller
+that is streaming the partial, there is no
+[FormBuilder](https://api.rubyonrails.org/v6.1.0/classes/ActionView/Helpers/FormBuilder.html)
+object.
+
+One way to get around this that I've borrowed from [Justin
+Searls](https://justin.searls.co/posts/instantiate-a-custom-rails-formbuilder-without-using-form_with/)
+is with a `FauxFormHelper`.
+
+```ruby
+module FauxFormHelper
+  FauxFormObject = Struct.new do
+    def errors
+    end
+
+    def method_missing(...)
+    end
+
+    def respond_to_missing?(...)
+      true
+    end
+  end
+
+  def faux_form
+    @faux_form ||= ActionView::Helpers::FormBuilder.new(
+      nil,
+      FauxFormObject.new,
+      self,
+      {}
+    )
+  end
+end
+```
+
+This module defines and exposes a `faux_form` object that controllers and views
+can access. Then my partial can recieve that form object as a parameter.