[Feature] Add a configuration option to transform props coming from Inertia

docs/guide/configuration.md

@@ -44,6 +44,33 @@ Inertia Rails supports setting any configuration option via environment variable
 
 Use `component_path_resolver` to customize component path resolution when [`default_render`](#default_render) config value is set to `true`. The value should be callable and will receive the `path` and `action` parameters, returning a string component path. See [Automatically determine component name](/guide/responses#automatically-determine-component-name).
 
+### `prop_transformer`
+
+**Default**: `->(props:) { props }`
+
+Use `prop_transformer` to apply a transformation to your props before they're sent to the view. One use-case this enables is to work with `snake_case` props within Rails while working with `camelCase` in your view:
+
+```ruby
+  inertia_config(
+    prop_transformer: lambda do |props:|
+      props.deep_transform_keys { |key| key.to_s.camelize(:lower) }
+    end
+  )
+```
+
+> [!NOTE]
+> This controls the props provided by Inertia Rails but does not concern itself with props coming _into_ Rails. You may want to add a global `before_action` to `ApplicationController`:
+
+```ruby
+before_action :underscore_params
+
+# ...
+
+def underscore_params
+  params.deep_transform_keys! { |key| key.to_s.underscore }
+end
+```
+
 ### `deep_merge_shared_data`
 
 **Default**: `false`

lib/inertia_rails/configuration.rb

@@ -12,6 +12,9 @@ class Configuration
       # Allows the user to hook into the default rendering behavior and change it to fit their needs
       component_path_resolver: ->(path:, action:) { "#{path}/#{action}" },
 
+      # A function that transforms the props before they are sent to the client.
+      prop_transformer: ->(props:) { props },
+
       # DEPRECATED: Let Rails decide which layout should be used based on the
       # controller configuration.
       layout: true,
@@ -89,6 +92,10 @@ def component_path_resolver(path:, action:)
       @options[:component_path_resolver].call(path: path, action: action)
     end
 
+    def prop_transformer(props:)
+      @options[:prop_transformer].call(props: props)
+    end
+
     OPTION_NAMES.each do |option|
       unless method_defined?(option)
         define_method(option) do

lib/inertia_rails/renderer.rb

@@ -90,10 +90,13 @@ def merge_props(shared_props, props)
     end
 
     def computed_props
-      merged_props = merge_props(shared_data, props)
-      deep_transform_props(merged_props).tap do |transformed_props|
-        transformed_props[:_inertia_meta] = meta_tags if meta_tags.present?
-      end
+      merge_props(shared_data, props)
+        # This performs the internal work of hydrating/filtering props
+        .then { |props| deep_transform_props(props) }
+        # Then we apply the user-defined prop transformer
+        .then { |props| configuration.prop_transformer(props: props) }
+        # Then we add meta tags after everything since they must not be transformed
+        .tap { |props| props[:_inertia_meta] = meta_tags if meta_tags.present? }
     end
 
     def page

spec/dummy/app/controllers/inertia_prop_transformer_controller.rb

@@ -0,0 +1,32 @@
+# frozen_string_literal: true
+
+class InertiaPropTransformerController < ApplicationController
+  inertia_config(
+    prop_transformer: lambda do |props:|
+      props.deep_transform_keys { |key| key.to_s.upcase }
+    end
+  )
+
+  def just_props
+    render inertia: 'TestComponent', props: {
+      lower_prop: 'lower_value',
+      parent_hash: {
+        lower_child_prop: 'lower_child_value',
+      },
+    }
+  end
+
+  def props_and_meta
+    render inertia: 'TestComponent',
+           props: {
+             lower_prop: 'lower_value',
+           },
+           meta: [
+             { name: 'description', content: "Don't transform me!" }
+           ]
+  end
+
+  def no_props
+    render inertia: 'TestComponent'
+  end
+end

spec/dummy/config/routes.rb

@@ -42,6 +42,9 @@
   get 'instance_props_test' => 'inertia_rails_mimic#instance_props_test'
   get 'default_render_test' => 'inertia_rails_mimic#default_render_test'
   get 'transformed_default_render_test' => 'transformed_inertia_rails_mimic#render_test'
+  get 'prop_transformer_test' => 'inertia_prop_transformer#just_props'
+  get 'prop_transformer_with_meta_test' => 'inertia_prop_transformer#props_and_meta'
+  get 'prop_transformer_no_props_test' => 'inertia_prop_transformer#no_props'
   get 'default_component_test' => 'inertia_rails_mimic#default_component_test'
   get 'default_component_with_props_test' => 'inertia_rails_mimic#default_component_with_props_test'
   get 'default_component_with_duplicated_props_test' => 'inertia_rails_mimic#default_component_with_duplicated_props_test'

spec/inertia/prop_transformer_spec.rb

@@ -0,0 +1,60 @@
+# frozen_string_literal: true
+
+require_relative '../../lib/inertia_rails/rspec'
+RSpec.describe 'props can be transformed', type: :request, inertia: true do
+  let(:headers) do
+    {
+      'X-Inertia' => true,
+      'X-Inertia-Partial-Component' => 'TestComponent',
+    }
+  end
+
+  context 'props are provided' do
+    it 'transforms the props' do
+      get prop_transformer_test_path, headers: headers
+
+      expect_inertia.to render_component('TestComponent')
+        .and have_exact_props({
+          'LOWER_PROP' => 'lower_value',
+          'PARENT_HASH' => {
+            'LOWER_CHILD_PROP' => 'lower_child_value',
+          },
+        })
+    end
+  end
+
+  context 'props and meta are provided' do
+    it 'transforms the props' do
+      get prop_transformer_with_meta_test_path, headers: headers
+
+      expect_inertia.to render_component('TestComponent')
+        .and include_props({
+          'LOWER_PROP' => 'lower_value',
+        })
+    end
+
+    it 'does not transform the meta' do
+      get prop_transformer_with_meta_test_path, headers: headers
+
+      expect(response.parsed_body['props']['_inertia_meta']).to eq(
+        [
+          {
+            'tagName' => 'meta',
+            'name' => 'description',
+            'content' => "Don't transform me!",
+            'headKey' => 'meta-name-description',
+          }
+        ]
+      )
+    end
+  end
+
+  context 'no props are provided' do
+    it 'does not error' do
+      get prop_transformer_no_props_test_path, headers: headers
+
+      expect_inertia.to render_component('TestComponent')
+        .and have_exact_props({})
+    end
+  end
+end