From 5411565d44d9a60ed00ba3158abcb6ed80129b33 Mon Sep 17 00:00:00 2001
From: Petrik <petrik@deheus.net>
Date: Tue, 19 May 2020 09:03:46 +0200
Subject: [PATCH] Add DidYouMean for ParameterMissing

If a parameter isn't found we can suggest similar params:

```
class BooksController < ActionController::Base
  def create
    params.require(:book).require(:name)
    head :ok
  end
end

post :create, params: { magazine: { name: "Mjallo!" } }

param is missing or the value is empty: book
Did you mean?  controller
               action
               magazine

post :create, params: { book: { title: "Mjallo!" } }

param is missing or the value is empty: name
Did you mean?  title

```
---
 .../metal/strong_parameters.rb                | 32 ++++++++++++++++---
 .../test/controller/required_params_test.rb   | 14 ++++++++
 2 files changed, 42 insertions(+), 4 deletions(-)

diff --git a/actionpack/lib/action_controller/metal/strong_parameters.rb b/actionpack/lib/action_controller/metal/strong_parameters.rb
index a9aeb28256..d5acb929db 100644
--- a/actionpack/lib/action_controller/metal/strong_parameters.rb
+++ b/actionpack/lib/action_controller/metal/strong_parameters.rb
@@ -19,12 +19,36 @@ module ActionController
   #   params.require(:a)
   #   # => ActionController::ParameterMissing: param is missing or the value is empty: a
   class ParameterMissing < KeyError
-    attr_reader :param # :nodoc:
+    attr_reader :param, :keys # :nodoc:
 
-    def initialize(param) # :nodoc:
+    def initialize(param, keys = nil) # :nodoc:
       @param = param
+      @keys  = keys
       super("param is missing or the value is empty: #{param}")
     end
+
+    class Correction
+      def initialize(error)
+        @error = error
+      end
+
+      def corrections
+        if @error.param && @error.keys
+          maybe_these = @error.keys
+
+          maybe_these.sort_by { |n|
+            DidYouMean::Jaro.distance(@error.param.to_s, n)
+          }.reverse.first(4)
+        else
+          []
+        end
+      end
+    end
+
+    # We may not have DYM, and DYM might not let us register error handlers
+    if defined?(DidYouMean) && DidYouMean.respond_to?(:correct_error)
+      DidYouMean.correct_error(self, Correction)
+    end
   end
 
   # Raised when a supplied parameter is not expected and
@@ -480,7 +504,7 @@ module ActionController
       if value.present? || value == false
         value
       else
-        raise ParameterMissing.new(key)
+        raise ParameterMissing.new(key, @parameters.keys)
       end
     end
 
@@ -617,7 +641,7 @@ module ActionController
           if block_given?
             yield
           else
-            args.fetch(0) { raise ActionController::ParameterMissing.new(key) }
+            args.fetch(0) { raise ActionController::ParameterMissing.new(key, @parameters.keys) }
           end
         }
       )
diff --git a/actionpack/test/controller/required_params_test.rb b/actionpack/test/controller/required_params_test.rb
index 4a83d07e7d..c4ec4fdf74 100644
--- a/actionpack/test/controller/required_params_test.rb
+++ b/actionpack/test/controller/required_params_test.rb
@@ -22,6 +22,20 @@ class ActionControllerRequiredParamsTest < ActionController::TestCase
     end
   end
 
+  if defined?(DidYouMean) && DidYouMean.respond_to?(:correct_error)
+    test "exceptions have suggestions for fix" do
+      error = assert_raise ActionController::ParameterMissing do
+        post :create, params: { magazine: { name: "Mjallo!" } }
+      end
+      assert_match "Did you mean?", error.message
+
+      error = assert_raise ActionController::ParameterMissing do
+        post :create, params: { book: { title: "Mjallo!" } }
+      end
+      assert_match "Did you mean?", error.message
+    end
+  end
+
   test "required parameters that are present will not raise" do
     post :create, params: { book: { name: "Mjallo!" } }
     assert_response :ok