Prevent invalid code when using dynamic finders with Ruby's reserved words.

The dynamic finder was creating the method signature with the parameters name,
which may have reserved words and this way creating invalid Ruby code.

Closes: #13261

    Example:

        # Before
        Dog.find_by_alias('dog name')

        # Was creating this method
        def self.find_by_alias(alias, options = {})

        # After
        Dog.find_by_alias('dog name')

        # Will create this method
        def self.find_by_alias(_alias, options = {})

activerecord/lib/active_record/dynamic_matchers.rb

@@ -84,13 +84,18 @@ module ActiveRecord
         "#{finder}(#{attributes_hash})"
       end
 
+      # The parameters in the signature may have reserved Ruby words, in order
+      # to prevent errors, we start each param name with `_`.
+      #
       # Extended in activerecord-deprecated_finders
       def signature
-        attribute_names.join(', ')
+        attribute_names.map { |name| "_#{name}" }.join(', ')
       end
 
+      # Given that the parameters starts with `_`, the finder needs to use the
+      # same parameter name.
       def attributes_hash
-        "{" + attribute_names.map { |name| ":#{name} => #{name}" }.join(',') + "}"
+        "{" + attribute_names.map { |name| ":#{name} => _#{name}" }.join(',') + "}"
       end
 
       def finder

activerecord/test/cases/finder_test.rb

@@ -12,6 +12,7 @@ require 'models/developer'
 require 'models/customer'
 require 'models/toy'
 require 'models/matey'
+require 'models/dog'
 
 class FinderTest < ActiveRecord::TestCase
   fixtures :companies, :topics, :entrants, :developers, :developers_projects, :posts, :comments, :accounts, :authors, :customers, :categories, :categorizations
@@ -641,6 +642,13 @@ class FinderTest < ActiveRecord::TestCase
     assert_raise(ActiveRecord::RecordNotFound) { Topic.find_by_title!("The First Topic!") }
   end
 
+  def test_find_by_on_attribute_that_is_a_reserved_word
+    dog_alias = 'Dog'
+    dog = Dog.create(alias: dog_alias)
+
+    assert_equal dog, Dog.find_by_alias(dog_alias)
+  end
+
   def test_find_by_one_attribute_that_is_an_alias
     assert_equal topics(:first), Topic.find_by_heading("The First Topic")
     assert_nil Topic.find_by_heading("The First Topic!")

activerecord/test/schema/schema.rb

@@ -245,6 +245,7 @@ ActiveRecord::Schema.define do
     t.integer :trainer_id
     t.integer :breeder_id
     t.integer :dog_lover_id
+    t.string  :alias
   end
 
   create_table :edges, force: true, id: false do |t|