Generate indexed names in input even when objects are not persisted

When you ask to generate multiple nested inputs using:

    field_for('comments[]', Comment.new) do |c|
      c.text_field :body

Rails should generated the names like `post[comments][][body]`.

To make sure we don't have regression the fake models now use the same
implementation of `#to_param` as `ActiveRecord::Base`

Fixes #26942

actionview/lib/action_view/helpers/tags/base.rb

@@ -16,7 +16,14 @@ module ActionView
           @skip_default_ids = options.delete(:skip_default_ids)
           @allow_method_names_outside_object = options.delete(:allow_method_names_outside_object)
           @options = options
-          @auto_index = Regexp.last_match ? retrieve_autoindex(Regexp.last_match.pre_match) : nil
+
+          if Regexp.last_match
+            @generate_indexed_names = true
+            @auto_index = retrieve_autoindex(Regexp.last_match.pre_match)
+          else
+            @generate_indexed_names = false
+            @auto_index = nil
+          end
         end
 
         # This is what child classes implement.
@@ -167,7 +174,11 @@ module ActionView
           end
 
           def name_and_id_index(options)
-            options.key?("index") ? options.delete("index") || "" : @auto_index
+            if options.key?("index")
+              options.delete("index") || ""
+            elsif @generate_indexed_names
+              @auto_index || ""
+            end
           end
 
           def skip_default_ids?

actionview/test/lib/controller/fake_models.rb

@@ -80,7 +80,7 @@ class Comment
   def to_key; id ? [id] : nil end
   def save; @id = 1; @post_id = 1 end
   def persisted?; @id.present? end
-  def to_param; @id.to_s; end
+  def to_param; @id && @id.to_s; end
   def name
     @id.nil? ? "new #{self.class.name.downcase}" : "#{self.class.name.downcase} ##{@id}"
   end
@@ -101,7 +101,7 @@ class Tag
   def to_key; id ? [id] : nil end
   def save; @id = 1; @post_id = 1 end
   def persisted?; @id.present? end
-  def to_param; @id; end
+  def to_param; @id && @id.to_s; end
   def value
     @id.nil? ? "new #{self.class.name.downcase}" : "#{self.class.name.downcase} ##{@id}"
   end
@@ -120,7 +120,7 @@ class CommentRelevance
   def to_key; id ? [id] : nil end
   def save; @id = 1; @comment_id = 1 end
   def persisted?; @id.present? end
-  def to_param; @id; end
+  def to_param; @id && @id.to_s; end
   def value
     @id.nil? ? "new #{self.class.name.downcase}" : "#{self.class.name.downcase} ##{@id}"
   end
@@ -136,7 +136,7 @@ class TagRelevance
   def to_key; id ? [id] : nil end
   def save; @id = 1; @tag_id = 1 end
   def persisted?; @id.present? end
-  def to_param; @id; end
+  def to_param; @id && @id.to_s; end
   def value
     @id.nil? ? "new #{self.class.name.downcase}" : "#{self.class.name.downcase} ##{@id}"
   end