ActiveRecord#respond_to? No longer allocates strings

This is an alternative to #34195

The active record `respond_to?` method needs to do two things if `super` does not say that the method exists. It has to see if the "name" being passed in represents a column in the table. If it does then it needs to pass it to `has_attribute?` to see if the key exists in the current object. The reason why this is slow is that `has_attribute?` needs a string and most (almost all) objects passed in are symbols.

The only time we need to allocate a string in this method is if the column does exist in the database, and since these are a limited number of strings (since column names are a finite set) then we can pre-generate all of them and use the same string. 

We generate a list hash of column names and convert them to symbols, and store the value as the string name. This allows us to both check if the "name" exists as a column, but also provides us with a string object we can use for the `has_attribute?` call. 

I then ran the test suite and found there was only one case where we're intentionally passing in a string and changed it to a symbol. (However there are tests where we are using a string key, but they don't ship with rails).

As re-written this method should never allocate unless the user passes in a string key, which is fairly uncommon with `respond_to?`.

This also eliminates the need to special case every common item that might come through the method via the `case` that was originally added in f80aa59 (by me) and then with an attempt to extend in #34195.

As a bonus this reduces 6,300 comparisons (in the CodeTriage app homepage) to 450 as we also no longer need to loop through the column array to check for an `include?`.

Author: schneems

activerecord/lib/active_record/attribute_methods.rb

@@ -261,21 +261,14 @@ def column_for_attribute(name)
     def respond_to?(name, include_private = false)
       return false unless super
 
-      case name
-      when :to_partial_path
-        name = "to_partial_path"
-      when :to_model
-        name = "to_model"
-      else
-        name = name.to_s
-      end
-
       # If the result is true then check for the select case.
       # For queries selecting a subset of columns, return false for unselected columns.
       # We check defined?(@attributes) not to issue warnings if called on objects that
       # have been allocated but not yet initialized.
-      if defined?(@attributes) && self.class.column_names.include?(name)
-        return has_attribute?(name)
+      if defined?(@attributes)
+        if name = self.class.symbol_column_to_string(name.to_sym)
+          return has_attribute?(name)
+        end
       end
 
       true

activerecord/lib/active_record/model_schema.rb

@@ -388,6 +388,11 @@ def column_names
         @column_names ||= columns.map(&:name)
       end
 
+      def symbol_column_to_string(name_symbol) # :nodoc:
+        @symbol_column_to_string_name_hash ||= column_names.index_by(&:to_sym)
+        @symbol_column_to_string_name_hash[name_symbol]
+      end
+
       # Returns an array of column objects where the primary id, all columns ending in "_id" or "_count",
       # and columns used for single table inheritance have been removed.
       def content_columns
@@ -477,6 +482,7 @@ def load_schema!
         def reload_schema_from_cache
           @arel_table = nil
           @column_names = nil
+          @symbol_column_to_string_name_hash = nil
           @attribute_types = nil
           @content_columns = nil
           @default_attributes = nil

activerecord/lib/active_record/nested_attributes.rb

@@ -426,7 +426,7 @@ def assign_nested_attributes_for_one_to_one_association(association_name, attrib
             existing_record.assign_attributes(assignable_attributes)
             association(association_name).initialize_attributes(existing_record)
           else
-            method = "build_#{association_name}"
+            method = :"build_#{association_name}"
             if respond_to?(method)
               send(method, assignable_attributes)
             else