1 files changed, 47 insertions, 15 deletions

diff --git a/activerecord/lib/active_record/attribute_methods.rb b/activerecord/lib/active_record/attribute_methods.rb
index f4a4e3f605..9d58a19304 100644
--- a/activerecord/lib/active_record/attribute_methods.rb
+++ b/activerecord/lib/active_record/attribute_methods.rb
@@ -1,4 +1,5 @@
 require 'active_support/core_ext/enumerable'
+require 'active_support/core_ext/string/filters'
 require 'mutex_m'
 require 'thread_safe'
 
@@ -82,7 +83,7 @@ module ActiveRecord
         generated_attribute_methods.synchronize do
           return false if @attribute_methods_generated
           superclass.define_attribute_methods unless self == base_class
-          super(column_names)
+          super(attribute_names)
           @attribute_methods_generated = true
         end
         true
@@ -90,7 +91,7 @@ module ActiveRecord
 
       def undefine_attribute_methods # :nodoc:
         generated_attribute_methods.synchronize do
-          super if @attribute_methods_generated
+          super if defined?(@attribute_methods_generated) && @attribute_methods_generated
           @attribute_methods_generated = false
         end
       end
@@ -149,7 +150,7 @@ module ActiveRecord
         BLACKLISTED_CLASS_METHODS.include?(method_name.to_s) || class_method_defined_within?(method_name, Base)
       end
 
-      def class_method_defined_within?(name, klass, superklass = klass.superclass) # :nodoc
+      def class_method_defined_within?(name, klass, superklass = klass.superclass) # :nodoc:
         if klass.respond_to?(name, true)
           if superklass.respond_to?(name, true)
             klass.method(name).owner != superklass.method(name).owner
@@ -184,14 +185,15 @@ module ActiveRecord
       #   # => ["id", "created_at", "updated_at", "name", "age"]
       def attribute_names
         @attribute_names ||= if !abstract_class? && table_exists?
-            column_names
+            attribute_types.keys
           else
             []
           end
       end
 
       # Returns the column object for the named attribute.
-      # Returns nil if the named attribute does not exist.
+      # Returns a +ActiveRecord::ConnectionAdapters::NullColumn+ if the
+      # named attribute does not exist.
       #
       #   class Person < ActiveRecord::Base
       #   end
@@ -201,21 +203,18 @@ module ActiveRecord
       #   # => #<ActiveRecord::ConnectionAdapters::Column:0x007ff4ab083980 @name="name", @sql_type="varchar(255)", @null=true, ...>
       #
       #   person.column_for_attribute(:nothing)
-      #   # => nil
+      #   # => #<ActiveRecord::ConnectionAdapters::NullColumn:0xXXX @name=nil, @sql_type=nil, @cast_type=#<Type::Value>, ...>
       def column_for_attribute(name)
-        column = columns_hash[name.to_s]
-        if column.nil?
-          ActiveSupport::Deprecation.warn \
-            "`column_for_attribute` will return a null object for non-existent columns " \
-            "in Rails 5.0. Use `has_attribute?` if you need to check for an attribute's existence."
+        name = name.to_s
+        columns_hash.fetch(name) do
+          ConnectionAdapters::NullColumn.new(name)
         end
-        column
       end
     end
 
     # A Person object with a name attribute can ask <tt>person.respond_to?(:name)</tt>,
     # <tt>person.respond_to?(:name=)</tt>, and <tt>person.respond_to?(:name?)</tt>
-    # which will all return +true+. It also define the attribute methods if they have
+    # which will all return +true+. It also defines the attribute methods if they have
     # not been generated.
     #
     #   class Person < ActiveRecord::Base
@@ -329,7 +328,7 @@ module ActiveRecord
     #   task.attribute_present?(:title)   # => true
     #   task.attribute_present?(:is_done) # => true
     def attribute_present?(attribute)
-      value = read_attribute(attribute)
+      value = _read_attribute(attribute)
       !value.nil? && !(value.respond_to?(:empty?) && value.empty?)
     end
 
@@ -370,6 +369,39 @@ module ActiveRecord
       write_attribute(attr_name, value)
     end
 
+    # Returns the name of all database fields which have been read from this
+    # model. This can be useful in development mode to determine which fields
+    # need to be selected. For performance critical pages, selecting only the
+    # required fields can be an easy performance win (assuming you aren't using
+    # all of the fields on the model).
+    #
+    # For example:
+    #
+    # class PostsController < ActionController::Base
+    #   after_action :print_accessed_fields, only: :index
+    #
+    #   def index
+    #     @posts = Post.all
+    #   end
+    #
+    #   private
+    #
+    #   def print_accessed_fields
+    #     p @posts.first.accessed_fields
+    #   end
+    # end
+    #
+    # Which allows you to quickly change your code to:
+    #
+    # class PostsController < ActionController::Base
+    #   def index
+    #     @posts = Post.select(:id, :title, :author_id, :updated_at)
+    #   end
+    # end
+    def accessed_fields
+      @attributes.accessed
+    end
+
     protected
 
     def clone_attribute_value(reader_method, attribute_name) # :nodoc:
@@ -430,7 +462,7 @@ module ActiveRecord
     end
 
     def typecasted_attribute_value(name)
-      read_attribute(name)
+      _read_attribute(name)
     end
   end
 end