7 files changed, 88 insertions, 1 deletions
diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index d5c141630e..45c3cb0690 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,3 +1,9 @@
+*   Add `ActiveRecord::Base#accessed_fields`, which can be used to quickly
+    discover which fields were read from a model when you are looking to only
+    select the data you need from the database.
+
+    *Sean Griffin*
+
 *   Introduce the `:if_exists` option for `drop_table`.
 
     Example:
diff --git a/activerecord/lib/active_record/attribute.rb b/activerecord/lib/active_record/attribute.rb
index 51e4fae62e..73bb059495 100644
--- a/activerecord/lib/active_record/attribute.rb
+++ b/activerecord/lib/active_record/attribute.rb
@@ -51,7 +51,7 @@ module ActiveRecord
     end
 
     def changed_in_place_from?(old_value)
-      type.changed_in_place?(old_value, value)
+      has_been_read? && type.changed_in_place?(old_value, value)
     end
 
     def with_value_from_user(value)
@@ -78,6 +78,10 @@ module ActiveRecord
       false
     end
 
+    def has_been_read?
+      defined?(@value)
+    end
+
     def ==(other)
       self.class == other.class &&
         name == other.name &&
diff --git a/activerecord/lib/active_record/attribute_methods.rb b/activerecord/lib/active_record/attribute_methods.rb
index 8f165fb1dc..b2db0ceae7 100644
--- a/activerecord/lib/active_record/attribute_methods.rb
+++ b/activerecord/lib/active_record/attribute_methods.rb
@@ -369,6 +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 devleopment 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:
diff --git a/activerecord/lib/active_record/attribute_set.rb b/activerecord/lib/active_record/attribute_set.rb
index 66fcaf6945..fdce68ce45 100644
--- a/activerecord/lib/active_record/attribute_set.rb
+++ b/activerecord/lib/active_record/attribute_set.rb
@@ -64,6 +64,10 @@ module ActiveRecord
       end
     end
 
+    def accessed
+      attributes.select { |_, attr| attr.has_been_read? }.keys
+    end
+
     protected
 
     attr_reader :attributes
diff --git a/activerecord/test/cases/attribute_methods_test.rb b/activerecord/test/cases/attribute_methods_test.rb
index 4036294121..ea2b94cbf4 100644
--- a/activerecord/test/cases/attribute_methods_test.rb
+++ b/activerecord/test/cases/attribute_methods_test.rb
@@ -937,6 +937,16 @@ class AttributeMethodsTest < ActiveRecord::TestCase
     assert model.id_came_from_user?
   end
 
+  def test_accessed_fields
+    model = @target.first
+
+    assert_equal [], model.accessed_fields
+
+    model.title
+
+    assert_equal ["title"], model.accessed_fields
+  end
+
   private
 
   def new_topic_like_ar_class(&block)
diff --git a/activerecord/test/cases/attribute_set_test.rb b/activerecord/test/cases/attribute_set_test.rb
index ba53612d30..8025c7c4d2 100644
--- a/activerecord/test/cases/attribute_set_test.rb
+++ b/activerecord/test/cases/attribute_set_test.rb
@@ -186,5 +186,16 @@ module ActiveRecord
       attributes.freeze
       assert_equal({ foo: "1" }, attributes.to_hash)
     end
+
+    test "#accessed_attributes returns only attributes which have been read" do
+      builder = AttributeSet::Builder.new(foo: Type::Value.new, bar: Type::Value.new)
+      attributes = builder.build_from_database(foo: "1", bar: "2")
+
+      assert_equal [], attributes.accessed
+
+      attributes.fetch_value(:foo)
+
+      assert_equal [:foo], attributes.accessed
+    end
   end
 end
diff --git a/activerecord/test/cases/attribute_test.rb b/activerecord/test/cases/attribute_test.rb
index 39a976fcc8..eac73e11e8 100644
--- a/activerecord/test/cases/attribute_test.rb
+++ b/activerecord/test/cases/attribute_test.rb
@@ -169,5 +169,24 @@ module ActiveRecord
       second = Attribute.from_user(:foo, 1, Type::Integer.new)
       assert_not_equal first, second
     end
+
+    test "an attribute has not been read by default" do
+      attribute = Attribute.from_database(:foo, 1, Type::Value.new)
+      assert_not attribute.has_been_read?
+    end
+
+    test "an attribute has been read when its value is calculated" do
+      attribute = Attribute.from_database(:foo, 1, Type::Value.new)
+      attribute.value
+      assert attribute.has_been_read?
+    end
+
+    test "an attribute can not be mutated if it has not been read,
+      and skips expensive calculations" do
+      type_which_raises_from_all_methods = Object.new
+      attribute = Attribute.from_database(:foo, "bar", type_which_raises_from_all_methods)
+
+      assert_not attribute.changed_in_place_from?("bar")
+    end
   end
 end