Introduce `ActiveRecord::Base#accessed_fields`

This method can be used to see all of the fields on a model which have
been read. This can be useful during development mode to quickly find
out which fields need to be selected. For performance critical pages, if
you are not using all of the fields of a database, an easy performance
win is only selecting the fields which you need. By calling this method
at the end of a controller action, it's easy to determine which fields
need to be selected.

While writing this, I also noticed a place for an easy performance win
internally which I had been wanting to introduce. You cannot mutate a
field which you have not read. Therefore, we can skip the calculation of
in place changes if we have never read from the field. This can
significantly speed up methods like `#changed?` if any of the fields
have an expensive mutable type (like `serialize`)

```
Calculating -------------------------------------
 #changed? with serialized column (before)
                       391.000  i/100ms
 #changed? with serialized column (after)
                         1.514k i/100ms
-------------------------------------------------
 #changed? with serialized column (before)
                          4.243k (± 3.7%) i/s -     21.505k
 #changed? with serialized column (after)
                         16.789k (± 3.2%) i/s -     84.784k
```

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:

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 &&

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:

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

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)

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

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