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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 ```
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@ -1,3 +1,9 @@
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* Add `ActiveRecord::Base#accessed_fields`, which can be used to quickly
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discover which fields were read from a model when you are looking to only
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select the data you need from the database.
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*Sean Griffin*
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* Introduce the `:if_exists` option for `drop_table`.
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Example:
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@ -51,7 +51,7 @@ module ActiveRecord
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end
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def changed_in_place_from?(old_value)
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type.changed_in_place?(old_value, value)
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has_been_read? && type.changed_in_place?(old_value, value)
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end
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def with_value_from_user(value)
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@ -78,6 +78,10 @@ module ActiveRecord
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false
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end
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def has_been_read?
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defined?(@value)
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end
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def ==(other)
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self.class == other.class &&
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name == other.name &&
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@ -369,6 +369,39 @@ module ActiveRecord
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write_attribute(attr_name, value)
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end
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# Returns the name of all database fields which have been read from this
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# model. This can be useful in devleopment mode to determine which fields
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# need to be selected. For performance critical pages, selecting only the
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# required fields can be an easy performance win (assuming you aren't using
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# all of the fields on the model).
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#
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# For example:
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#
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# class PostsController < ActionController::Base
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# after_action :print_accessed_fields, only: :index
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#
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# def index
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# @posts = Post.all
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# end
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#
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# private
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#
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# def print_accessed_fields
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# p @posts.first.accessed_fields
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# end
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# end
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#
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# Which allows you to quickly change your code to:
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#
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# class PostsController < ActionController::Base
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# def index
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# @posts = Post.select(:id, :title, :author_id, :updated_at)
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# end
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# end
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def accessed_fields
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@attributes.accessed
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end
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protected
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def clone_attribute_value(reader_method, attribute_name) # :nodoc:
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@ -64,6 +64,10 @@ module ActiveRecord
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end
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end
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def accessed
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attributes.select { |_, attr| attr.has_been_read? }.keys
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end
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protected
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attr_reader :attributes
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@ -937,6 +937,16 @@ class AttributeMethodsTest < ActiveRecord::TestCase
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assert model.id_came_from_user?
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end
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def test_accessed_fields
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model = @target.first
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assert_equal [], model.accessed_fields
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model.title
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assert_equal ["title"], model.accessed_fields
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end
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private
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def new_topic_like_ar_class(&block)
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@ -186,5 +186,16 @@ module ActiveRecord
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attributes.freeze
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assert_equal({ foo: "1" }, attributes.to_hash)
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end
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test "#accessed_attributes returns only attributes which have been read" do
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builder = AttributeSet::Builder.new(foo: Type::Value.new, bar: Type::Value.new)
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attributes = builder.build_from_database(foo: "1", bar: "2")
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assert_equal [], attributes.accessed
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attributes.fetch_value(:foo)
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assert_equal [:foo], attributes.accessed
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end
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end
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end
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@ -169,5 +169,24 @@ module ActiveRecord
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second = Attribute.from_user(:foo, 1, Type::Integer.new)
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assert_not_equal first, second
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end
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test "an attribute has not been read by default" do
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attribute = Attribute.from_database(:foo, 1, Type::Value.new)
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assert_not attribute.has_been_read?
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end
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test "an attribute has been read when its value is calculated" do
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attribute = Attribute.from_database(:foo, 1, Type::Value.new)
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attribute.value
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assert attribute.has_been_read?
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end
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test "an attribute can not be mutated if it has not been read,
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and skips expensive calculations" do
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type_which_raises_from_all_methods = Object.new
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attribute = Attribute.from_database(:foo, "bar", type_which_raises_from_all_methods)
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assert_not attribute.changed_in_place_from?("bar")
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end
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end
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end
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