JBuilder 2.11.3 introduces an extra DB query when rendering collection partials

[cupakromer]

Ruby Version: 2.7.4
Rails Version: 5.2.6
JBuilder Version: 2.11.3

This was introduced by #501 with the options[:collection].empty? call:

jbuilder/lib/jbuilder/jbuilder_template.rb

Line 144 in b94394b

if options.key?(:collection) && (options[:collection].nil? || options[:collection].empty?)

When the collection object is an Active Record association relation/collection the empty? call introduces a SELECT 1 query statement from this code in ActiveRecord::Relation#empty?:

    # Returns true if there are no records.
    def empty?
      return @records.empty? if loaded?
      !exists?
    end

This SQL statement occurs for the case when the collection is not yet loaded; which is very common for the main collection on index actions. The exists? code is run which is defined in ActiveRecord::FinderMethods#exists?. This causes SQL to be executed via connection.select_one.

The extra SQL statement is an unnecessary DB trip across the network when there is data.

For comparison, in JBuild 2.11.2 the same process passes the collection to array! which ends up loading the full collection without this select_one call.

This is easily reproduced with a controller action such as:

# app/controllers/widgets_controller.rb
class WidgetsController < ApplicationController
  # GET /widgets or /widgets.json
  def index
    @widgets = Widget.all
  end
end

# app/views/widgets/index.json.jbuilder
json.array! @widgets, partial: "widgets/widget", as: :widget

# app/views/widgets/_widget.json.jbuilder
json.extract! widget, :id, :name, :created_at, :updated_at
json.url widget_url(widget, format: :json)

When hit without data the Rails default logger shows:

Started GET "/widgets.json" for 127.0.0.1 at 2021-11-16 17:02:20 -0500
Processing by WidgetsController#index as JSON
  Rendering widgets/index.json.jbuilder
  Widget Exists (1.3ms)  SELECT  1 AS one FROM "widgets" LIMIT $1  [["LIMIT", 1]]
  ↳ app/views/widgets/index.json.jbuilder:1
  Rendered widgets/index.json.jbuilder (3.1ms)
Completed 200 OK in 7ms (Views: 4.4ms | ActiveRecord: 1.3ms)

When run with data, the SELECT 1 and the SELECT * are both present:

Started GET "/widgets.json" for 127.0.0.1 at 2021-11-16 17:04:04 -0500
Processing by WidgetsController#index as JSON
  Rendering widgets/index.json.jbuilder
  Widget Exists (0.3ms)  SELECT  1 AS one FROM "widgets" LIMIT $1  [["LIMIT", 1]]
  ↳ app/views/widgets/index.json.jbuilder:1
  Widget Load (0.2ms)  SELECT "widgets".* FROM "widgets"
  ↳ app/views/widgets/index.json.jbuilder:1
  Rendered widgets/_widget.json.jbuilder (0.5ms)
  Rendered widgets/_widget.json.jbuilder (0.2ms)
  Rendered widgets/_widget.json.jbuilder (0.2ms)
  Rendered widgets/index.json.jbuilder (4.9ms)
Completed 200 OK in 9ms (Views: 6.7ms | ActiveRecord: 0.5ms)

[cristianzanfir]

Also experiencing the same issue with the additional SELECT COUNT(*). The .empty? method call triggers this query.
Do you think there is a way to improve this? Cheers 🍻

[tf]

Also before, the collection option worked for anything implementing Enumerable. I now see failures, since the empty? method is undefined.

[lacco]

Same error for me, 2.11.2 works, but 2.11.3 is broken.

As pointed out by @tf , just implementing Enumerable was enough.
With the recent change, empty? method needs to be implemented as well.

[dhh]

cc @yuki24

[yuki24]

#524 should address this issue.