Ignore empty condition on #construct_relation_for_exists

[r7kamura]

Summary

At fc0e335,

relation = relation.where(conditions)

was rewritten to:

relation.where!(condition)

This change accidentally changed the result of Topic.exists?({}) from true to false.

To fix this regression, I moved the blank check logic (opts.blank?) from #where to #where!.

I think #where! should be identical to #where, except that instead of returning a new relation, it adds the condition to the existing relation.

Details

Steps to reproduce

begin
  require "bundler/inline"
rescue LoadError
  $stderr.puts "Bundler version 1.10 or later is required. Please update your Bundler"
  raise
end

gemfile(true) do
  source "https://rubygems.org"

  git_source(:github) { |repo| "https://github.com/#{repo}.git" }

  gem "rails", github: "rails/rails"
  gem "sqlite3"
end

require "active_record"
require "minitest/autorun"

ActiveRecord::Base.establish_connection(adapter: "sqlite3", database: ":memory:")

ActiveRecord::Schema.define do
  create_table :topics, force: true
end

ActiveRecord::Base.logger = Logger.new(STDOUT)

class Topic < ActiveRecord::Base
end

class ExistsTest < Minitest::Test
  def setup
    Topic.create!
  end

  def test_exists_with_empty_hash_argument
    assert_equal true, Topic.exists?({})
  end
end

Expected behavior

-- create_table(:topics, {:force=>true})
   -> 0.0024s
Run options: --seed 39899

# Running:

D, [2018-10-27T14:57:50.240167 #60154] DEBUG -- :    (0.0ms)  begin transaction
D, [2018-10-27T14:57:50.240357 #60154] DEBUG -- :   Topic Create (0.1ms)  INSERT INTO "topics" DEFAULT VALUES
D, [2018-10-27T14:57:50.240578 #60154] DEBUG -- :    (0.0ms)  commit transaction
D, [2018-10-27T14:57:50.241155 #60154] DEBUG -- :   Topic Exists (0.1ms)  SELECT 1 AS one FROM "topics" LIMIT ?  [["LIMIT", 1]]
.

Finished in 0.003765s, 265.6042 runs/s, 265.6042 assertions/s.
1 runs, 1 assertions, 0 failures, 0 errors, 0 skips

Actual behavior

-- create_table(:topics, {:force=>true})
   -> 0.0024s
Run options: --seed 53123

# Running:

D, [2018-10-27T14:58:16.323575 #60237] DEBUG -- :    (0.0ms)  begin transaction
D, [2018-10-27T14:58:16.323774 #60237] DEBUG -- :   Topic Create (0.1ms)  INSERT INTO "topics" DEFAULT VALUES
D, [2018-10-27T14:58:16.323973 #60237] DEBUG -- :    (0.0ms)  commit transaction
D, [2018-10-27T14:58:16.324506 #60237] DEBUG -- :   Topic Exists (0.1ms)  SELECT 1 AS one FROM "topics" WHERE (1=0) LIMIT ?  [["LIMIT", 1]]
F

Failure:
ExistsTest#test_exists_with_empty_hash_argument [/Users/r7kamura/Desktop/test.rb:37]:
Expected: true
  Actual: false


rails test Users/r7kamura/Desktop/test.rb:36



Finished in 0.003548s, 281.8489 runs/s, 281.8489 assertions/s.
1 runs, 1 assertions, 1 failures, 0 errors, 0 skips

[rails-bot]

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[kamipo]

This changes the existing where behavior which return the same instance if empty args, it may affect to any apps.

How about just fixing construct_relation_for_exists?

diff --git a/activerecord/lib/active_record/relation/finder_methods.rb b/activerecord/lib/active_record/relation/finder_methods.rb
index 6f420fe6bb..55c387f7b7 100644
--- a/activerecord/lib/active_record/relation/finder_methods.rb
+++ b/activerecord/lib/active_record/relation/finder_methods.rb
@@ -363,7 +363,7 @@ def construct_relation_for_exists(conditions)
 
         case conditions
         when Array, Hash
-          relation.where!(conditions)
+          relation.where!(conditions) unless condition.empty?
         else
           relation.where!(primary_key => conditions) unless conditions == :none
         end

[kamipo]

Note that where! is private API unlike where, people should not rely on the private API.

[r7kamura]

I agree that we shouldn't change the existent #where behavior.

So here we can choose the one of following solutions I think:

1. Change #construct_relation_for_exists (as you suggested)
2. Change only #where! (and leave elsif opts.blank? in #where)

The advantage of 2nd option is we can still keep the same behavior on #where and #where! with blank argument. What do you think?

[kamipo]

I'm not prefer to add extra / duplicated args checking #where! which is called in #where, and we'd not guarantee that #where! behave like #where since that is private API.

[r7kamura]

Okay, it means it's a special feature of #where to handle blank conditions, and #where! doesn't have this feature.

In this case, just fixing #construct_relation_for_exists is the best solution 🥇

[kamipo]

Looks good to me.
Can you squash your commits and include the context (Summary part in this PR) in the commit message?

[kamipo]

ah I realized that 2nd commit includes the context. Please ignore about the commit message.

[r7kamura]

Squashed the 2 commits into 4694fcf that has all the context of this PR in its commit message.

[kamipo]

Thanks!