Deprecation warning for `MultiJson` when using multi_json 1.21.0 or later

[kymmt90]

When using multi_json 1.21.0 or later, the following warning is displayed:

The MultiJson constant is deprecated and will be removed in v2.0. Use MultiJSON instead.

MultiJson is deprecated in multi_json 1.21.0 and later; MultiJSON should be used instead.

https://github.com/sferik/multi_json/blob/v1.21.1/CHANGELOG.md#deprecated

Deprecated

• The MultiJson constant in favor of MultiJSON.

The issue is likely caused by the fact that lib/grape/json.rb only uses ::MultiJson for backend detection.

Steps to reproduce

Confirmed with v3.2.1.

1. Add gem 'multi_json' to Gemfile and run bundle install
2. Add require 'multi_json' to the beginning of lib/grape/json.rb
3. Run bundle exec rspec spec/grape/api_spec.rb --fail-fast

diff --git a/Gemfile b/Gemfile
index 468c73fd..298c0b47 100644
--- a/Gemfile
+++ b/Gemfile
@@ -7,6 +7,7 @@ gemspec
group :development, :test do
gem 'builder', require: false 
gem 'bundler'
+gem 'multi_json' 
gem 'rake' 
gem 'rubocop', '1.86.0', require: false 
gem 'rubocop-performance', '1.26.1', require: false
diff --git a/lib/grape/json.rb b/lib/grape/json.rb
index a3001453..cb76e21c 100644
--- a/lib/grape/json.rb
+++ b/lib/grape/json.rb
@@ -1,5 +1,7 @@ 
# frozen_string_literal: true

+require 'multi_json'
+ 
module Grape 
if defined?(::MultiJson) 
Json = ::MultiJson

Test run results:

Failures: 

1) Grape::API.route put allows a(n) Hash json object in params 
Failure/Error: __send__ verb, '/', Grape::Json.dump(object), 'CONTENT_TYPE' => 'application/json' 

DeprecatedWarningHandler::DeprecationWarning: 
The MultiJson constant is deprecated and will be removed in v2.0. Use MultiJSON instead. 
# ./spec/grape/api_spec.rb:492:in 'block (6 levels) in <top (required)>'

Finished in 0.09947 seconds (files took 0.21904 seconds to load)
81 examples, 1 failure

Failed examples:

rspec ./spec/grape/api_spec.rb[1:11:9:13] # Grape::API.route put allows a(n) Hash json object in params

Randomized with seed 13003

[Huaian666]

hmm interesting approach. different from what ive seen before. digging into the source

[dblock]

Please open a PR fixing the deprecation!

[ericproulx]

Worth flagging that multi_json 1.21.0 deprecated more than the constant — MultiJSON.dump and MultiJSON.load are deprecated too (in favor of generate/parse), all removed in 2.0:

https://github.com/sferik/multi_json/blob/v1.21.1/CHANGELOG.md#deprecated

Since Grape calls Grape::Json.dump/.load internally, swapping the constant alone (::MultiJson → ::MultiJSON) would just trade the constant warning for these method warnings. #2764 detects ::MultiJSON and routes Grape's dump/load to MultiJSON.generate/parse (byte-identical output), clearing all three deprecations without changing behavior.