Drop multi_json dependency and fix Time serialization regression (#404)

Switching from MultiJson.dump to the standard JSON library in a prior
release broke Time serialization for apps using ActiveSupport: Time
values were no longer encoded as ISO 8601 strings because the standard
JSON backend does not invoke ActiveSupport's as_json chain.

Replace MultiJson.dump(serializable_hash(...)) with
serializable_hash(...).to_json so that ActiveSupport's to_json
extension is invoked, restoring ISO 8601 output (e.g.
"2012-02-27T00:00:00.000Z") for Time and other ActiveSupport-enhanced
types. Remove the multi_json runtime dependency from the gemspec as it
is no longer used.

Regression specs assert the exact ISO 8601 string rather than
delegating the expectation to as_json on the same value, so a joint
regression in both methods would still be caught.

Author: numbata

CHANGELOG.md

@@ -7,6 +7,7 @@
 #### Fixes
 
 * Your contribution here.
+* [#404](https://github.com/ruby-grape/grape-entity/pull/404): Drop `MultiJson` dependency, use `Hash#to_json` for ActiveSupport-aware serialization - [@numbata](https://github.com/numbata).
 
 ### 1.0.4 (2026-04-17)
 

grape-entity.gemspec

@@ -25,7 +25,6 @@ Gem::Specification.new do |s|
   s.required_ruby_version = '>= 3.0'
 
   s.add_dependency 'activesupport', '>= 3.0.0'
-  s.add_dependency 'multi_json', '>= 1.0'
 
   s.files         = Dir['lib/**/*.rb', 'CHANGELOG.md', 'LICENSE', 'README.md']
   s.require_paths = ['lib']

lib/grape_entity/entity.rb

@@ -1,7 +1,5 @@
 # frozen_string_literal: true
 
-require 'multi_json'
-
 module Grape
   # An Entity is a lightweight structure that allows you to easily
   # represent data from your application in a consistent and abstracted
@@ -594,7 +592,7 @@ def is_defined_in_entity?(attribute)
 
     def to_json(options = {})
       options = options.to_h if options&.respond_to?(:to_h)
-      MultiJson.dump(serializable_hash(options))
+      serializable_hash(options).to_json
     end
 
     def to_xml(options = {})

spec/grape_entity/entity_spec.rb

@@ -1886,7 +1886,7 @@ class NoPathCharacterEntity < Grape::Entity
         entity = fresh_class.new(model)
         json = entity.to_json
         parsed = JSON.parse(json)
-        expect(parsed['birthday']).to eq(attributes[:birthday].as_json)
+        expect(parsed['birthday']).to eq('2012-02-27T00:00:00.000Z')
       end
 
       it 'serializes Time values in nested exposures via as_json' do
@@ -1895,7 +1895,7 @@ class NoPathCharacterEntity < Grape::Entity
         end
         entity = fresh_class.new(model)
         parsed = JSON.parse(entity.to_json)
-        expect(parsed['details']['birthday']).to eq(attributes[:birthday].as_json)
+        expect(parsed['details']['birthday']).to eq('2012-02-27T00:00:00.000Z')
       end
     end