Feature: Handle symbol-to-proc wrappers (&:) that refer to a method created using delegation or method missing

These methods have an arity of -1

Author: marcrohloff

CHANGELOG.md

@@ -1,5 +1,7 @@
 ### Next Release
 
+* [#406](https://github.com/ruby-grape/grape-entity/pull/406): Handle symbol-to-proc wrappers (`&:method_name`) where the method uses `delegate` or `method_missing`
+
 #### Features
 
 * Your contribution here.

lib/grape_entity/entity.rb

@@ -548,7 +548,8 @@ def ensure_block_arity!(block)
       end
 
       arity = object.method(origin_method_name).arity
-      return if arity.zero?
+      # functions defined using `delegate` or `method_missing` have an arity of -1
+      return if arity <= 0
 
       raise ArgumentError, <<~MSG
         Cannot use `&:#{origin_method_name}` because that method expects #{arity} argument#{'s' if arity != 1}.

spec/grape_entity/entity_spec.rb

@@ -400,6 +400,14 @@ class BogusEntity < Grape::Entity
 
         describe 'blocks' do
           class SomeObject
+            class SomeObjectDelegate
+              def method_using_delegation
+                'delegated-result'
+              end
+            end
+
+            delegate :method_using_delegation, to: :delegate_object
+
             def method_without_args
               'result'
             end
@@ -415,6 +423,23 @@ def method_with_multiple_args(_object, _options)
             def raises_argument_error
               raise ArgumentError, 'something different'
             end
+
+            def method_missing(method, ...)
+              return 'missing-result' if method.to_sym == :method_using_missing
+
+              super
+            end
+
+            def delegate_object
+              @delegate_object ||= SomeObjectDelegate.new
+            end
+
+            private
+
+            def respond_to_missing?(method, include_private = false)
+              method.to_sym == :method_using_missing ||
+                super
+            end
           end
 
           describe 'with block passed in' do
@@ -459,6 +484,28 @@ def raises_argument_error
             end
           end
 
+          context 'with block passed in via & that uses `missing_method`' do
+            specify do
+              subject.expose :using_missing, &:method_using_missing
+
+              object = SomeObject.new
+              expect(object.method(:method_using_missing).arity).to eq(-1)
+              value = subject.represent(object).value_for(:using_missing)
+              expect(value).to eq('missing-result')
+            end
+          end
+
+          context 'with block passed in via & that uses `delegate`' do
+            specify do
+              subject.expose :using_delegation, &:method_using_delegation
+
+              object = SomeObject.new
+              expect(object.method(:method_using_delegation).arity).to eq(-1)
+              value = subject.represent(object).value_for(:using_delegation)
+              expect(value).to eq('delegated-result')
+            end
+          end
+
           context 'with block passed in via &' do
             specify do
               subject.expose :that_method_with_one_arg, &:method_with_one_arg