[ISSUE-2321] Updates documentation on re-mounted configuration for params (#2339)

* [ISSUE-2321] Updates documentation on re-mounted configuration for params

* Adds changelog

* Extra space on changelog

* More updates to changelog

Author: myxoh

CHANGELOG.md

@@ -12,6 +12,7 @@
 
 #### Fixes
 
+* [#2339](https://github.com/ruby-grape/grape/pull/2339): Documentation and specs for remountable configuration in params - [@myxoh](https://github.com/myxoh).
 * [#2328](https://github.com/ruby-grape/grape/pull/2328): Don't cache Class.instance_methods - [@byroot](https://github.com/byroot).
 * [#2337](https://github.com/ruby-grape/grape/pull/2337): Fix: allow custom validators that do not end with _validator - [@ericproulx](https://github.com/ericproulx).
 * Your contribution here.

README.md

@@ -526,15 +526,15 @@ end
 ```ruby
 class BasicAPI < Grape::API
   desc 'Statuses index' do
-    params: mounted { configuration[:entity] || API::Entities::Status }.documentation
+    params: (configuration[:entity] || API::Entities::Status).documentation
   end
   params do
-    requires :all, using: mounted { configuration[:entity] || API::Entities::Status }.documentation
+    requires :all, using: (configuration[:entity] || API::Entities::Status).documentation
   end
   get '/statuses' do
     statuses = Status.all
     type = current_user.admin? ? :full : :default
-    present statuses, with: mounted { configuration[:entity] || API::Entities::Status }, type: type
+    present statuses, with: (configuration[:entity] || API::Entities::Status), type: type
   end
 end
 

spec/grape/api_remount_spec.rb

@@ -147,6 +147,42 @@ def app
         end
       end
 
+      context 'when the params are configured via a configuration' do
+        subject(:a_remounted_api) do
+          Class.new(described_class) do
+            params do
+              requires configuration[:required_attr_name], type: String
+            end
+
+            get(mounted { configuration[:endpoint] }) do
+              status 200
+            end
+          end
+        end
+
+        context 'when the configured param is my_attr' do
+          it 'requires the configured params' do
+            root_api.mount a_remounted_api, with: {
+              required_attr_name: 'my_attr',
+              endpoint: 'test'
+            }
+            get 'test?another_attr=1'
+            expect(last_response.status).to eq 400
+            get 'test?my_attr=1'
+            expect(last_response.status).to eq 200
+
+            root_api.mount a_remounted_api, with: {
+              required_attr_name: 'another_attr',
+              endpoint: 'test_b'
+            }
+            get 'test_b?another_attr=1'
+            expect(last_response.status).to eq 200
+            get 'test_b?my_attr=1'
+            expect(last_response.status).to eq 400
+          end
+        end
+      end
+
       context 'when executing a standard block within a `mounted` block with all dynamic params' do
         subject(:a_remounted_api) do
           Class.new(described_class) do