From d29eb793098a871aec02cd4903af0bf5f76ddd49 Mon Sep 17 00:00:00 2001
From: Nicolas Klein <myxoh10@gmail.com>
Date: Fri, 5 Jul 2019 16:46:00 +0100
Subject: [PATCH 1/2] Makes sure Grape::API behaves as a Rack::App by calling:
 'call' on the first mounted instance rather than the base instance (which is
 never mounted) which will have the environment information as to where it was
 mounted

---
 lib/grape/api.rb                    | 16 ++++++++++++++--
 spec/grape/integration/rack_spec.rb | 22 ++++++++++++++++++++++
 2 files changed, 36 insertions(+), 2 deletions(-)

diff --git a/lib/grape/api.rb b/lib/grape/api.rb
index d4b70ae3e1..4cc7d702af 100644
--- a/lib/grape/api.rb
+++ b/lib/grape/api.rb
@@ -48,9 +48,13 @@ def override_all_methods!
       # (http://www.rubydoc.info/github/rack/rack/master/file/SPEC) for more.
       # NOTE: This will only be called on an API directly mounted on RACK
       def call(*args, &block)
-        base_instance.call(*args, &block)
+        instance_for_rack = if never_mounted?
+          base_instance
+        else
+          mounted_instances.first
+        end
+        instance_for_rack.call(*args, &block)
       end
-
       # Allows an API to itself be inheritable:
       def make_inheritable(api)
         # When a child API inherits from a parent API.
@@ -147,6 +151,14 @@ def evaluate_arguments(configuration, *args)
           end
         end
       end
+
+      def never_mounted?
+        mounted_instances.empty?
+      end
+
+      def mounted_instances
+        instances - [base_instance]
+      end
     end
   end
 end
diff --git a/spec/grape/integration/rack_spec.rb b/spec/grape/integration/rack_spec.rb
index e7c9f56659..a1e49a6d81 100644
--- a/spec/grape/integration/rack_spec.rb
+++ b/spec/grape/integration/rack_spec.rb
@@ -31,4 +31,26 @@
       input.unlink
     end
   end
+
+  context 'when the app is mounted' do
+    def app
+      @main_app ||= Class.new(Grape::API) do
+        get 'ping'
+      end
+    end
+
+    let!(:base) do
+      app_to_mount = app
+      Class.new(Grape::API) do
+        namespace 'namespace' do
+          mount app_to_mount
+        end
+      end
+    end
+
+    it 'finds the app on the namespace' do
+      get '/namespace/ping'
+      expect(last_response.status).to eq 200
+    end
+  end
 end

From 0efc8c7d2f9634c30729bea0316bbe2a0109b021 Mon Sep 17 00:00:00 2001
From: Nicolas Klein <myxoh10@gmail.com>
Date: Fri, 5 Jul 2019 16:50:28 +0100
Subject: [PATCH 2/2] Addresses code style issues

---
 CHANGELOG.md     | 1 +
 lib/grape/api.rb | 9 +++++----
 2 files changed, 6 insertions(+), 4 deletions(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index bf42e32681..c4ea50fdb5 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,6 +7,7 @@
 #### Fixes
 
 * Your contribution here.
+* [#1893](https://github.com/ruby-grape/grape/pull/1893): Allows `Grape::API` to behave like a Rack::app in some instances where it was misbehaving - [@myxoh](https://github.com/myxoh).
 
 ### 1.2.4 (2019/06/13)
 
diff --git a/lib/grape/api.rb b/lib/grape/api.rb
index 4cc7d702af..a830bb834b 100644
--- a/lib/grape/api.rb
+++ b/lib/grape/api.rb
@@ -49,12 +49,13 @@ def override_all_methods!
       # NOTE: This will only be called on an API directly mounted on RACK
       def call(*args, &block)
         instance_for_rack = if never_mounted?
-          base_instance
-        else
-          mounted_instances.first
-        end
+                              base_instance
+                            else
+                              mounted_instances.first
+                            end
         instance_for_rack.call(*args, &block)
       end
+
       # Allows an API to itself be inheritable:
       def make_inheritable(api)
         # When a child API inherits from a parent API.