Raise an error if JSON encoding adapters are missing

lib/faraday/error.rb

@@ -34,5 +34,7 @@ def inspect
     class ConnectionFailed < ClientError;   end
     class ResourceNotFound < ClientError;   end
     class ParsingError     < ClientError;   end
+
+    class MissingDependency < StandardError; end
   end
 end

lib/faraday/request/json.rb

@@ -3,7 +3,11 @@ class Request::JSON < Request::UrlEncoded
     self.mime_type = 'application/json'.freeze
 
     class << self
-      attr_accessor :adapter
+      attr_writer :adapter
+
+      def adapter
+        @adapter or raise Error::MissingDependency, "No JSON adapter available. Install either activesupport or yajl-ruby."
+      end
     end
 
     # loads the JSON encoder either from yajl-ruby or activesupport

test/request_middleware_test.rb

@@ -42,6 +42,37 @@ def test_json_skips_encoding_for_strings
     assert_equal '{"a":"b"}', response.body
   end
 
+  def test_json_fails_with_useful_message_when_no_json_adapter_available
+    without_json_adapter do
+      expected_msg = "No JSON adapter available. Install either activesupport or yajl-ruby."
+      # assert_raise doesn't work to check the message (at least on 1.8.7)
+      begin
+        @conn.post('/echo', { :fruit => %w[apples oranges] }, 'content-type' => 'application/json')
+        fail "Exception not raised"
+      rescue Faraday::Error::MissingDependency => e
+        assert_equal expected_msg, e.message
+      end
+    end
+  end
+
+  def test_url_encoded_does_not_fail_when_no_json_adapter_available
+    without_json_adapter do
+      assert_nothing_raised {
+        @conn.post('/echo', { :fruit => %w[apples oranges] })
+      }
+    end
+  end
+
+  def without_json_adapter
+    original_adapter = Faraday::Request::JSON.adapter
+    Faraday::Request::JSON.adapter = nil
+    begin
+      yield
+    ensure
+      Faraday::Request::JSON.adapter = original_adapter
+    end
+  end
+
   def test_url_encoded_no_header
     response = @conn.post('/echo', { :fruit => %w[apples oranges] })
     assert_equal 'application/x-www-form-urlencoded', response.headers['Content-Type']