accept format query parameter like rails to steer the response format

Author: Neeti Raj

lib/grape/middleware/formatter.rb

@@ -19,7 +19,7 @@ def headers
       end
 
       def before
-        fmt = format_from_extension || options[:format] || format_from_header || options[:default_format]
+        fmt = format_from_extension || format_from_params || options[:format] || format_from_header || options[:default_format]
         if content_types.key?(fmt)
           if !env['rack.input'].nil? and (body = env['rack.input'].read).strip.length != 0
             parser = parser_for fmt
@@ -50,6 +50,15 @@ def format_from_extension
         nil
       end
 
+      def format_from_params
+        if env['QUERY_STRING']
+          format_query = env['QUERY_STRING'].split('&').reject{|q| !q.include?('format=')} 
+          return format_query[0].split('=').last.to_sym if (format_query && !format_query.empty?)
+        else
+          nil
+        end 
+      end
+
       def format_from_header
         mime_array.each do |t|
           if mime_types.key?(t)

spec/grape/middleware/formatter_spec.rb

@@ -68,6 +68,13 @@ def to_xml
       subject.env['api.format'].should == :json
     end
 
+    it 'should use the format parameter if one is provided' do 
+      subject.call({'PATH_INFO' => '/somewhere','QUERY_STRING' => 'format=json'})
+      subject.env['api.format'].should == :json
+      subject.call({'PATH_INFO' => '/somewhere','QUERY_STRING' => 'format=xml'})
+      subject.env['api.format'].should == :xml
+    end
+
     it 'should use the default format if none is provided' do
       subject.call({'PATH_INFO' => '/info'})
       subject.env['api.format'].should == :txt