Fix RUBY-2932 map_reduce changes broke read preference passing from Mongoid (#2433)

Co-authored-by: Oleg Pudeyev <code@olegp.name>

Author: p-mongo

lib/mongo/collection/view/builder/map_reduce.rb

@@ -115,11 +115,14 @@ def map_reduce_command
             end
             command.update(view_options)
             command.update(Utils.slice_hash(options, :collation))
+
             # Read preference isn't simply passed in the command payload
-            # (it may need to be converted to wire protocol flags)
-            # so remove it here and hopefully it's handled elsewhere.
-            # If not, RUBY-2706.
-            command.delete(:read)
+            # (it may need to be converted to wire protocol flags).
+            # Ideally it should be removed here, however due to Mongoid 7
+            # using this method and requiring :read to be returned from it,
+            # we cannot do this just yet - see RUBY-2932.
+            #command.delete(:read)
+
             command.merge!(Options::Mapper.transform_documents(options, MAPPINGS))
             command
           end

lib/mongo/collection/view/map_reduce.rb

@@ -248,7 +248,20 @@ def new(options)
         end
 
         def initial_query_op(session)
-          Operation::MapReduce.new(map_reduce_spec(session))
+          spec = map_reduce_spec(session)
+          # Read preference isn't simply passed in the command payload
+          # (it may need to be converted to wire protocol flags).
+          # Passing it in command payload produces errors on at least
+          # 5.0 mongoses.
+          # In the future map_reduce_command should remove :read
+          # from its return value, however we cannot do this right now
+          # due to Mongoid 7 relying on :read being returned as part of
+          # the command - see RUBY-2932.
+          # Delete :read here for now because it cannot be sent to mongos this way.
+          spec = spec.dup
+          spec[:selector] = spec[:selector].dup
+          spec[:selector].delete(:read)
+          Operation::MapReduce.new(spec)
         end
 
         def valid_server?(server)

spec/mongo/collection/view/map_reduce_spec.rb

@@ -865,4 +865,20 @@
       end
     end
   end
+
+  describe '#map_reduce_spec' do
+    context 'when read preference is given' do
+      let(:view_options) do
+        { read: {mode: :secondary} }
+      end
+
+      context 'selector' do
+        # For compatibility with released versions of Mongoid, this method
+        # must return read preference under the :read key.
+        it 'contains read preference' do
+          map_reduce_spec[:selector][:read].should == {'mode' => :secondary}
+        end
+      end
+    end
+  end
 end