serialize to replace toBson in document sources and pipeline

20 files changed, 152 insertions, 246 deletions

diff --git a/src/mongo/db/commands/pipeline_command.cpp b/src/mongo/db/commands/pipeline_command.cpp
index b69226e4a08..280e6f9796b 100644
--- a/src/mongo/db/commands/pipeline_command.cpp
+++ b/src/mongo/db/commands/pipeline_command.cpp
@@ -234,9 +234,7 @@ namespace mongo {
                 // important because sharded aggregations rely on this ability.
                 // Skipping when inShard because this has already been through the
                 // transformation (and this unsets pCtx->inShard).
-                BSONObjBuilder bb;
-                pPipeline->toBson(&bb);
-                parsed = bb.obj();
+                parsed = pPipeline->serialize().toBson();
                 pPipeline = Pipeline::parseCommand(errmsg, parsed, pCtx);
                 verify(pPipeline);
             }
@@ -295,31 +293,20 @@ namespace mongo {
             */
             intrusive_ptr<Pipeline> pShardSplit = pPipeline->splitForSharded();
 
-            /*
-            Write the split pipeline as we would in order to transmit it to
-            the shard servers.
-            */
-            BSONObjBuilder shardBuilder;
-            pShardSplit->toBson(&shardBuilder);
-            BSONObj shardBson(shardBuilder.done());
-
-            DEV (log() << "\n---- shardBson\n" <<
-                 shardBson.jsonString(Strict, 1) << "\n----\n");
-
-            /* for debugging purposes, show what the pipeline now looks like */
-            DEV {
-                BSONObjBuilder pipelineBuilder;
-                pPipeline->toBson(&pipelineBuilder);
-                BSONObj pipelineBson(pipelineBuilder.done());
-                (log() << "\n---- pipelineBson\n" <<
-                 pipelineBson.jsonString(Strict, 1) << "\n----\n");
-            }
+            // Write the split pipeline as we would in order to transmit it to the shard servers.
+            Document shardCmd = pShardSplit->serialize();
+
+            DEV log() << "\n---- shardDescription\n" << shardCmd.toString() << "\n----\n";
+
+            // for debugging purposes, show what the pipeline now looks like
+            DEV log() << "\n---- pipelineDescription\n" << pPipeline->serialize() << "\n----\n";
 
             /* on the shard servers, create the local pipeline */
             intrusive_ptr<ExpressionContext> pShardCtx =
                 new ExpressionContext(InterruptStatusMongod::status, NamespaceString(ns));
+            BSONObj shardObj = shardCmd.toBson();
             intrusive_ptr<Pipeline> pShardPipeline(
-                Pipeline::parseCommand(errmsg, shardBson, pShardCtx));
+                Pipeline::parseCommand(errmsg, shardObj, pShardCtx));
             if (!pShardPipeline.get()) {
                 return false;
             }
diff --git a/src/mongo/db/pipeline/document_source.cpp b/src/mongo/db/pipeline/document_source.cpp
index 1cfb2d8c689..e91dc62e102 100644
--- a/src/mongo/db/pipeline/document_source.cpp
+++ b/src/mongo/db/pipeline/document_source.cpp
@@ -18,6 +18,7 @@
 
 #include "mongo/db/pipeline/document_source.h"
 #include "mongo/db/pipeline/expression_context.h"
+#include "mongo/db/pipeline/value.h"
 
 namespace mongo {
 
@@ -55,18 +56,11 @@ namespace mongo {
         }
     }
 
-    void DocumentSource::addToBsonArray(
-        BSONArrayBuilder *pBuilder, bool explain) const {
-        BSONObjBuilder insides;
-        sourceToBson(&insides, explain);
-
-/* No statistics at this time
-        if (explain) {
-            insides.append("nOut", nOut);
+    void DocumentSource::serializeToArray(vector<Value>& array, bool explain) const {
+        Value entry = serialize(explain);
+        if (!entry.missing()) {
+            array.push_back(entry);
         }
-*/
-
-        pBuilder->append(insides.done());
     }
 
     BSONObj DocumentSource::depsToProjection(const set<string>& deps) {
diff --git a/src/mongo/db/pipeline/document_source.h b/src/mongo/db/pipeline/document_source.h
index 99ae26b0b82..8ce6e3b57bc 100644
--- a/src/mongo/db/pipeline/document_source.h
+++ b/src/mongo/db/pipeline/document_source.h
@@ -159,19 +159,13 @@ namespace mongo {
         static Document documentFromBsonWithDeps(const BSONObj& object, const ParsedDeps& deps);
 
         /**
-          Add the DocumentSource to the array builder.
-
-          The default implementation calls sourceToBson() in order to
-          convert the inner part of the object which will be added to the
-          array being built here.
-
-          A subclass may choose to overwrite this rather than addToBsonArray
-          if it should output multiple stages.
-
-          @param pBuilder the array builder to add the operation to.
-          @param explain create explain output
+         * In the default case, serializes the DocumentSource and adds it to the vector<Value>.
+         *
+         * A subclass may choose to overwrite this, rather than serialize,
+         * if it should output multiple stages (eg, $sort sometimes also outputs a $limit).
          */
-        virtual void addToBsonArray(BSONArrayBuilder *pBuilder, bool explain=false) const;
+
+        virtual void serializeToArray(vector<Value>& array, bool explain = false) const;
 
         /// Returns true if doesn't require an input source (most DocumentSources do).
         virtual bool isValidInitialSource() const { return false; }
@@ -182,18 +176,6 @@ namespace mongo {
          */
         DocumentSource(const intrusive_ptr<ExpressionContext> &pExpCtx);
 
-        /**
-          Create an object that represents the document source.  The object
-          will have a single field whose name is the source's name.  This
-          will be used by the default implementation of addToBsonArray()
-          to add this object to a pipeline being represented in BSON.
-
-          @param pBuilder a blank object builder to write to
-          @param explain create explain output
-         */
-        virtual void sourceToBson(BSONObjBuilder *pBuilder,
-                                  bool explain) const = 0;
-
         /*
           Most DocumentSources have an underlying source they get their data
           from.  This is a convenience for them.
@@ -219,6 +201,16 @@ namespace mongo {
           This is *not* unsigned so it can be passed to BSONObjBuilder.append().
          */
         long long nRowsOut;
+
+    private:
+        /**
+         * Create a Value that represents the document source.
+         *
+         * This is used by the default implementation of serializeToArray() to add this object
+         * to a pipeline being serialized. Returning a missing() Value results in no entry
+         * being added to the array for this stage (DocumentSource).
+         */
+        virtual Value serialize(bool explain = false) const = 0;
     };
 
     /** This class marks DocumentSources that should be split between the router and the shards
@@ -281,6 +273,7 @@ namespace mongo {
     public:
         // virtuals from DocumentSource
         virtual boost::optional<Document> getNext();
+        virtual Value serialize(bool explain = false) const;
         virtual void setSource(DocumentSource *pSource);
         virtual bool isValidInitialSource() const { return true; }
 
@@ -302,10 +295,6 @@ namespace mongo {
             BSONElement *pBsonElement,
             const intrusive_ptr<ExpressionContext> &pExpCtx);
 
-    protected:
-        // virtuals from DocumentSource
-        virtual void sourceToBson(BSONObjBuilder *pBuilder, bool explain) const;
-
     private:
         DocumentSourceBsonArray(BSONElement *pBsonElement,
             const intrusive_ptr<ExpressionContext> &pExpCtx);
@@ -320,6 +309,7 @@ namespace mongo {
     public:
         // virtuals from DocumentSource
         virtual boost::optional<Document> getNext();
+        virtual Value serialize(bool explain = false) const;
         virtual void setSource(DocumentSource *pSource);
         virtual bool isValidInitialSource() const { return true; }
 
@@ -337,10 +327,6 @@ namespace mongo {
             const ShardOutput& shardOutput,
             const intrusive_ptr<ExpressionContext>& pExpCtx);
 
-    protected:
-        // virtuals from DocumentSource
-        virtual void sourceToBson(BSONObjBuilder *pBuilder, bool explain) const;
-
     private:
         DocumentSourceCommandShards(const ShardOutput& shardOutput,
             const intrusive_ptr<ExpressionContext>& pExpCtx);
@@ -374,6 +360,7 @@ namespace mongo {
         // virtuals from DocumentSource
         virtual ~DocumentSourceCursor();
         virtual boost::optional<Document> getNext();
+        virtual Value serialize(bool explain = false) const;
         virtual void setSource(DocumentSource *pSource);
         virtual bool coalesce(const intrusive_ptr<DocumentSource>& nextSource);
         virtual bool isValidInitialSource() const { return true; }
@@ -440,9 +427,6 @@ namespace mongo {
 
         /// returns -1 for no limit
         long long getLimit() const;
-    protected:
-        // virtuals from DocumentSource
-        virtual void sourceToBson(BSONObjBuilder *pBuilder, bool explain) const;
 
     private:
         DocumentSourceCursor(
@@ -531,6 +515,7 @@ namespace mongo {
         virtual const char *getSourceName() const;
         virtual GetDepsReturn getDependencies(set<string>& deps) const;
         virtual void dispose();
+        virtual Value serialize(bool explain = false) const;
 
         /**
           Create a new grouping DocumentSource.
@@ -592,10 +577,6 @@ namespace mongo {
 
         static const char groupName[];
 
-    protected:
-        // virtuals from DocumentSource
-        virtual void sourceToBson(BSONObjBuilder *pBuilder, bool explain) const;
-
     private:
         DocumentSourceGroup(const intrusive_ptr<ExpressionContext> &pExpCtx);
 
@@ -660,6 +641,7 @@ namespace mongo {
     public:
         // virtuals from DocumentSource
         virtual const char *getSourceName() const;
+        virtual Value serialize(bool explain = false) const;
 
         /**
           Create a filter.
@@ -686,10 +668,6 @@ namespace mongo {
 
         static const char matchName[];
 
-    protected:
-        // virtuals from DocumentSource
-        virtual void sourceToBson(BSONObjBuilder *pBuilder, bool explain) const;
-
         // virtuals from DocumentSourceFilterBase
         virtual bool accept(const Document& pDocument) const;
 
@@ -710,6 +688,7 @@ namespace mongo {
         virtual void setSource(DocumentSource *pSource);
         virtual const char *getSourceName() const;
         virtual void dispose();
+        virtual Value serialize(bool explain = false) const;
         virtual bool isValidInitialSource() const { return true; }
 
         static intrusive_ptr<DocumentSource> createFromBson(
@@ -721,9 +700,6 @@ namespace mongo {
             const intrusive_ptr<ExpressionContext> &pExpCtx);
 
         static const char name[];
-    protected:
-        // virtuals from DocumentSource
-        virtual void sourceToBson(BSONObjBuilder *pBuilder, bool explain) const;
 
     private:
 
@@ -757,6 +733,7 @@ namespace mongo {
         virtual ~DocumentSourceOut();
         virtual boost::optional<Document> getNext();
         virtual const char *getSourceName() const;
+        virtual Value serialize(bool explain = false) const;
 
         // Virtuals for SplittableDocumentSource
         virtual intrusive_ptr<DocumentSource> getShardSource() { return NULL; }
@@ -780,10 +757,6 @@ namespace mongo {
 
         static const char outName[];
 
-    protected:
-        // virtuals from DocumentSource
-        virtual void sourceToBson(BSONObjBuilder *pBuilder, bool explain) const;
-
     private:
         DocumentSourceOut(const NamespaceString& outputNs,
                           const intrusive_ptr<ExpressionContext> &pExpCtx);
@@ -807,6 +780,7 @@ namespace mongo {
         virtual boost::optional<Document> getNext();
         virtual const char *getSourceName() const;
         virtual void optimize();
+        virtual Value serialize(bool explain = false) const;
 
         virtual GetDepsReturn getDependencies(set<string>& deps) const;
 
@@ -829,10 +803,6 @@ namespace mongo {
         /** projection as specified by the user */
         BSONObj getRaw() const { return _raw; }
 
-    protected:
-        // virtuals from DocumentSource
-        virtual void sourceToBson(BSONObjBuilder *pBuilder, bool explain) const;
-
     private:
         DocumentSourceProject(const intrusive_ptr<ExpressionContext>& pExpCtx,
                               const intrusive_ptr<ExpressionObject>& exprObj);
@@ -859,8 +829,7 @@ namespace mongo {
         static intrusive_ptr<DocumentSource> createFromBson(BSONElement* bsonElement,
                 const intrusive_ptr<ExpressionContext>& expCtx);
 
-    protected:
-        virtual void sourceToBson(BSONObjBuilder* pBuilder, bool explain) const;
+        virtual Value serialize(bool explain = false) const;
 
     private:
         DocumentSourceRedact(const intrusive_ptr<ExpressionContext>& expCtx,
@@ -877,7 +846,7 @@ namespace mongo {
         // virtuals from DocumentSource
         virtual boost::optional<Document> getNext();
         virtual const char *getSourceName() const;
-        virtual void addToBsonArray(BSONArrayBuilder *pBuilder, bool explain=false) const;
+        virtual void serializeToArray(vector<Value>& array, bool explain = false) const;
         virtual bool coalesce(const intrusive_ptr<DocumentSource> &pNextSource);
         virtual void dispose();
 
@@ -902,13 +871,8 @@ namespace mongo {
         */
         void addKey(const string &fieldPath, bool ascending);
 
-        /**
-          Write out an object whose contents are the sort key.
-
-          @param pBuilder initialized object builder.
-          @param fieldPrefix specify whether or not to include the field prefix
-         */
-        void sortKeyToBson(BSONObjBuilder *pBuilder, bool usePrefix) const;
+        /// Write out a Document whose contents are the sort key.
+        Document serializeSortKey() const;
 
         /**
           Create a sorting DocumentSource from BSON.
@@ -937,15 +901,14 @@ namespace mongo {
         intrusive_ptr<DocumentSourceLimit> getLimitSrc() const { return limitSrc; }
 
         static const char sortName[];
-    protected:
-        // virtuals from DocumentSource
-        virtual void sourceToBson(BSONObjBuilder *pBuilder, bool explain) const {
-            verify(false); // should call addToBsonArray instead
-        }
 
     private:
         DocumentSourceSort(const intrusive_ptr<ExpressionContext> &pExpCtx);
 
+        virtual Value serialize(bool explain = false) const {
+            verify(false); // should call addToBsonArray instead
+        }
+
         /*
           Before returning anything, this source must fetch everything from
           the underlying source and group it.  populate() is used to do that
@@ -992,6 +955,7 @@ namespace mongo {
         virtual boost::optional<Document> getNext();
         virtual const char *getSourceName() const;
         virtual bool coalesce(const intrusive_ptr<DocumentSource> &pNextSource);
+        virtual Value serialize(bool explain = false) const;
 
         virtual GetDepsReturn getDependencies(set<string>& deps) const {
             return SEE_NEXT; // This doesn't affect needed fields
@@ -1032,10 +996,6 @@ namespace mongo {
 
         static const char limitName[];
 
-    protected:
-        // virtuals from DocumentSource
-        virtual void sourceToBson(BSONObjBuilder *pBuilder, bool explain) const;
-
     private:
         DocumentSourceLimit(const intrusive_ptr<ExpressionContext> &pExpCtx,
                             long long limit);
@@ -1051,6 +1011,7 @@ namespace mongo {
         virtual boost::optional<Document> getNext();
         virtual const char *getSourceName() const;
         virtual bool coalesce(const intrusive_ptr<DocumentSource> &pNextSource);
+        virtual Value serialize(bool explain = false) const;
 
         virtual GetDepsReturn getDependencies(set<string>& deps) const {
             return SEE_NEXT; // This doesn't affect needed fields
@@ -1090,10 +1051,6 @@ namespace mongo {
 
         static const char skipName[];
 
-    protected:
-        // virtuals from DocumentSource
-        virtual void sourceToBson(BSONObjBuilder *pBuilder, bool explain) const;
-
     private:
         DocumentSourceSkip(const intrusive_ptr<ExpressionContext> &pExpCtx);
 
@@ -1108,6 +1065,7 @@ namespace mongo {
         // virtuals from DocumentSource
         virtual boost::optional<Document> getNext();
         virtual const char *getSourceName() const;
+        virtual Value serialize(bool explain = false) const;
 
         virtual GetDepsReturn getDependencies(set<string>& deps) const;
 
@@ -1127,10 +1085,6 @@ namespace mongo {
 
         static const char unwindName[];
 
-    protected:
-        // virtuals from DocumentSource
-        virtual void sourceToBson(BSONObjBuilder *pBuilder, bool explain) const;
-
     private:
         DocumentSourceUnwind(const intrusive_ptr<ExpressionContext> &pExpCtx);
 
@@ -1154,6 +1108,7 @@ namespace mongo {
         virtual void setSource(DocumentSource *pSource); // errors out since this must be first
         virtual bool coalesce(const intrusive_ptr<DocumentSource> &pNextSource);
         virtual bool isValidInitialSource() const { return true; }
+        virtual Value serialize(bool explain = false) const;
 
         // Virtuals for SplittableDocumentSource
         virtual intrusive_ptr<DocumentSource> getShardSource();
@@ -1171,10 +1126,6 @@ namespace mongo {
         static intrusive_ptr<DocumentSourceGeoNear> create(
             const intrusive_ptr<ExpressionContext> &pCtx);
 
-    protected:
-        // virtuals from DocumentSource
-        virtual void sourceToBson(BSONObjBuilder *pBuilder, bool explain) const;
-
     private:
         DocumentSourceGeoNear(const intrusive_ptr<ExpressionContext> &pExpCtx);
 
diff --git a/src/mongo/db/pipeline/document_source_bson_array.cpp b/src/mongo/db/pipeline/document_source_bson_array.cpp
index 0f13d3ab4c8..fc40915859e 100644
--- a/src/mongo/db/pipeline/document_source_bson_array.cpp
+++ b/src/mongo/db/pipeline/document_source_bson_array.cpp
@@ -55,13 +55,10 @@ namespace mongo {
         return pSource;
     }
 
-    void DocumentSourceBsonArray::sourceToBson(
-        BSONObjBuilder *pBuilder, bool explain) const {
-
+    Value DocumentSourceBsonArray::serialize(bool explain) const {
         if (explain) {
-            BSONObj empty;
-
-            pBuilder->append("bsonArray", empty);
+            return Value(DOC("bsonArray" << Document()));
         }
+        return Value();
     }
 }
diff --git a/src/mongo/db/pipeline/document_source_command_shards.cpp b/src/mongo/db/pipeline/document_source_command_shards.cpp
index f11978468bf..4537c37b2fc 100644
--- a/src/mongo/db/pipeline/document_source_command_shards.cpp
+++ b/src/mongo/db/pipeline/document_source_command_shards.cpp
@@ -26,9 +26,8 @@ namespace mongo {
         verify(false);
     }
 
-    void DocumentSourceCommandShards::sourceToBson(
-        BSONObjBuilder *pBuilder, bool explain) const {
-        /* this has no BSON equivalent */
+    Value DocumentSourceCommandShards::serialize(bool explain) const {
+        // this has no BSON equivalent
         verify(false);
     }
 
diff --git a/src/mongo/db/pipeline/document_source_cursor.cpp b/src/mongo/db/pipeline/document_source_cursor.cpp
index 50fbc876ef3..c16238d096a 100644
--- a/src/mongo/db/pipeline/document_source_cursor.cpp
+++ b/src/mongo/db/pipeline/document_source_cursor.cpp
@@ -190,28 +190,25 @@ namespace mongo {
         return false;
     }
 
-    void DocumentSourceCursor::sourceToBson(
-        BSONObjBuilder *pBuilder, bool explain) const {
-
-        /* this has no analog in the BSON world, so only allow it for explain */
-        if (explain)
-        {
-            BSONObj bsonObj;
+    Value DocumentSourceCursor::serialize(bool explain) const {
+        // we never parse a documentSourceCursor, so we do not serialize it
+        if (explain) {
+            MutableDocument result;
             
-            pBuilder->append("query", _query);
+            result.setField("query", Value(_query));
 
             if (!_sort.isEmpty()) {
-                pBuilder->append("sort", _sort);
+                result.setField("sort", Value(_sort));
             }
 
             if (_limit) {
-                pBuilder->append("limit", _limit->getLimit());
+                result.setField("limit", Value(_limit->getLimit()));
             }
 
             BSONObj projectionSpec;
             if (_projection) {
                 projectionSpec = _projection->getSpec();
-                pBuilder->append("projection", projectionSpec);
+                result.setField("projection", Value(projectionSpec));
             }
 
             // construct query for explain
@@ -227,8 +224,10 @@ namespace mongo {
                                                                   ? &projectionSpec
                                                                   : NULL));
 
-            pBuilder->append("cursor", explainResult);
+            result.setField("cursor", Value(explainResult));
+            return result.freezeToValue();
         }
+        return Value();
     }
 
     DocumentSourceCursor::DocumentSourceCursor(const string& ns,
diff --git a/src/mongo/db/pipeline/document_source_geo_near.cpp b/src/mongo/db/pipeline/document_source_geo_near.cpp
index f6f8305c271..7c2476985e8 100644
--- a/src/mongo/db/pipeline/document_source_geo_near.cpp
+++ b/src/mongo/db/pipeline/document_source_geo_near.cpp
@@ -67,32 +67,34 @@ namespace mongo {
                                           limit);
     }
 
-    void DocumentSourceGeoNear::sourceToBson(BSONObjBuilder *pBuilder, bool explain) const {
-        BSONObjBuilder geoNear (pBuilder->subobjStart("$geoNear"));
+    Value DocumentSourceGeoNear::serialize(bool explain) const {
+        MutableDocument result;
 
         if (coordsIsArray) {
-            geoNear.appendArray("near", coords);
+            result.setField("near", Value(BSONArray(coords)));
         }
         else {
-            geoNear.append("near", coords);
+            result.setField("near", Value(coords));
         }
 
-        geoNear.append("distanceField", distanceField->getPath(false)); // not in buildGeoNearCmd
-        geoNear.append("limit", limit);
+        // not in buildGeoNearCmd
+        result.setField("distanceField", Value(distanceField->getPath(false)));
+
+        result.setField("limit", Value(limit));
 
         if (maxDistance > 0)
-            geoNear.append("maxDistance", maxDistance);
+            result.setField("maxDistance", Value(maxDistance));
 
-        geoNear.append("query", query);
-        geoNear.append("spherical", spherical);
-        geoNear.append("distanceMultiplier", distanceMultiplier);
+        result.setField("query", Value(query));
+        result.setField("spherical", Value(spherical));
+        result.setField("distanceMultiplier", Value(distanceMultiplier));
 
         if (includeLocs)
-            geoNear.append("includeLocs", includeLocs->getPath(false));
+            result.setField("includeLocs", Value(includeLocs->getPath(false)));
 
-        geoNear.append("uniqueDocs", uniqueDocs);
+        result.setField("uniqueDocs", Value(uniqueDocs));
 
-        geoNear.doneFast();
+        return Value(DOC(getSourceName() << result.freeze()));
     }
 
     BSONObj DocumentSourceGeoNear::buildGeoNearCmd() const {
diff --git a/src/mongo/db/pipeline/document_source_group.cpp b/src/mongo/db/pipeline/document_source_group.cpp
index 486af593028..2ece0f36d98 100644
--- a/src/mongo/db/pipeline/document_source_group.cpp
+++ b/src/mongo/db/pipeline/document_source_group.cpp
@@ -109,18 +109,17 @@ namespace mongo {
         pSource->dispose();
     }
 
-    void DocumentSourceGroup::sourceToBson(BSONObjBuilder* pBuilder, bool explain) const {
+    Value DocumentSourceGroup::serialize(bool explain) const {
         MutableDocument insides;
 
-        /* add the _id */
+        // add the _id
         insides["_id"] = pIdExpression->serialize();
 
-        /* add the remaining fields */
+        // add the remaining fields
         const size_t n = vFieldName.size();
         for(size_t i = 0; i < n; ++i) {
             intrusive_ptr<Accumulator> accum = vpAccumulatorFactory[i]();
-            insides[vFieldName[i]] = Value(
-                    DOC(accum->getOpName() << vpExpression[i]->serialize()));
+            insides[vFieldName[i]] = Value(DOC(accum->getOpName() << vpExpression[i]->serialize()));
         }
 
         if (_doingMerge) {
@@ -130,7 +129,7 @@ namespace mongo {
             insides["$doingMerge"] = Value(true);
         }
 
-        *pBuilder << groupName << insides.freeze();
+        return Value(DOC(getSourceName() << insides.freeze()));
     }
 
     DocumentSource::GetDepsReturn DocumentSourceGroup::getDependencies(set<string>& deps) const {
diff --git a/src/mongo/db/pipeline/document_source_limit.cpp b/src/mongo/db/pipeline/document_source_limit.cpp
index f19da5171c0..cad0893c382 100644
--- a/src/mongo/db/pipeline/document_source_limit.cpp
+++ b/src/mongo/db/pipeline/document_source_limit.cpp
@@ -63,9 +63,8 @@ namespace mongo {
         return pSource->getNext();
     }
 
-    void DocumentSourceLimit::sourceToBson(
-        BSONObjBuilder *pBuilder, bool explain) const {
-        pBuilder->append("$limit", limit);
+    Value DocumentSourceLimit::serialize(bool explain) const {
+        return Value(DOC(getSourceName() << limit));
     }
 
     intrusive_ptr<DocumentSourceLimit> DocumentSourceLimit::create(
diff --git a/src/mongo/db/pipeline/document_source_match.cpp b/src/mongo/db/pipeline/document_source_match.cpp
index 476abbf946e..d1e502bc7d8 100644
--- a/src/mongo/db/pipeline/document_source_match.cpp
+++ b/src/mongo/db/pipeline/document_source_match.cpp
@@ -30,10 +30,8 @@ namespace mongo {
         return matchName;
     }
 
-    void DocumentSourceMatch::sourceToBson(
-        BSONObjBuilder *pBuilder, bool explain) const {
-        const BSONObj *pQuery = matcher.getQuery();
-        pBuilder->append(matchName, *pQuery);
+    Value DocumentSourceMatch::serialize(bool explain) const {
+        return Value(DOC(getSourceName() << Document(*matcher.getQuery())));
     }
 
     bool DocumentSourceMatch::accept(const Document& input) const {
diff --git a/src/mongo/db/pipeline/document_source_merge_cursors.cpp b/src/mongo/db/pipeline/document_source_merge_cursors.cpp
index e45716999be..1d4ed9347b3 100644
--- a/src/mongo/db/pipeline/document_source_merge_cursors.cpp
+++ b/src/mongo/db/pipeline/document_source_merge_cursors.cpp
@@ -67,12 +67,13 @@ namespace mongo {
         return new DocumentSourceMergeCursors(cursorIds, pExpCtx);
     }
 
-    void DocumentSourceMergeCursors::sourceToBson(BSONObjBuilder* builder, bool explain) const {
-        BSONArrayBuilder cursors(builder->subarrayStart("$mergeCursors"));
+    Value DocumentSourceMergeCursors::serialize(bool explain) const {
+        vector<Value> cursors;
         for (size_t i = 0; i < _cursorIds.size(); i++) {
-            cursors.append(BSON("host" << _cursorIds[i].first.toString()
-                             << "id" << _cursorIds[i].second));
+            cursors.push_back(Value(DOC("host" << Value(_cursorIds[i].first.toString())
+                                     << "id" << _cursorIds[i].second)));
         }
+        return Value(DOC(getSourceName() << Value(cursors)));
     }
 
     DocumentSourceMergeCursors::CursorAndConnection::CursorAndConnection(
diff --git a/src/mongo/db/pipeline/document_source_out.cpp b/src/mongo/db/pipeline/document_source_out.cpp
index f76a6d74a3a..bd04811779d 100644
--- a/src/mongo/db/pipeline/document_source_out.cpp
+++ b/src/mongo/db/pipeline/document_source_out.cpp
@@ -157,10 +157,10 @@ namespace mongo {
         return new DocumentSourceOut(outputNs, pExpCtx);
     }
 
-    void DocumentSourceOut::sourceToBson(BSONObjBuilder *pBuilder, bool explain) const {
+    Value DocumentSourceOut::serialize(bool explain) const {
         massert(17000, "$out shouldn't have different db than input",
                 _outputNs.db() == pExpCtx->ns.db());
 
-        pBuilder->append("$out", _outputNs.coll());
+        return Value(DOC(getSourceName() << _outputNs.coll()));
     }
 }
diff --git a/src/mongo/db/pipeline/document_source_project.cpp b/src/mongo/db/pipeline/document_source_project.cpp
index d751eb1c51d..5db8afde329 100644
--- a/src/mongo/db/pipeline/document_source_project.cpp
+++ b/src/mongo/db/pipeline/document_source_project.cpp
@@ -82,8 +82,8 @@ namespace mongo {
         pEO = dynamic_pointer_cast<ExpressionObject>(pE);
     }
 
-    void DocumentSourceProject::sourceToBson(BSONObjBuilder* pBuilder, bool explain) const {
-        *pBuilder << projectName << pEO->serialize();
+    Value DocumentSourceProject::serialize(bool explain) const {
+        return Value(DOC(getSourceName() << pEO->serialize()));
     }
 
     intrusive_ptr<DocumentSource> DocumentSourceProject::createFromBson(
diff --git a/src/mongo/db/pipeline/document_source_redact.cpp b/src/mongo/db/pipeline/document_source_redact.cpp
index 04d2b4f644b..21f5aed1b00 100644
--- a/src/mongo/db/pipeline/document_source_redact.cpp
+++ b/src/mongo/db/pipeline/document_source_redact.cpp
@@ -124,8 +124,8 @@ namespace mongo {
         _expression = _expression->optimize();
     }
 
-    void DocumentSourceRedact::sourceToBson(BSONObjBuilder* pBuilder, bool explain) const {
-        *pBuilder << redactName << _expression.get()->serialize();
+    Value DocumentSourceRedact::serialize(bool explain) const {
+        return Value(DOC(getSourceName() << _expression.get()->serialize()));
     }
 
     intrusive_ptr<DocumentSource> DocumentSourceRedact::createFromBson(
diff --git a/src/mongo/db/pipeline/document_source_skip.cpp b/src/mongo/db/pipeline/document_source_skip.cpp
index 8758ff743db..3d85085f521 100644
--- a/src/mongo/db/pipeline/document_source_skip.cpp
+++ b/src/mongo/db/pipeline/document_source_skip.cpp
@@ -65,9 +65,8 @@ namespace mongo {
         return pSource->getNext();
     }
 
-    void DocumentSourceSkip::sourceToBson(
-        BSONObjBuilder *pBuilder, bool explain) const {
-        pBuilder->append("$skip", _skip);
+    Value DocumentSourceSkip::serialize(bool explain) const {
+        return Value(DOC(getSourceName() << _skip));
     }
 
     intrusive_ptr<DocumentSourceSkip> DocumentSourceSkip::create(
diff --git a/src/mongo/db/pipeline/document_source_sort.cpp b/src/mongo/db/pipeline/document_source_sort.cpp
index cc01796b600..fd6ddd81d73 100644
--- a/src/mongo/db/pipeline/document_source_sort.cpp
+++ b/src/mongo/db/pipeline/document_source_sort.cpp
@@ -43,31 +43,17 @@ namespace mongo {
         return _output->next().second;
     }
 
-    void DocumentSourceSort::addToBsonArray(BSONArrayBuilder *pBuilder, bool explain) const {
-        if (explain) { // always one obj for combined $sort + $limit
-            BSONObjBuilder sortObj (pBuilder->subobjStart());
-            BSONObjBuilder insides (sortObj.subobjStart(sortName));
-            BSONObjBuilder sortKey (insides.subobjStart("sortKey"));
-            sortKeyToBson(&sortKey, false);
-            sortKey.doneFast();
-
-            if (explain && limitSrc) {
-                insides.appendNumber("limit", limitSrc->getLimit());
-            }
-            insides.doneFast();
-            sortObj.doneFast();
+    void DocumentSourceSort::serializeToArray(vector<Value>& array, bool explain) const {
+        if (explain) { // always one Value for combined $sort + $limit
+            array.push_back(Value(
+                    DOC(getSourceName() << DOC("sortKey" << serializeSortKey()
+                                            << "limit" << (limitSrc ? Value(limitSrc->getLimit())
+                                                                    : Value())))));
         }
-        else { // one obj for $sort + maybe one obj for $limit
-            {
-                BSONObjBuilder sortObj (pBuilder->subobjStart());
-                BSONObjBuilder insides (sortObj.subobjStart(sortName));
-                sortKeyToBson(&insides, false);
-                insides.doneFast();
-                sortObj.doneFast();
-            }
-
+        else { // one Value for $sort and maybe a Value for $limit
+            array.push_back(Value(DOC(getSourceName() << serializeSortKey())));
             if (limitSrc) {
-                limitSrc->addToBsonArray(pBuilder, explain);
+                limitSrc->serializeToArray(array);
             }
         }
     }
@@ -101,8 +87,9 @@ namespace mongo {
         vAscending.push_back(ascending);
     }
 
-    void DocumentSourceSort::sortKeyToBson(BSONObjBuilder* pBuilder, bool usePrefix) const {
-        /* add the key fields */
+    Document DocumentSourceSort::serializeSortKey() const {
+        MutableDocument keyObj;
+        // add the key fields
         const size_t n = vSortKey.size();
         for(size_t i = 0; i < n; ++i) {
             // get the field name out of each ExpressionFieldPath
@@ -111,10 +98,12 @@ namespace mongo {
             verify(withVariable.getFieldName(0) == "ROOT");
             const string fieldPath = withVariable.tail().getPath(false);
 
-            /* append a named integer based on the sort order */
-            pBuilder->append(fieldPath, (vAscending[i] ? 1 : -1));
+            // append a named integer based on the sort order
+            keyObj.setField(fieldPath, Value(vAscending[i] ? 1 : -1));
         }
+        return keyObj.freeze();
     }
+
     DocumentSource::GetDepsReturn DocumentSourceSort::getDependencies(set<string>& deps) const {
         for(size_t i = 0; i < vSortKey.size(); ++i) {
             vSortKey[i]->addDependencies(deps);
diff --git a/src/mongo/db/pipeline/document_source_unwind.cpp b/src/mongo/db/pipeline/document_source_unwind.cpp
index eeb046aa0a4..a3ff87591d7 100644
--- a/src/mongo/db/pipeline/document_source_unwind.cpp
+++ b/src/mongo/db/pipeline/document_source_unwind.cpp
@@ -126,10 +126,9 @@ namespace mongo {
         return out;
     }
 
-    void DocumentSourceUnwind::sourceToBson(
-        BSONObjBuilder *pBuilder, bool explain) const {
+    Value DocumentSourceUnwind::serialize(bool explain) const {
         verify(_unwindPath);
-        pBuilder->append(unwindName, _unwindPath->getPath(true));
+        return Value(DOC(getSourceName() << _unwindPath->getPath(true)));
     }
 
     DocumentSource::GetDepsReturn DocumentSourceUnwind::getDependencies(set<string>& deps) const {
diff --git a/src/mongo/db/pipeline/pipeline.cpp b/src/mongo/db/pipeline/pipeline.cpp
index 9d3dd259e13..d22f2be3633 100644
--- a/src/mongo/db/pipeline/pipeline.cpp
+++ b/src/mongo/db/pipeline/pipeline.cpp
@@ -401,33 +401,36 @@ namespace mongo {
         return true;
     }
 
-    void Pipeline::toBson(BSONObjBuilder *pBuilder) const {
-        /* create an array out of the pipeline operations */
-        BSONArrayBuilder arrayBuilder;
+    Document Pipeline::serialize() const {
+        MutableDocument serialized;
+        // create an array out of the pipeline operations
+        vector<Value> array;
         for(SourceContainer::const_iterator iter(sources.begin()),
                                             listEnd(sources.end());
                                         iter != listEnd;
                                         ++iter) {
             intrusive_ptr<DocumentSource> pSource(*iter);
-            pSource->addToBsonArray(&arrayBuilder);
+            pSource->serializeToArray(array);
         }
 
-        /* add the top-level items to the command */
-        pBuilder->append(commandName, getCollectionName());
-        pBuilder->append(pipelineName, arrayBuilder.arr());
+        // add the top-level items to the command
+        serialized.setField(commandName, Value(getCollectionName()));
+        serialized.setField(pipelineName, Value(array));
 
         if (explain) {
-            pBuilder->append(explainName, explain);
+            serialized.setField(explainName, Value(explain));
         }
 
         if (pCtx->extSortAllowed) {
-            pBuilder->append("allowDiskUsage", true);
+            serialized.setField("allowDiskUsage", Value(true));
         }
 
         bool btemp;
         if ((btemp = getSplitMongodPipeline())) {
-            pBuilder->append(splitMongodPipelineName, btemp);
+            serialized.setField(splitMongodPipelineName, Value(btemp));
         }
+
+        return serialized.freeze();
     }
 
     void Pipeline::stitch() {
@@ -482,7 +485,8 @@ namespace mongo {
         }
     }
 
-    void Pipeline::writeExplainOps(BSONArrayBuilder *pArrayBuilder) const {
+    vector<Value> Pipeline::writeExplainOps() const {
+        vector<Value> array;
         for(SourceContainer::const_iterator iter(sources.begin()),
                                             listEnd(sources.end());
                                         iter != listEnd;
@@ -493,17 +497,13 @@ namespace mongo {
             if (dynamic_cast<DocumentSourceBsonArray*>(pSource.get()))
                 continue;
 
-            pSource->addToBsonArray(pArrayBuilder, true);
+            pSource->serializeToArray(array, true);
         }
+        return array;
     }
 
     void Pipeline::writeExplainShard(BSONObjBuilder &result) const {
-        BSONArrayBuilder opArray; // where we'll put the pipeline ops
-
-        // next, add the pipeline operators
-        writeExplainOps(&opArray);
-
-        result.appendArray(serverPipelineName, opArray.arr());
+        result << serverPipelineName << Value(writeExplainOps());
     }
 
     void Pipeline::writeExplainMongos(BSONObjBuilder &result) const {
@@ -525,12 +525,9 @@ namespace mongo {
             opBuilder.doneFast();
         }
 
-        BSONArrayBuilder mongosOpArray; // where we'll put the pipeline ops
-        writeExplainOps(&mongosOpArray);
-
         // now we combine the shard pipelines with the one here
         result.append(serverPipelineName, shardOpArray.arr());
-        result.append(mongosPipelineName, mongosOpArray.arr());
+        result << mongosPipelineName << Value(writeExplainOps());
     }
 
     void Pipeline::addInitialSource(intrusive_ptr<DocumentSource> source) {
diff --git a/src/mongo/db/pipeline/pipeline.h b/src/mongo/db/pipeline/pipeline.h
index b7333e59686..cb8bc6ae5f2 100644
--- a/src/mongo/db/pipeline/pipeline.h
+++ b/src/mongo/db/pipeline/pipeline.h
@@ -20,6 +20,7 @@
 
 #include "mongo/pch.h"
 
+#include "mongo/db/pipeline/value.h"
 #include "mongo/util/intrusive_counter.h"
 #include "mongo/util/timer.h"
 
@@ -98,7 +99,7 @@ namespace mongo {
 
           @param the builder to write the command to
         */
-        void toBson(BSONObjBuilder *pBuilder) const;
+        Document serialize() const;
 
         /** Stitch together the source pointers (by calling setSource) for each source in sources.
          *  Must be called after optimize and addInitialSource but before trying to get results.
@@ -167,13 +168,11 @@ namespace mongo {
 
         Pipeline(const intrusive_ptr<ExpressionContext> &pCtx);
 
-        /*
-          Write the pipeline's operators to the given array, with the
-          explain flag true (for DocumentSource::addToBsonArray()).
-
-          @param pArrayBuilder where to write the ops to
+        /**
+         * Write the pipeline's operators to a vector<Value>, with the
+         * explain flag true (for DocumentSource::serializeToArray()).
          */
-        void writeExplainOps(BSONArrayBuilder *pArrayBuilder) const;
+        vector<Value> writeExplainOps() const;
 
         /*
           Write the pipeline's operators to the given result document,
diff --git a/src/mongo/db/pipeline/pipeline_d.cpp b/src/mongo/db/pipeline/pipeline_d.cpp
index 8a8c2f88ba9..e6119fb01ec 100644
--- a/src/mongo/db/pipeline/pipeline_d.cpp
+++ b/src/mongo/db/pipeline/pipeline_d.cpp
@@ -119,24 +119,21 @@ namespace {
           index scan.
         */
         intrusive_ptr<DocumentSourceSort> pSort;
-        BSONObjBuilder sortBuilder;
+        BSONObj sortObj;
         if (!sources.empty()) {
             const intrusive_ptr<DocumentSource> &pSC = sources.front();
             pSort = dynamic_cast<DocumentSourceSort *>(pSC.get());
 
             if (pSort) {
-                /* build the sort key */
-                pSort->sortKeyToBson(&sortBuilder, false);
+                // build the sort key
+                sortObj = pSort->serializeSortKey().toBson();
             }
         }
 
-        /* Create the sort object; see comments on the query object above */
-        BSONObj sortObj = sortBuilder.obj();
-
-        /* get the full "namespace" name */
+        // get the full "namespace" name
         string fullName(dbName + "." + pPipeline->getCollectionName());
 
-        /* for debugging purposes, show what the query and sort are */
+        // for debugging purposes, show what the query and sort are
         DEV {
             (log() << "\n---- query BSON\n" <<
              queryObj.jsonString(Strict, 1) << "\n----\n");