SERVER-60755 Distinguish between null, missing, and undefined in $graphLookup

1 files changed, 27 insertions, 0 deletions

diff --git a/src/mongo/db/pipeline/document_source_graph_lookup.cpp b/src/mongo/db/pipeline/document_source_graph_lookup.cpp
index df0dbe22e13..60e74b9c043 100644
--- a/src/mongo/db/pipeline/document_source_graph_lookup.cpp
+++ b/src/mongo/db/pipeline/document_source_graph_lookup.cpp
@@ -401,6 +401,11 @@ boost::optional<BSONObj> DocumentSourceGraphLookUp::makeMatchStageFromFrontier(
     //
     // We wrap the query in a $match so that it can be parsed into a DocumentSourceMatch when
     // constructing a pipeline to execute.
+
+    // $match stages will conflate null, and undefined values. Keep track of which ones are
+    // present and eliminate documents that would match the others later.
+    bool matchNull = false;
+    bool matchUndefined = false;
     BSONObjBuilder match;
     {
         BSONObjBuilder query(match.subobjStart("$match"));
@@ -417,11 +422,33 @@ boost::optional<BSONObj> DocumentSourceGraphLookUp::makeMatchStageFromFrontier(
                     {
                         BSONArrayBuilder in(subObj.subarrayStart("$in"));
                         for (auto&& value : _frontier) {
+                            if (value.getType() == BSONType::jstNULL) {
+                                matchNull = true;
+                            } else if (value.getType() == BSONType::Undefined) {
+                                matchUndefined = true;
+                            }
                             in << value;
                         }
                     }
                 }
             }
+            // We never want to see documents where the 'connectToField' is missing.
+            auto existsMatch = BSON(_connectToField.fullPath() << BSON("$exists" << true));
+            andObj << existsMatch;
+            // If matching null or undefined, make sure we don't match the other one.
+            if (matchNull || matchUndefined) {
+                if (!matchUndefined) {
+                    auto notUndefined =
+                        BSON(_connectToField.fullPath() << BSON("$not" << BSON("$type"
+                                                                               << "undefined")));
+                    andObj << notUndefined;
+                } else if (!matchNull) {
+                    auto notUndefined =
+                        BSON(_connectToField.fullPath() << BSON("$not" << BSON("$type"
+                                                                               << "null")));
+                    andObj << notUndefined;
+                }
+            }
         }
     }