SERVER-64764 Fix '$$ROOT' handling in $expr rewrite

3 files changed, 18 insertions, 3 deletions

diff --git a/jstests/core/expr.js b/jstests/core/expr.js
index 362e06c9a55..c599bcd757e 100644
--- a/jstests/core/expr.js
+++ b/jstests/core/expr.js
@@ -24,8 +24,9 @@ const isMongos = (hello.msg === "isdbgrid");
 //
 
 coll.drop();
-assert.commandWorked(coll.insert({a: 0}));
+assert.commandWorked(coll.insert({_id: 0, a: 0}));
 assert.eq(1, coll.aggregate([{$match: {$expr: {$eq: ["$a", 0]}}}]).itcount());
+assert.eq(1, coll.aggregate([{$match: {$expr: {$eq: ["$$ROOT", {_id: 0, a: 0}]}}}]).itcount());
 assert.throws(function() {
     coll.aggregate([{$match: {$expr: {$eq: ["$a", "$$unbound"]}}}]);
 });
diff --git a/src/mongo/db/matcher/rewrite_expr.cpp b/src/mongo/db/matcher/rewrite_expr.cpp
index a523853a639..e61ddeaab78 100644
--- a/src/mongo/db/matcher/rewrite_expr.cpp
+++ b/src/mongo/db/matcher/rewrite_expr.cpp
@@ -233,8 +233,10 @@ bool RewriteExpr::_canRewriteComparison(
 
     for (auto operand : operandList) {
         if (auto exprFieldPath = dynamic_cast<ExpressionFieldPath*>(operand.get())) {
-            if (exprFieldPath->isVariableReference()) {
-                // This field path refers to a variable rather than a local document field path.
+            if (exprFieldPath->isVariableReference() || exprFieldPath->isROOT()) {
+                // Rather than a local document field path, this field path refers to either a
+                // variable or the full document itself. Neither of which can be expressed in the
+                // match language.
                 return false;
             }
 
diff --git a/src/mongo/db/matcher/rewrite_expr_test.cpp b/src/mongo/db/matcher/rewrite_expr_test.cpp
index 9fa64b3c70b..948add02c26 100644
--- a/src/mongo/db/matcher/rewrite_expr_test.cpp
+++ b/src/mongo/db/matcher/rewrite_expr_test.cpp
@@ -163,6 +163,18 @@ TEST(RewriteExpr, EqWithComparisonToMissingDoesNotRewriteToMatch) {
     testExprRewrite(expr, expectedMatch);
 }
 
+TEST(RewriteExpr, EqWithComparisonToRootDoesNotRewriteToMatch) {
+    BSONObj expr = fromjson("{$expr: {$eq: ['$$ROOT', {a: 1}]}}");
+    const BSONObj expectedMatch;
+    testExprRewrite(expr, expectedMatch);
+}
+
+TEST(RewriteExpr, EqWithComparisonToCurrentDoesNotRewriteToMatch) {
+    BSONObj expr = fromjson("{$expr: {$eq: ['$$CURRENT', 2]}}");
+    const BSONObj expectedMatch;
+    testExprRewrite(expr, expectedMatch);
+}
+
 TEST(RewriteExpr, EqWithComparisonToArrayDoesNotRewriteToMatch) {
     BSONObj expr = fromjson("{$expr: {$eq: ['$x', [1, 2, 3]]}}");
     const BSONObj expectedMatch;