SERVER-43350 $lookup with no local default or user-specified collation should explicitly set the simple collation on the foreign expression context

1 files changed, 8 insertions, 1 deletions

diff --git a/src/mongo/db/pipeline/document_source_lookup.cpp b/src/mongo/db/pipeline/document_source_lookup.cpp
index a4b1bf4221a..575d3335ba4 100644
--- a/src/mongo/db/pipeline/document_source_lookup.cpp
+++ b/src/mongo/db/pipeline/document_source_lookup.cpp
@@ -61,7 +61,14 @@ DocumentSourceLookUp::DocumentSourceLookUp(NamespaceString fromNs,
     const auto& resolvedNamespace = expCtx->getResolvedNamespace(_fromNs);
     _resolvedNs = resolvedNamespace.ns;
     _resolvedPipeline = resolvedNamespace.pipeline;
-    _fromExpCtx = expCtx->copyWith(_resolvedNs);
+
+    // We always set an explicit collator on the copied expression context, even if the collator is
+    // null (i.e. the simple collation). Otherwise, in the situation where the user did not specify
+    // a collation and the simple collation was inherited from the local collection, the execution
+    // machinery will incorrectly interpret the null collator and empty user collation as an
+    // indication that it should inherit the foreign collection's collation.
+    _fromExpCtx = expCtx->copyWith(
+        _resolvedNs, boost::none, expCtx->getCollator() ? expCtx->getCollator()->clone() : nullptr);
 
     _fromExpCtx->subPipelineDepth += 1;
     uassert(ErrorCodes::MaxSubPipelineDepthExceeded,