SERVER-29620 Add shorthand syntax for retrieving the record count for a collection using the agg stage

2 files changed, 28 insertions, 1 deletions

diff --git a/src/mongo/db/stats/storage_stats.cpp b/src/mongo/db/stats/storage_stats.cpp
index fca73b4a473..4d45dc02b4b 100644
--- a/src/mongo/db/stats/storage_stats.cpp
+++ b/src/mongo/db/stats/storage_stats.cpp
@@ -108,4 +108,24 @@ Status appendCollectionStorageStats(OperationContext* opCtx,
 
     return Status::OK();
 }
+
+Status appendCollectionRecordCount(OperationContext* opCtx,
+                                   const NamespaceString& nss,
+                                   BSONObjBuilder* result) {
+    AutoGetCollectionForReadCommand ctx(opCtx, nss);
+    if (!ctx.getDb()) {
+        return {ErrorCodes::BadValue,
+                str::stream() << "Database [" << nss.db().toString() << "] not found."};
+    }
+
+    Collection* collection = ctx.getCollection();
+    if (!collection) {
+        return {ErrorCodes::BadValue,
+                str::stream() << "Collection [" << nss.toString() << "] not found."};
+    }
+
+    result->appendNumber("count", static_cast<long long>(collection->numRecords(opCtx)));
+
+    return Status::OK();
+}
 }  // namespace mongo
diff --git a/src/mongo/db/stats/storage_stats.h b/src/mongo/db/stats/storage_stats.h
index 0f62229c80a..f3a3db99b9b 100644
--- a/src/mongo/db/stats/storage_stats.h
+++ b/src/mongo/db/stats/storage_stats.h
@@ -38,10 +38,17 @@ namespace mongo {
 
 /**
  * Appends to 'builder' storage related statistics for the collection represented by 'nss'.
- * Used by both the collStats command and the $collStats aggregation stage.
  */
 Status appendCollectionStorageStats(OperationContext* opCtx,
                                     const NamespaceString& nss,
                                     const BSONObj& param,
                                     BSONObjBuilder* builder);
+
+/**
+ * Appends the collection record count to 'builder' for the collection represented by 'nss'.
+ */
+Status appendCollectionRecordCount(OperationContext* opCtx,
+                                   const NamespaceString& nss,
+                                   BSONObjBuilder* builder);
+
 };  // namespace mongo