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  1. Core Server
  2. SERVER-51886

$lookup + $merge pipeline may fail to resolve views correctly when collection names collide

    • Fully Compatible
    • ALL
    • v4.4, v4.2
    • Query 2020-11-30, Query 2020-12-14

      Getting the namespace involved in a cross-database $merge stage causes the StringMap<ResolvedNamespace> to store a key for the target collection name with an empty pipeline. But that collection name may also be the name for a collection or view in the database the aggregate command is being run against. For certain pipelines, this may cause a $lookup on a view to instead act as if it were performing the $lookup on an empty collection without erroring.

      if (involvedNs.db() != request.getNamespaceString().db()) {
          // If the involved namespace is not in the same database as the aggregation, it must be
          // from a $merge to a collection in a different database. Since we cannot write to
          // views, simply assume that the namespace is a collection.
          resolvedNamespaces[involvedNs.coll()] = {involvedNs, std::vector<BSONObj>{}};

      Note that because LiteParsedPipeline::getInvolvedNamespaces() returns a stdx::unordered_set<NamespaceString> that namespaces may be resolved in different orders across multiple aggregate command executions. And also note that because resolveInvolvedNamespaces() skips resolving a namespace if its collection name already exists in the StringMap, this issue would only manifest if the $merge namespace was resolved before the $lookup namespace.

            mihai.andrei@mongodb.com Mihai Andrei
            max.hirschhorn@mongodb.com Max Hirschhorn
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