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  1. Drivers
  2. DRIVERS-806

$meta:"sortKey" expressions can generate BSON with duplicate field names which are not correctly handled by Document/Value class

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      This change affects how clients will see the results of a {$meta: "sortKey"} projection or a {$meta: "sortKey"} aggregation expression. From a quick search, I didn't find any documentation for this feature, but if there are examples in the documentation, they should be upgraded to show the new format. A release note about the change might be warranted as well.

      Previously, the result of {$meta: "sortKey"} would look like:

      {"": "first component", "second component", ..., "": "last component"}

      With this change in place the same sort key will look like this:
      ["first component", "second component", ..., "last component"]

      I don't think any drivers are relying on the format of the sort key, but if they are, they will likely need minor compatibility fixes. Message me or the server-query Slack channel, and we can work out the details. Thanks!

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      This change affects how clients will see the results of a {$meta: "sortKey"} projection or a {$meta: "sortKey"} aggregation expression. From a quick search, I didn't find any documentation for this feature, but if there are examples in the documentation, they should be upgraded to show the new format. A release note about the change might be warranted as well. Previously, the result of {$meta: "sortKey"} would look like: {"": "first component", "second component", ..., "": "last component"} With this change in place the same sort key will look like this: ["first component", "second component", ..., "last component"] I don't think any drivers are relying on the format of the sort key, but if they are, they will likely need minor compatibility fixes. Message me or the server-query Slack channel, and we can work out the details. Thanks!

      Downstream Change Summary

      This change affects how clients will see the results of a {$meta: "sortKey"} projection or a {$meta: "sortKey"} aggregation expression. From a quick search, I didn't find any documentation for this feature, but if there are examples in the documentation, they should be upgraded to show the new format. A release note about the change might be warranted as well.

      Previously, the result of {$meta: "sortKey"} would look like:

      Unknown macro: {""}

      With this change in place the same sort key will look like this:
      ["first component", "second component", ..., "last component"]

      I don't think any drivers are relying on the format of the sort key, but if they are, they will likely need minor compatibility fixes. Message me or the server-query Slack channel, and we can work out the details. Thanks!

      Description of Linked Ticket

      If a document has a compound sort key (e.g.:

      {"": 1, "": 2}

      ) and an expression tries to manipulate it, the result may only take one of the fields into account. For example, if the user tries to use {$objectToArray: {$meta: "sortKey"}}, the resulting array only has the first element (

      {k: "", v: 1}

      ).

      I haven't encountered any cases yet, but it's possible that as we use Document/Value more we'll run into internal cases where we are manipulating Documents with duplicate fields.

      In general, there is no invariant that all field names in a BSONObj are unique, but the only safe way to have a Document with duplicate field names is when you create it from a BSONObj and convert back to a BSONObj with no transformations (so that the lazy conversion behavior just returns the original BSONObj).

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            Reporter:
            backlog-server-pm Backlog - Core Eng Program Management Team
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