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Type:
Task
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Resolution: Gone away
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Priority:
Major - P3
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Affects Version/s: None
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Labels:None
Description
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 {$meta: "sortKey"} would look like:
With this change in place the same sort key will look like this:
["first component", "second component", ..., "last component"]
I don't think any drivers are relying on the format of the sort key, but if they are, they will likely need minor compatibility fixes. Message me or the server-query Slack channel, and we can work out the details. Thanks!
Description of Linked Ticket
If a document has a compound sort key (e.g.:
{"": 1, "": 2}) and an expression tries to manipulate it, the result may only take one of the fields into account. For example, if the user tries to use {$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).
Scope of changes
Impact to Other Docs
MVP (Work and Date)
Resources (Scope or Design Docs, Invision, etc.)
- documents
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SERVER-43669 $meta:"sortKey" expressions can generate BSON with duplicate field names which are not correctly handled by Document/Value class
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- Closed
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