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Type:
Improvement
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Resolution: Done
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Priority:
Major - P3
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Affects Version/s: None
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Component/s: Aggregation Framework
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Labels:
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Fully Compatible
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v3.4
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Query 2017-03-27
The $project stage supports both inclusion and exclusion based projections. However, there is no way to conditionally exclude a field based on the result of some computation. In order to support this use case, we will add a new system variable REMOVE. When a field f is set to the value of an expression, and that expression evaluates to $$REMOVE, f is excluded from the resulting document.
Suppose, for example, that you wanted to include the field middleInitial, unless its value was the empty string. This could be achieved as shown below:
> db.names.drop();
> db.names.insert({last: "Smith", middleInitial: "H", first: "John"});
> db.names.insert({last: "Doe", middleInitial: "", first: "Jane"});
// Here we have removed the middleInitial field, but only in those documents where it contained the empty string.
> db.names.aggregate([{$project: {
_id: 0,
last: 1,
middleInitial: {$cond: {if: {$eq: ["", "$middleInitial"]}, then: "$$REMOVE", else: "$middleInitial"}},
first: 1
}}]);
{ "last" : "Smith", "first" : "John", "middleInitial" : "H" }
{ "last" : "Doe", "first" : "Jane" }
- causes
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SERVER-32819 Tests in jstests/aggregation/variables/ directory aren't running in aggregation passthroughs
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- Closed
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- is documented by
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DOCS-10022 Docs for SERVER-27614: provide aggregation expression to remove field: $$REMOVE
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- Closed
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