In some cases it is possible to query for the undefined type. If full document matching is bypassed, such a query can match an empty array, which is stored in the index using the undefined type.
> c.ensureIndex( { a:1 } ) > c.save( { a:[] } ) > c.find( { a:{ $in:[ undefined ] } } ) { "_id" : ObjectId("4f7a117c37365e9167f24ed3"), "a" : [ ] } >
- is related to
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SERVER-1160 {a: undefined} broken with index
- Closed
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SERVER-12024 re-implement $in in terms of $or
- Backlog
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SERVER-5063 $in on first compound key element and limit efficiency
- Closed