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Type:
Question
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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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None
Observed behavior: When a subfield is excluded, its parent field(s) will be included even if they would have otherwise been excluded.
Expected behavior: tbd, but with a find() projection an assertion is currently raised in this case.
c = db.c;
c.drop();
c.save( { x:1, a:{b:1,c:1} } );
c.save( { x:1, a:{c:1} } );
// No 'a' field is included.
printjson( c.aggregate( { $project:{ 'x':1 } } ) );
// The 'a' field is implicitly included due to the implementation of 'a.b' being excluded.
printjson( c.aggregate( { $project:{ 'x':1, 'a.b':0 } } ) );
// An assertion is raised because one field is included and another is excluded.
printjson( c.find( {}, { 'x':1, 'a.b':0 } ).toArray() );
Another effect of this behavior is that you cannot exclude both a field and its parent. So following on the above example you can't { $project:
{ a:0, 'a.b':0 }}. A human probably wouldn't write a projection like this, but a computer might.