our queries with sorts aren't returning results that really should be returned. I believe this happens when the index used by the sort doesn't include the record(s) that should be returned. here's a simple way to reproduce the problem:

> db.version()
1.6.5
> db.foo.insert(

{a: 1, b: 2}

)

> db.foo.find({}).sort(

{c: -1}

)

{ "_id" : ObjectId("4d49a87bff97b37303687cfa"), "a" : 1, "b" : 2 }

// all ok so far

> db.foo.find({c: {$exists: false}}).sort(

{c: -1}

)

{ "_id" : ObjectId("4d49a87bff97b37303687cfa"), "a" : 1, "b" : 2 }

// still ok

> db.foo.ensureIndex(

{c: -1}

) // now we have an index on the sort key

> db.foo.find({c: {$exists: false}}).sort(

{c: -1}

) // which makes my result disappear

> db.foo.find({}).sort(

{c: -1}

) // still ok without the $exists

{ "_id" : ObjectId("4d49a87bff97b37303687cfa"), "a" : 1, "b" : 2 }

> db.foo.find({c: {$not: {$exists: true}}}).sort(

{c: -1}

) // and ok with a convoluted $not $exists

{ "_id" : ObjectId("4d49a87bff97b37303687cfa"), "a" : 1, "b" : 2 }

this didn't happen in 10.6.3 (not sure in 10.6.4).