Problem:
A different number of documents are returned if an array is indexed or not.

Reproduce:

> db.perm.save(

{ what : "1", permissions : [ "perm-access-help" ] }

)
> db.perm.save(

{ what : "2", permissions : [ "perm-access-admin-area" ] }

)
> db.perm.save(

{ what : "3", permissions : [ "perm-access-admin-area","perm-access-help" ] }

)
> db.perm.save(

{ what : "4", permissions : [ ] }

)
> db.perm.save(

{ what : "5" }

)

Query with out the index, returns four documents
> db.perm.find( { permissions :

{ $in:["perm-access-help", "perm-access-admin-area", null] }

} )

{ "_id" : ObjectId("4cb64a7540292d4539c3ceeb"), "what" : "1", "permissions" : [ "perm-access-help" ] } { "_id" : ObjectId("4cb64a8040292d4539c3ceec"), "what" : "2", "permissions" : [ "perm-access-admin-area" ] } { "_id" : ObjectId("4cb64a8b40292d4539c3ceed"), "what" : "3", "permissions" : [ "perm-access-admin-area", "perm-access-help" ] } { "_id" : ObjectId("4cb64aa140292d4539c3ceef"), "what" : "5" }

Adding the index returns 5 documents
> db.perm.ensureIndex(

{permissions : 1}

)
> db.perm.find( { permissions :

{ $in:["perm-access-help", "perm-access-admin-area", null] }

} )

{ "_id" : ObjectId("4cb64a9940292d4539c3ceee"), "what" : "4", "permissions" : [ ] } { "_id" : ObjectId("4cb64aa140292d4539c3ceef"), "what" : "5" } { "_id" : ObjectId("4cb64a8040292d4539c3ceec"), "what" : "2", "permissions" : [ "perm-access-admin-area" ] } { "_id" : ObjectId("4cb64a8b40292d4539c3ceed"), "what" : "3", "permissions" : [ "perm-access-admin-area", "perm-access-help" ] } { "_id" : ObjectId("4cb64a7540292d4539c3ceeb"), "what" : "1", "permissions" : [ "perm-access-help" ] }

Business Case:

• reliability
  The behavior of the query should not change depending on whether an index is present or not.