While performing an academic experiment based on the random generation of a large set of tests to exercise the Query Language of MongoDB (all of which syntactically valid), a possible bug was detected in some preliminary trial tests.

Comparing an array field to an array of values (e.g., {$gt: ["a", "b"] or ["a"]}) instead of a single value (e.g., "a" or "b") on a hint()'ed query returns erroneous results.

• hint()'ing on the _id field (with or without creating a custom index with ensureIndex) returns results but cursor.count() and cursor.size() both return 0.
• hint()'ing on a custom index returns no results and cursor.count() and cursor.size() both return 0.
• without hint()'ing but with the custom index created no results are returned and cursor.count() and cursor.size() both return 0.
• the same query but without hint()'ing and with no custom indexes created results in the array portion of the query being ignored.

Examples:

find($and: [{array: {$gt: [10]}}, {notArray: {$lt: 1000}}]) results in the same as

find({notArray: {$lt: 1000}}) meaning that the array portion of the query is ignored

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ensureIndex({notArray: 1})

find($and: [{array: {$gt: [10]}}, {notArray: {$lt: 1000}}]) no results and cursor.count() and cursor.size() both return 0

find($and: [{array: {$gt: [10]}}, {notArray: {$lt: 1000}}]).hint("id") shows results in the shell but cursor.count() and cursor.size() both return 0

find($and: [{array: {$gt: [10]}}, {notArray: {$lt: 1000}}]).hint("notArray_1") no results and cursor.count() and cursor.size() both return 0