Syntax

{$in: [<arbitrary expression>, <arbitrary array expression>]}

Examples

db.coll.insert([
  {_id: 0, x: "Bob"}
]);
> db.coll.aggregate([{
  $project: {
    hasBobOrRob: {$in: ["$x", ["Bob", "Rob"]]}
  }
}]);
{_id: 0, hasBobOrRob: true}

// Example 2
> db.coll.insert([
  {_id: 0, x: "aaa"},
  {_id: 1, x: /^a/}
]);
> db.coll.aggregate([{
  $project: {
    matches: {$in: ["$x", [/^a/]]}
  }
}])
{_id: 0, matches: false}
{_id: 1, matches: true}
// Note the intentional inconsistency.
> db.foo.find({x: {$in: [/^a/]}})
{_id: 0, x: "aaa"}
{_id: 1, x: /^a/}

Notes

• Should result in the same values as a $in in a MatchExpression (except with regexes).
• Should not do regex matching if the values in the array are regexes.
• This is different from what the query system does, a query predicate of {x: {$in: [/^a/]} will find strings which start with "a". This is arguably a bug in query, and will not be duplicated here.

Errors

• If the second expression is not an array.
• If there are not two arguments passed to the expression.

Old Description
Currently expressions like $eq, $ne, $gt, etc are supported but there is no $in available to test set membership.