Engineering Ticket Description:

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.