Need a type conversion mechanism for booleans, ISODates, ObjectID

{$toISODate: <arbitrary expression>}

{$toObjectId: <arbitrary expression>}

{$toBoolean: <arbitrary expression>}

// Example 1

> db.coll.insert([

  {_id: 0, x: 1},

  {_id: 1, x: 0}

]);

> db.coll.aggregate([{

  $project: {

    boolean: {$toBoolean: "$x"}

  }])

{_id: 0, boolean: true}

{_id: 1, boolean: false}

// Example 2

> db.coll.insert([

  {_id: 0, x: "5769a41ba1db3f11dba6d4d3"}

]);

> db.coll.aggregate([{

  $project: {

    objectID: {$toObjectID: "$x"}

  }])

{_id: 0, objectID: ObjectId("5769a41ba1db3f11dba6d4d3")}

// Example 3

> db.coll.insert([

  {_id: 0, x: "2011-01-01"}

]);

> db.coll.aggregate([{

  $project: {

    isoDate: {$toISODate: ["$x", "YYYY-MM-DD"]}

  }])

{_id: 0, isoDate: ISODate("2011-11-11T00:00:00Z")}

{$coerceToBoolean: "xxx"} // false

{$coerceToBoolean: ["xxx", true]} // true

{$coerceToBoolean: ["5.6abc"]} // false

{$coerceToObjectID: ["5769a41ba1db3f11dba6d4d3"]} // ObjectID("5769a41ba1db3f11dba6d4d3")

{$coerceToObjectID: ["x", 2]} // 2

{$coerceToISODate: ["2011-11-11", "YYYY-MM-DD", ISODate("2014-11-11T00:00:00Z")]} // ISODate("2011-11-11T00:00:00Z")