Docs for SERVER-10689: Add a $switch expression

Engineering Ticket Description:

Syntax

    {$project: {

        fieldName: {

            $switch: {

                branches: [

                    {

                        case: <expression>,

                        then: <expression>

                    },

                    …

                ],

                default: <optional, expression>

            }

        }

    }}

Examples

> db.coll.insert([

  {_id: 0, x: 0}

  {_id: 1, x: -1}

  {_id: 2, x: 2}

]);

> db.coll.aggregate([{

  $project: {

    z: {

      $switch: {

        branches: [

          {

            case: {$eq: ["$x", 0]},

            then: "Equal to zero."

          },

          {

            case: {$eq: [{$abs: "$x"}, 1]},

            then: "Equal to plus or minus one."

          },

        ],

        default: "Something else."

      }

    }

  }

}])

{_id: 0, z: "Equal to zero."}

{_id: 1, z: "Equal to plus or minus one."}

{_id: 2, z: "Something else."}

Notes

• Allows $project expressions to be written without convoluted $conds.
• If "case" does not return a boolean, the value will be coerced to a bool.
• Since each case does not have to be mutually exclusive, an input will enter the first branch it matches.
  Errors
• 'branches' is missing or not an array with at least one entry.
• An entry in 'branches' does not contain 'case'
• An entry in 'branches' does not contain 'then'
• If 'default' is not specified, and an input doesn't match, an error will be thrown.

Old Description:
Currently if you want to do a multi-way switch statement equivalent in $project you have to do a very messy embedded sequence of {$cond:[]} statements (emulating an if-elif-elif-elif type sequence) which is very unreadable.

It would be nice to have an operator to do something like:

 'field: {$case:[ {condition1: val1}, {cond2: val2}, etc]}' 

where the first condition that matches sets the value of field to appropriate val.

Asya