Add a $switch expression

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    • Fully Compatible
    • Query 13 (04/22/16)
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      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

            Assignee:
            Benjamin Murphy (Inactive)
            Reporter:
            Asya Kamsky
            Votes:
            7 Vote for this issue
            Watchers:
            14 Start watching this issue

              Created:
              Updated:
              Resolved: