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Type:
New Feature
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Resolution: Done
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Priority:
Major - P3
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Affects Version/s: None
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Component/s: Aggregation Framework
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Labels:
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Fully Compatible
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Query 13 (04/22/16)
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
- is depended on by
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- Closed
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- is documented by
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DOCS-9556 Docs for SERVER-10689: Add a $switch expression
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- Closed
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- is related to
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- Closed
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