Details
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Type:
New Feature
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Status: Closed
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Priority:
Major - P3
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Resolution: Duplicate
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Affects Version/s: None
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Fix Version/s: None
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Component/s: Aggregation Framework
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Labels:None
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Sprint:Integration 17 (07/15/16)
Description
Syntax:
$transform: {
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root: <expression evaluates to object>,
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set: {<fieldname>: <arbitrary expression>}
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}
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Examples
// ===== Example #1 - Promoting a subfield to the top level =====
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>db.example.insert({_id: 0, a1: {b: 1}, a2: 2})
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>db.example.aggregate([{$transform: {root: “$a1”}]);
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{b: 1}
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// ===== Example #2 - Creating a new field =====
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>db.example.insert({_id: 0, a1: {b: 1}, a2: 2})
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>db.example.aggregate([{$transform: {set: {a3: 3}}}])
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{_id: 0, a1: {b: 1}, a2: 2, a3: 3}
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// ===== Example #3 - Creating new calculated fields =====
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>db.example.insert({_id: 0, a1: {b: 1}, a2: 2})
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>db.example.aggregate([{$transform: {set: {a3: {$gt: ["$a2", 3]}, a4: 4 }}}])
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{_id: 0, a1: {b: 1}, a2: 2, a3: false, a4: 4}
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// ===== Example #4 - Promoting a subfield to the top level and then creating new fields =====
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>db.example.insert({_id: 0, a1: {b: 1}, a2: 2 })
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>db.example.aggregate([{$transform: {root: "$a1", set: {c: 3}}}])
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{b: 1, c: 3}
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// ===== Example #5 - Error: using value of a field that doesn't exist anymore =====
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>db.example.insert({_id: 0, a1: {b: 1}, a2: 2 })
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>db.example.aggregate([{$transform: {root: "$a1", set: {c: "$a2"}}}])
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Error
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// Allowing this is a non-goal of this ticket.
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This command will be another stage in the aggregation pipeline. Either the root or set fields (or both) must be present in the $transform. The transformation replaces the document with the root field first, and then sets the fields specified in set on the contents of the replaced document.
Attachments
Issue Links
- duplicates
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- Closed
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- Closed
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