$project का उपयोग
$project हर document में fields choose, rename, और compute करता है।
In this page:
Syntax
{ $project: { field1: 1, field2: 1, _id: 0, computed: { $multiply: ["$a", "$b"] } } }$project
1 से fields include करें, 0 से exclude करें, और $concat, $multiply, और $cond जैसे expressions से नई fields compute करें। यह अगले stage या final output के लिए documents reshape करता है। $addFields और $set बाकी रखते हुए computed fields add करते हैं।
Note:
$addFields existing fields रखता है; $project shape replace करता है।
उदाहरण: $project
test> db.people.insertMany([{ _id: 1, first: "Ada", last: "Lovelace", born: 1815 }, { _id: 2, first: "Alan", last: "Turing", born: 1912 }])
{ acknowledged: true, insertedIds: { '0': 1, '1': 2 } }
test> db.people.aggregate([{ $project: { _id: 0, fullName: { $concat: ["$first", " ", "$last"] } } }])
[ { fullName: 'Ada Lovelace' }, { fullName: 'Alan Turing' } ]
test> db.people.aggregate([{ $project: { _id: 0, born: 1, century: { $ceil: { $divide: ["$born", 100] } } } }])
[ { born: 1815, century: 19 }, { born: 1912, century: 20 } ]
test> db.people.aggregate([{ $addFields: { initials: { $concat: [{ $substr: ["$first", 0, 1] }, { $substr: ["$last", 0, 1] }] } } }, { $project: { _id: 0, initials: 1, born: 1 } }])
[ { born: 1815, initials: 'AL' }, { born: 1912, initials: 'AT' } ]⚠️ Run this in your own terminal or Node.js environment.
Related Topics
{# common_mistakes/chapter_summary/browser_support: on Hindi pages the
view already swaps in the hi_ translation fields (or blanks these
out if untranslated), so this renders correctly for both languages
without a lang_code check here. #}
आम गलतियां
- यह भूल जाना कि _id default से included है
- Include और exclude mix करना
- जब $addFields simpler हो $project इस्तेमाल करना
चैप्टर सारांश
- $project documents reshape करता है
- 1 include करता है और 0 exclude करता है
- Expressions नई fields compute करते हैं
- $addFields बाकी रखता है
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