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Type:
New Feature
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Resolution: Duplicate
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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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A $findOne operator should be useful when data must be compiled from multiple collections.
This is not like a "Collection JOIN Collection".
It is more towards "From this document LOOKUP that document".
$findOne: {Options, Criteria, Projection}
Options is
{Collection:"name", Database:"db"}Criteria and Projection will be similar to db.collection.findOne() method.
Take following sales order document for example:
db.sales.findOne()
{
"_id" : 1,
"refcd" : "S1",
"date" : ISODate("2010-11-26T11:18:16.463Z"),
"cust" : {
"cid" : 10,
"doc" : "customer"
},
"lineitem" : [
{
"rowid" : 1,
"prod" : {
"pid" : 20,
"unitprice" : 41.37,
"doc" : "product"
},
"qty" : 26,
"linetotal" : 1075.62
}
],
"amount" : 1075.62
}
To get the details from customer & product details for this document:
db.sales.aggregate([
{ $project:
{
"_id":1,
"refcd":1,
"date":1,
"cust.cid":1,
"cust.doc":1,
"cust.ref":{$let: {vars:{}, in:{$findOne:{
{Collection:"$cust.doc"},
{"_id":"$cust.cid"},
{"_id":0}
}
}
}
},
"lineitem":1,
"lineitem_ref": {$map: { input: "$lineitem",
as: "li",
in: {$findOne:{
{Collection:"$$li.prod.doc"},
{"_id":"$$li.prod.pid"},
{"_id":0}
}
}
}
},
"amount":1
}
}
])
- duplicates
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- Closed
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