MONGODB redact array - mongodb

Question

i just want to understand $redact in mongodb
suppose i have a collection like .
db.tab12.find()
{ "_id" : "1", "name" : "jan", "passport" : [ "usa" ] }
{ "_id" : "2", "name" : "jaan", "passport" : [ "usa", "canada" ] }
{ "_id" : "3", "name" : "jon", "passport" : [ "germany" ] }
and i run the following command
db.tab12.aggregate({$match:{"name":{$regex:"a"}}},{$redact:{$cond:{if:{$in:["$country",["canada"]]},then:"$$DESCEND",else:"$$PRUNE" }} } )
I get no result, mongodb lacks examples on net i think.

Answer 1

Let's imagine that you have collections PC and events. The PC is generating a different type of events and you want just to take the events which are from type:1
PC {
"_id": ObjectId,
"version": "SomeVersion",
"location": "Sofia",
"price": 220,
events: [1,2,3,4,5]
}
events{
"_id":1,
"type": 1,
"ts": "1999-01-01"
}
events{
"_id":2,
"securiy:" true,
"type": 2,
"ts": "2015-01-01"
}
PCs.aggregate([
{
$match: {
"location": "Sofia"
}
},
{
$unwind: {
path: '$events'
}
},
{
$project: {
events: '$pcs.events'
}
},
{
$unwind: {
path: '$events'
}
},
{
$lookup: {
from: 'events',
localField: 'event',
foreignField: '_id',
as: 'events'
}
},
{
$match: {
$and: [{
'events.ts': {
$gt: new Date("1970-01-01")
}
},
{
'events.ts': {
$lt: new Date("2000-02-02")
}
}
]
}
},
{
$redact: {
$cond: {
if: { $eq: [ '$events.type', 1 ] },
then: '$$KEEP',
else: '$$PRUNE'
}
}
},
this will remove all the other type of events, which are not passing the condition. After this step you can make some kind of grouping and for example if the events have some data (for example price), you can sum it.
{
$group: {
_id: {
isSecurityEvent: '$events.security',
},
totalEvents: { $sum: 1 },
}
}
}
])
From the mongo documentation you have 3 types of operators:
System Variable Description
$$DESCEND $redact returns the fields at
the current document level, excluding embedded documents. To include
embedded documents and embedded documents within arrays, apply the
$cond expression to the embedded documents to determine access for
these embedded documents.
$$PRUNE $redact excludes all fields at this current
document/embedded document level, without further inspection of any of
the excluded fields. This applies even if the excluded field contains
embedded documents that may have different access levels.
$$KEEP $redact returns or keeps all fields at this current
document/embedded document level, without further inspection of the
fields at this level. This applies even if the included field contains
embedded documents that may have different access levels.