MongoDB extra parameter in $lookup, is it possible? - mongodb

Question

I want to find all active=true from property, and aggregate each record with translation
BUT!! I only want the english text "gb", not the danish one "dk"
property
{
"_id" : "111",
"unique" : "ATL-D406",
"name" : "Atlantis",
"active" : true
},
{
"_id" : "222",
"unique" : "WAT-606",
"name" : "Wong Amat Tower",
"active" : true
}
translation
{
"_id" : "aaa",
"language" : "gb",
"property" : "111",
"texts" : "Great Condo with Pool View and Balcony"
},
{
"_id" : "bbb",
"language" : "dk",
"property" : "111",
"texts" : "Lækker Lejlighed med Pool udsigt og Balkon"
},
{
"_id" : "ccc",
"language" : "gb",
"property" : "222",
"texts" : "Luxury with Direct Beach Front"
},
{
"_id" : "ddd",
"language" : "dk",
"property" : "222",
"texts" : "Luksus direkte på Stranden"
}
As far as I can see, Mongodb only allow one field matching in the $lookup
Is there a way to do this except for the very complex $redact $$KEEP $$PRUNE (which seem like a very complex way of doing it)
Or would I be better off just doing two seperate finds?
Or is there a way to combine the result from two independant finds and project the fields you want?
Like:
- find all property where active = true
- find all translate where language = "gb"
- combine them and project some fields...
db.getCollection("property").aggregate(
// Find all active properties
{
$match: {active:true}
},
// Find matching translation record
{
$lookup: {
from: "translation",
localField: "_id",
foreignField: "property",
as: "translate"
}
}
)
The only way I can think of is to make two finds from js, and then do all the work there.

Answer 1

you have to other solutions (than $$KEEP and $$PRUNE) :
unwind results and match the langage you want :
add an $unwind stage and then filter documents on translate.langage field :
db.getCollection("property").aggregate([
{ $match: {active:true}},
{ $lookup: {
from: "translation",
localField: "_id",
foreignField: "property",
as: "translate"
}
},
{ $unwind: "$translate"},
{ $match : {"translate.language": "dk"}}
])
split the translation collection in multiple collection
The other solution would be to have one translation collection by langage, for example translation_en, translation_dk etc.
So you would just have to do the $lookup on the corresponding langage collection :
db.getCollection("property").aggregate([
{ $match: {active:true}},
{ $lookup: {
from: "translation_dk",
localField: "_id",
foreignField: "property",
as: "translate"
}
}
])
Note that the second option would be faster