MongoDB $lookup with nested object with nested array - mongodb

Question

I have 2 collections.
cases
_id: ObjectId.
name: string.
info: {
[here can be many different fields with diff types]
relatedEntities: [
{ role: string;
id: ObjectId;
} <--- here can be a lot of entities
]
}
entities
_id: ObjectId.
type: string,
name: string,
info: {
[here can be many different fields with diff types]
}
I need to retrieve all cases and for each case.info.entities object I need to have field data which will equal to entity document ( case.info.entities.id === entity_id)
Example what I need to have
_id: ObjectId.
name: string.
info: {
[here can be many different fields with diff types]
entities: [
{ role: string;
id: ObjectId;
data: {
_id: ObjectId.
type: string,
name: string,
info: {
[here can be many different fields with diff types]
}
}
} <--- here can be a lot of entities
]
}
How to do it in a proper way?
At the moment I implemented this is that way:
{ $unwind: "$info.relatedEntities" },
{ $lookup: {
"from": "entities",
"localField": "info.relatedEntities.entity",
"foreignField": "_id",
"as": "info.relatedEntities.entityObject"
}},
{ $group: {
"_id": "$_id",
"templateType":{$first: "$templateType"},
"info":{$first: "$info"},
"relatedEntities": {
$push: "$info.relatedEntities"
}
}}
It's working, but required additional parsing when data is retrieved, but I'd like to do it without workarounds..

Answer 1

You have done almost everything. Based on your query and model I have given below query (Field names might be different). Hope it helps.
db.cases.aggregate([
{ $unwind: '$info.relatedEntities' },
{ $lookup: {
from: 'entities',
localField: 'info.relatedEntities.entity',
foreignField: '_id',
as: 'info.relatedEntities.entityObject'
}
},
{ $group: {
_id: {
_id : '$_id',
templateType : '$templateType',
name : '$name',
info : {
address : "$info.address",
}
},
'relatedEntities': {
$push: {
role : '$info.relatedEntities.role',
entity : '$info.relatedEntities.entity',
data : { $arrayElemAt: [ '$info.relatedEntities.entityObject', 0 ] }
}
}
}
},
{
$project : {
_id : '$_id._id',
name : '$_id.name',
templateType : '$_id.templateType',
info : {
address : '$_id.info.address',
entities : '$relatedEntities'
}
}
}
]).pretty()