Project Fields in Array of Array Objects in Mongodb - mongodb

Question

I have this document:
[
{
"name": "Report1",
"specifications": [
{
"parameters": [
{
"name": "feature",
"value": [
"13"
]
},
{
"name": "security",
"value": [
"XXXX-695"
]
},
{
"name": "imageURL",
"value": [
"football.jpg"
],
}
]
}
]
},
{
"name": "Report2",
"specifications": [
{
"parameters": [
{
"name": "feature",
"value": [
"67"
]
},
{
"name": "imageURL",
"value": [
"basketball.jpg"
],
},
{
"name": "security",
"value": [
"XXXX-123"
]
}
]
}
]
}
]
I want to obtain specifications[0].parameters.value[0] where parameters.name = "imageUrl". Like that:
[
{
"imageparam": "football.jpg",
"name": "Report1"
},
{
"imageparam": "basketball.jpg",
"name": "Report2"
}
]
I use MongoDB 3.6.3 with MongoDB Compass. I want to use aggregation (to add a pipeline in MongoDb Compass) so I could write finally this aggregation but it has 5 $project stage. Is there any more efficient or better solution:
db.collection.aggregate([{$project: {
_id: 0,
name: 1,
specifications: {$arrayElemAt: ["$specifications", 0]}
}}, {$project: {
name: 1,
imageparam: {
$filter: {
input: '$specifications.parameters',
as: 'param',
cond: {
$eq: [
'$$param.name',
'imageURL'
]
}
}
}
}}, {$project: {
name: 1,
imageparam: {$arrayElemAt: ["$imageparam",0]}
}}, {$project: {
name: 1,
imageparam: "$imageparam.value"
}}, {$project: {
name: 1,
imageparam: {$arrayElemAt: ["$imageparam",0]}
}}])
This is playground.

Answer 1

You can reduce it to 2 stages, might be there will be other options as well,
pass directly $arrayElemAt of specifications.parameters to $filter input and find the matching value for imageURL
use can use $addFields or $set stage to get first element from return result
db.collection.aggregate([
{
$project: {
_id: 0,
name: 1,
imageparam: {
$arrayElemAt: [
{
$filter: {
input: { $arrayElemAt: ["$specifications.parameters", 0] },
cond: { $eq: ["$$this.name", "imageURL"] }
}
},
0
]
}
}
},
{
$addFields: {
imageparam: { $arrayElemAt: ["$imageparam.value", 0] }
}
}
])
Playground
The second option you can do it in single stage using $let to bind the variables for use in the specified expression,
db.collection.aggregate([
{
$project: {
_id: 0,
name: 1,
imageparam: {
$let: {
vars: {
param: {
$arrayElemAt: [
{
$filter: {
input: { $arrayElemAt: ["$specifications.parameters", 0] },
cond: { $eq: ["$$this.name", "imageURL"] }
}
},
0
]
}
},
in: { $arrayElemAt: ["$$param.value", 0] }
}
}
}
}
])
Playground