How can I do a lookup based on conditional selection? - mongodb

Question

I have 2 collection based on collection1 I need to fetch from collection2
collection1
[
{
"_id": ObjectId("5ce7454f77af2d1143f84c38"),
"menu_name": "mainmenu1",
"sub_menus": [
{
"name": "submenu1",
"project": [
"All"
]
},
{
"name": "submenu2",
"project": [
"p2"
]
}
]
}
]
based on project field I need to fetch the record. If the project field is "All", I need to fetch all the projects under that submenu. if it is specific project only those project I need to fetch.
Here is my collection2
collection2
"project": [
{
"project_name": "p1",
"sub_menus": "submenu1",
},
{
"project_name": "p2",
"sub_menus": "submenu2",
}
{
"project_name": "p2",
"sub_menus": "submenu1",
},
{
"project_name": "p3",
"sub_menus": "submenu2",
}
{
"project_name": "p3",
"sub_menus": "submenu1",
},
{
"project_name": "p4",
"sub_menus": "submenu2",
}
]
https://mongoplayground.net/p/qH9fuJorq6z.
Can I do a conditional lookup?
Expected Result is
[
{
"_id": ObjectId("5ce7454f77af2d1143f84c38"),
"menu_name": "mainmenu1",
"sub_menus": [
{
"projectData": [
{
"project_name": "p1"
},
{
"project_name": "p2"
},
{
"project_name": "p3"
}
],
"sub_menu_name": "submenu1"
},
{
"projectData": [
{
"project_name": "p2"
}
],
"sub_menu_name": "submenu2"
}
]
}
]

Answer 1

Yes, you can define your own matching condition for $lookup pipeline but since your structure is deeply nested you need to flatten your sub_menus using $reduce before you run your $lookup. Once you bring all projects that match to any submenu you can use $map with $filter to put them into releval sub_menu:
db.collection1.aggregate([
{
$addFields: {
sub_menus_flat: {
$reduce: {
input: "$sub_menus",
initialValue: [],
in: {
$concatArrays: [
"$$value",
{ $map: { input: "$$this.project", as: "p", in: { name: "$$this.name", project: "$$p" } } }
]
}
}
}
}
},
{
$lookup: {
from: "collection2",
let: { sub_menus_flat: "$sub_menus_flat" },
pipeline: [
{
$match: {
$expr: {
$anyElementTrue: {
$map: {
input: "$$sub_menus_flat",
in: {
$and: [
{ $eq: [ "$$this.name", "$sub_menus" ] },
{ $in: [ "$$this.project", [ "All", "$project_name" ] ] }
]
}
}
}
}
}
}
],
as: "projects"
}
},
{
$project: {
_id: 1,
menu_name: 1,
sub_menus: {
$map: {
input: "$sub_menus",
in: {
sub_menu_name: "$$this.name",
projectData: {
$filter: {
input: "$projects",
as: "p",
cond: {
$and: [
{ $eq: [ "$$p.sub_menus", "$$this.name" ] }
]
}
}
}
}
}
}
}
},
{
$project: {
"sub_menus.projectData._id": 0,
"sub_menus.projectData.sub_menus": 0
}
}
])
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