MongoDB lookup when foreign field is an array of ids - mongodb

Question

I have two collections, fruit and salesman . I want my query to return all fruit with comma separated salesman.
Salesman document have array of fruit id
fruit document have id, name ,........
salesman table have id,name, fruits[apple_id,mango_id.......],...
db.getCollection('fruit').aggregate([{ "$unwind": "$fruits" }, { "$lookup": {
"from": "salesman",
"localField": "fruits",
"foreignField": "_id",
"as": "fruitObjects"
}},
{ "$unwind": "$fruitObjects" } ])
even this query is not giving result of $fruitObjects..?
Fruit Document
{
"_id" : ObjectId("5b101caddcab7850a4ba32eb"),
"name" : "Mango"
}
{
"_id" : ObjectId("5b101caddcab7850a4ba32ec"),
"name" : "Pears"
}
{
"_id" : ObjectId("5b101caddcab7850a4ba32de"),
"name" : "apple"
}
{
"_id" : ObjectId("5b101caddcab7850a4ba32fe"),
"name" : "guava"
}
Salesman document
{
"_id" : ObjectId("5b101caddcab7850a4ba3257"),
"name" : "xyz",
"fruits":["5b101caddcab7850a4ba32ec","5b101caddcab7850a4ba32de","5b101caddcab7850a4ba32fe"]
}
{
"_id" : ObjectId("5b101caddcab7850a4ba3258"),
"name" : "abc",
"fruits":["5b101caddcab7850a4ba32eb","5b101caddcab7850a4ba32de"]
}
{
"_id" : ObjectId("5b101caddcab7850a4ba3259"),
"name" : "def",
"fruits":["5b101caddcab7850a4ba32ec"]
}
{
"_id" : ObjectId("5b101caddcab7850a4ba3260"),
"name" : "zxc",
"fruits":["5b101caddcab7850a4ba32ec","5b101caddcab7850a4ba32de","5b101caddcab7850a4ba32eb"]
}
``````````````````````````

Answer 1

Yes #barrypicker is correct - types of fields didn't match which is resulting in empty on salesman, Please try to store both of same type. Meanwhile you can actually convert one to other type - while querying each time, Also you need not to do $unwind, Please try below query :
Query 1:
db.fruit.aggregate([
{
$lookup:
{
from: "salesman",
let: { fruitName: { $toString: '$_id' } },
pipeline: [
{
$match:
{
$expr:
{ $in: ["$$fruitName", "$fruits"] }
}
}, { $project: { name: 1, _id: 0 } }
],
as: "salesman"
}
}])
Result for Query 1:
/* 1 */
{
"_id" : ObjectId("5b101caddcab7850a4ba32eb"),
"name" : "Mango",
"salesman" : [
{
"name" : "abc"
},
{
"name" : "zxc"
}
]
}
/* 2 */
{
"_id" : ObjectId("5b101caddcab7850a4ba32ec"),
"name" : "Pears",
"salesman" : [
{
"name" : "def"
},
{
"name" : "zxc"
}
]
}
/* 3 */
{
"_id" : ObjectId("5b101caddcab7850a4ba32de"),
"name" : "apple",
"salesman" : [
{
"name" : "abc"
},
{
"name" : "zxc"
}
]
}
/* 4 */
{
"_id" : ObjectId("5b101caddcab7850a4ba32fe"),
"name" : "guava",
"salesman" : []
}
Or if you want those names to be in an array :
Query 2 :
db.fruit.aggregate([
{
$lookup:
{
from: "salesman",
let: { fruitName: { $toString: '$_id' } },
pipeline: [
{
$match:
{
$expr:
{ $in: ["$$fruitName", "$fruits"] }
}
}, { $project: { name: 1, _id: 0 } }
],
as: "salesmanList"
}
}, {
$project: {
name: 1, salesman: {
$map:
{
input: "$salesmanList",
as: "each",
in: '$$each.name'
}
}
}
}])
Result for Query 2:
/* 1 */
{
"_id" : ObjectId("5b101caddcab7850a4ba32eb"),
"name" : "Mango",
"salesman" : [
"abc",
"zxc"
]
}
/* 2 */
{
"_id" : ObjectId("5b101caddcab7850a4ba32ec"),
"name" : "Pears",
"salesman" : [
"def",
"zxc"
]
}
/* 3 */
{
"_id" : ObjectId("5b101caddcab7850a4ba32de"),
"name" : "apple",
"salesman" : [
"abc",
"zxc"
]
}
/* 4 */
{
"_id" : ObjectId("5b101caddcab7850a4ba32fe"),
"name" : "guava",
"salesman" : []
}
fruit collection :
/* 1 */
{
"_id" : ObjectId("5b101caddcab7850a4ba32eb"),
"name" : "Mango"
}
/* 2 */
{
"_id" : ObjectId("5b101caddcab7850a4ba32ec"),
"name" : "Pears"
}
/* 3 */
{
"_id" : ObjectId("5b101caddcab7850a4ba32de"),
"name" : "apple"
}
/* 4 */
{
"_id" : ObjectId("5b101caddcab7850a4ba32fe"),
"name" : "guava"
}
salesman collection :
/* 1 */
{
"_id" : ObjectId("5b101caddcab7850a4ba3258"),
"name" : "abc",
"fruits" : [
"5b101caddcab7850a4ba32eb",
"5b101caddcab7850a4ba32de"
]
}
/* 2 */
{
"_id" : ObjectId("5b101caddcab7850a4ba3259"),
"name" : "def",
"fruits" : [
"5b101caddcab7850a4ba32ec"
]
}
/* 3 */
{
"_id" : ObjectId("5b101caddcab7850a4ba3260"),
"name" : "zxc",
"fruits" : [
"5b101caddcab7850a4ba32ec",
"5b101caddcab7850a4ba32de",
"5b101caddcab7850a4ba32eb"
]
}
If you want the entire object from salesman then you could remove { $project: { name: 1, _id: 0 } } in $lookup.
Ref : $lookup, $map

Answer 2

Does this help?
db.salesman.aggregate([
{ $unwind : "$fruits" },
{ $addFields : { "fruitObjectId": { $toObjectId: "$fruits" } } },
{ $lookup : {
"from" : "fruit",
"localField" : "fruitObjectId",
"foreignField" : "_id",
"as" : "fruitObjects"
}
}
])
Or, perhaps from the opposite perspective?
db.fruit.aggregate([
{ $project : {
_id : { $toString : "$_id" },
name : 1
}
},
{ $lookup : {
"from" : "salesman",
"localField" : "_id",
"foreignField" : "fruits",
"as" : "fruitObjects"
}
}
])