Using mongodb $lookup on a single collection - mongodb

Question

I have a collection with documents like this
{
"_id" : ObjectId("5773ac6a486f811694711875"),
"bsk" : {
"bskItems" : [
{
"id" : 4,
"bskItemLineType" : "SaleItem",
"product" : {
"description" : "reblochon"
}
},
{
"id" : 5,
"bskItemLineType" : "SaleItem",
"product" : {
"description" : "Pinot Noir"
}
},
{
"id" : 13,
"bskItemLineType" : "PromotionItem",
"promotionApplied" : {
"bskIds" : [
4,
5
]
}
},
{
"id" : 8,
"bskItemLineType" : "SaleItem",
"product" : {
"description" : "Food"
}
},
{
"id" : 10,
"bskItemLineType" : "SubTotalItem"
},
{
"id" : 12,
"bskItemLineType" : "TenderItem"
},
{
"id" : 14,
"bskItemLineType" : "ChangeDue"
}
]
}
}
I want an output where I can see the "promotionsApplied" and the descriptions of the items they applied to. For the document above the "promotionsApplied" were to "bsk.BskItems.id" 4 and 5 so I would like the output to be:
{
"_id": xxxxx,
"promotionAppliedto : "reblochon"
},
{
"_id": xxxxx,
"promotionAppliedto : "Pinot Noir"
}
the query below:
db.getCollection('DSTest').aggregate([
{$project:{"bsk.bskItems.product.description":1,"bsk.bskItems.id":1}},
{$unwind: "$bsk.bskItems"},
])
gets me the descriptions
db.getCollection('DSTest').aggregate([
{$project:{"bsk.bskItems.promotionApplied.bskIds":1}},
{$unwind: "$bsk.bskItems"},
{$unwind:"$bsk.bskItems.promotionApplied.bskIds"},
])
gets me the promotions applied. I was hoping to be able to use $lookup to join the two based on _id and bsk.bskItems.promotionApplied.bskIds and _id and bsk.bskItems.id, but I can't figure out how.

Answer 1

I don't know if you solved your problem or if this is relevant anymore but I figured out your question:
db.DSTest.aggregate([
{
$unwind: "$bsk.bskItems"
},
{
$project: {
baItId: { $ifNull: [ "$bsk.bskItems.id", 0 ] },
"bsk": {
"bskItems": {
"promotionApplied": {
"bskIds": { $ifNull: [ "$bsk.bskItems.promotionApplied.bskIds", [0] ] }
}
}
},
"product": { $ifNull: [ "$bsk.bskItems.product.description", "" ] },
}
},
{
$unwind: "$bsk.bskItems.promotionApplied.bskIds"
},
{
$project: {
baItId: 1,
proAppliedId:
{
$cond: { if: { $eq: [ "$bsk.bskItems.promotionApplied.bskIds", 0 ] }, then: "$baItId", else: "$bsk.bskItems.promotionApplied.bskIds" }
},
product: 1
}
},
{
$group: {
_id: { proAppliedId: "$proAppliedId", docId: "$_id"},
product: { $push: { "p": "$product" } },
groupCount: { $sum: 1 }
}
},
{
$unwind: "$product"
},
{
$match: {
"product.p": {$ne: ""}, "groupCount": { $gt: 1}
}
},
{
$project: {
_id: "$_id.docId",
"promotionAppliedto": "$product.p"
}
}
])
With the dummy document you gave this is the result I get:
{
"_id" : ObjectId("5773ac6a486f811694711875"),
"promotionAppliedto" : "reblochon"
}
{
"_id" : ObjectId("5773ac6a486f811694711875"),
"promotionAppliedto" : "Pinot Noir"
}
But my advise is to put some thought in your database structure next time. You had apples and pears, so we had to make an Asian pear in order to get to this result. Also from the aggregation levels you see it was not an easy job. That could have been much easier if you had separated the arrays that contained the field product from the ones that contained the field promotionApplied.
To break it down and explain what is happening step by step:
{
$unwind: "$bsk.bskItems"
}
By unwinding we are flattening our array. We need this in order to access the fields inside the array and do operations on them . More about $unwind
{
$project: {
baItId: { $ifNull: [ "$bsk.bskItems.id", 0 ] },
"bsk": {
"bskItems": {
"promotionApplied": {
"bskIds": { $ifNull: [ "$bsk.bskItems.promotionApplied.bskIds", [0] ] }
}
}
},
"product": { $ifNull: [ "$bsk.bskItems.product.description", "" ] },
}
}
baItId: { $ifNull: [ "$bsk.bskItems.id", 0 ] }
With this line we just make sure that every document gets an basket item id. In your case they all do, I just added it to make sure. And if some document didn't have a value for that field we set it to 0 (you can set it to -1 or whatever you want)
"bsk": {
"bskItems": {
"promotionApplied": {
"bskIds": { $ifNull: [ "$bsk.bskItems.promotionApplied.bskIds", [0] ] }
}
}
}
Here we are creating an array for the field "$bsk.bskItems.promotionApplied.bskIds". Since not all documents have this field we have to add to them all, otherwise we are comparing oranges with apples.
"product": { $ifNull: [ "$bsk.bskItems.product.description", "" ] }
As said before, we have to make our documents look all alike so we also add $bsk.bskItems.product.description to the ones that don't have this field. Those who don't have the field we set it to an empty string
Now all our documents have the same structure and we can start with the actual sorting out.
{
$unwind: "$bsk.bskItems.promotionApplied.bskIds"
}
Since we want to access the ids inside $bsk.bskItems.promotionApplied.bskIds we have to unwind this array as well.
{
$project: {
baItId: 1,
proAppliedId:
{
$cond: { if: { $eq: [ "$bsk.bskItems.promotionApplied.bskIds", 0 ] }, then: "$baItId", else: "$bsk.bskItems.promotionApplied.bskIds" }
},
product: 1
}
}
baItId: 1 and product: 1, are just being passed on. The proAppliedId will contain our bsk.bskItems.promotionApplied.bskIds. If they are 0 then the get the same id as the field $baItId, otherwise they keep their id.
{
$group: {
_id: { proAppliedId: "$proAppliedId", docId: "$_id"},
product: { $push: { "p": "$product" } },
groupCount: { $sum: 1 }
}
}
Now finally we can group our documents by $proAppliedId that we created in the previous aggregation pipeline.
We also push the product values in an array. So there will be now arrays that contain two entries.
One with the value that we look for and one with an empty string because we did that in a previous aggregation pipeline "product": { $ifNull: [ "$bsk.bskItems.product.description", "" ] }
We also create a new field called groupCount to count the documents that were grouped together.
{ $project: {
_id: "$_id.docId",
"promotionAppliedto": "$product.p" } }
In the final project we just build the final document by how we want it to look like.
Hope you understand now why thinking, were and how we save things, matter.

Answer 2

Using document type database - it will be better to store promotion metadtaa instead of only id.
Please see attached example
"promotionApplied" : [{
bskId : 4,
name : "name",
otherData : "otherData"
}, {
bskId : 5,
name : "name5",
otherData : "otherData5"
}
]