How to get either an existing document or newly added document - mongodb

Question

I have a table structure as given below.
{
"_id" : ObjectId("5870f1d9dd0ef6e62102e6b8"),
"product" : "product1",
"title" : "Alt Summit 1",
"category" : "category1",
"order" : 1
}
{
"_id" : ObjectId("5870f1d9dd0ef6e62102e6b9"),
"product" : "product2",
"title" : "Alt Summit 2",
"category" : "category1",
"order" : 2
}
{
"_id" : ObjectId("5870f1d9dd0ef6e62102e6ba"),
"product" : "product1",
"title" : "Alt Summit 1",
"category" : "category1",
"order" : 2,
"added_by" : user1
}
{
"_id" : ObjectId("5870f1d9dd0ef6e62102e6bb"),
"product" : "product2",
"title" : "Alt Summit 2",
"category" : "category1",
"order" : 1,
"added_by" : user1
}
{
"_id" : ObjectId("5870f1d9dd0ef6e62102e6bc"),
"product" : "product3",
"title" : "Alt Summit 3",
"category" : "category1",
"order" : 3,
"added_by" : user1
}
{
"_id" : ObjectId("5870f1d9dd0ef6e62102e6bd"),
"product" : "product4",
"title" : "Alt Summit 4",
"category" : "category1",
"order" : 4
}
I would like to explain the database structure first.
I have default products for every user on my website and users can add their own products to the list or they can re-order the existing list. when they add a new product it will be added by their username. when they re-order the products, I get the existing default records and insert by appending the username and change the order (you will understand that if you observe the above data).
Now how can I get the products for a category specific to a user.
For example if i need to get all the products for a user1 in category1 the output should be as below (sorted based on the order field). If you observe carefully the above data, the same product is been repeated with extra field named added_by and order changed. I want to get the order of user if user customizes it, If not the default order.
{
"_id" : ObjectId("5870f1d9dd0ef6e62102e6bb"),
"product" : "product2",
"title" : "Alt Summit 2",
"category" : "category1",
"order" : 1,
"added_by" : user1
}
{
"_id" : ObjectId("5870f1d9dd0ef6e62102e6ba"),
"product" : "product1",
"title" : "Alt Summit 1",
"category" : "category1",
"order" : 2,
"added_by" : user1
}
{
"_id" : ObjectId("5870f1d9dd0ef6e62102e6bc"),
"product" : "product3",
"title" : "Alt Summit 3",
"category" : "category1",
"order" : 3,
"added_by" : user1
}
{
"_id" : ObjectId("5870f1d9dd0ef6e62102e6bd"),
"product" : "product4",
"title" : "Alt Summit 4",
"category" : "category1",
"order" : 4
}
I am open for any suggestion on if I should change the structure of the table. But at any given time I should be able to get to the output which I mentioned above.

Answer 1

You can try the below aggregation with current Mongo 3.4 version.
$match documents for query criteria.
$group documents on product and pick the $first based on the earlier sort order while keeping the document with $$ROOT.
$replaceRoot to promote the root document to top level.
'$sort' by order field.
db.collection.aggregate([{
$match: {
"category": "category1",
$or: [{
"added_by": "user1"
}, {
"added_by": {
"$exists": false
}
}]
}
}, {
$group: {
"_id": "$product",
"root": {
"$first": "$$ROOT"
}
}
}, {
$replaceRoot: {
newRoot: "$root"
}
},{
$sort: {
"order": 1
}
}])
Update after OP's Comment
$group documents on product and $push all the products with $$ROOT.
$project stage to compare the $size if not $eq to 1 then $filter to pick the product with the added_by field.
db.collection.aggregate([{
$match: {
"category": "category1",
$or: [{
"added_by": "user1"
}, {
"added_by": {
"$exists": false
}
}]
}
}, {
$group: {
"_id": "$product",
"products": {
"$push": "$$ROOT"
}
}
}, {
$project: {
"_id": 0,
"product": {
"$arrayElemAt": [{
"$cond": [
{
$eq: [{$size: "$products"}, 1]
},
"$products",
{
"$filter": {
input: "$products",
as: "product",
cond: {
$ifNull: ["$$product.added_by", false]
}
}
}
]
},
0
]
}
}
}, {
$replaceRoot: {
newRoot: "$product"
}
}, {
$sort: {
"order": 1
}
}]);