How to Join a Collection to a Document using Mongocxx - join

Question

I have a single document (or collection, whichever would work best to get to the solution) named "main":
{
"field1":"value1",
"objects":[ {"key":1}, {"key":2} ]
}
I have another collection named "foreign" with the following three documents:
//document 1
{
"foreignKey":1,
"name": "Mike"
}
//document 2 {
"foreignKey":2,
"name": "Michael"
}
//document 3
{
"foreignKey":3,
"name": "Mick"
}
I want the joined results to be:
//results
{
"field1":"value1",
"objects":[
{
"foreignKey":1,
"name": "Mike"
},
{
"foreignKey":2,
"name": "Michael"
}
]
}
I've only found examples in MongoDB that almost accomplishes this but it's example only has an array of values; but I have an array of objects.
I don't know how to translate this to Mongo-cxx.
I copied the following example from the MongoDB site for convenience
------------------------------------------------
//Consider a collection orders with the following //document:
({ "_id" : 1, "item" : "MON1003", "price" : 350, "quantity" : 2, "specs" :
[ "27 inch", "Retina display", "1920x1080" ], "type" : "Monitor" }
//Another collection inventory contains the following //documents:
{ "_id" : 1, "sku" : "MON1003", "type" : "Monitor", "instock" : 120,
"size" : "27 inch", "resolution" : "1920x1080" }
{ "_id" : 2, "sku" : "MON1012", "type" : "Monitor", "instock" : 85,
"size" : "23 inch", "resolution" : "1280x800" }
{ "_id" : 3, "sku" : "MON1031", "type" : "Monitor", "instock" : 60,
"size" : "23 inch", "display_type" : "LED" }
(
//The following aggregation operation performs a join //on documents in the orders collection which match a //particular element of the specs array to the size //field in the inventory collection.
db.orders.aggregate([
//stage
{
$unwind: "$specs"
},
//stage
{
$lookup:
{
from: "inventory",
localField: "specs",
foreignField: "size",
as: "inventory_docs"
}
},
//stage
{
$match: { "inventory_docs": { $ne: [] } }
}
])
//The operation returns the following document:
{
"_id" : 1,
"item" : "MON1003",
"price" : 350,
"quantity" : 2,
"specs" : "27 inch",
"type" : "Monitor",
"inventory_docs" : [
{
"_id" : 1,
"sku" : "MON1003",
"type" : "Monitor",
"instock" : 120,
"size" : "27 inch",
"resolution" : "1920x1080"
}
]
}

Answer 1

Using the example you linked from MongoDB's $lookup with an array, and based on aggregation_examples.cpp and test/collection.cpp, the following code implements an aggregation operation with $unwind, $lookup and $match that returns the same document as in the example:
#include <iostream>
#include <bsoncxx/json.hpp>
#include <mongocxx/client.hpp>
#include <mongocxx/instance.hpp>
#include <mongocxx/uri.hpp>
using bsoncxx::builder::basic::kvp;
using bsoncxx::builder::basic::make_document;
using bsoncxx::builder::basic::array;
int main(int, char**)
{
std::cout << "Start program" << std::endl;
mongocxx::instance instance{};
mongocxx::client client{ mongocxx::uri{} };
mongocxx::database db = client["stack"];
mongocxx::collection colInventory = db["inventory"];
// Pipeline stages: $unwind, $lookup and $match
mongocxx::pipeline pipe{};
pipe.unwind("$specs");
pipe.lookup(
make_document(
kvp("from", colInventory.name()),
kvp("localField", "specs"),
kvp("foreignField", "size"),
kvp("as", "inventory_docs")
)
);
pipe.match(
make_document(
kvp("inventory_docs", make_document(kvp("$ne", array{})))
)
);
auto cursor = db["orders"].aggregate(pipe, mongocxx::options::aggregate{});
for (auto doc : cursor) {
std::cout << bsoncxx::to_json(doc) << std::endl;
}
std::cout << "End program" << std::endl;
}
Output:
Start program
{ "_id" : 1.0, "item" : "MON1003", "price" : 350.0, "quantity" : 2.0, "specs" : "27 inch", "type" : "Monitor", "inventory_docs" : [ { "_id" : 1.0, "sku" : "MON1003", "type" : "Monitor", "instock" : 120.0, "size" : "27 inch", "resolution" : "1920x1080" } ] }
End program
Pretty print:
{
"_id": 1.0,
"item": "MON1003",
"price": 350.0,
"quantity": 2.0,
"specs": "27 inch",
"type": "Monitor",
"inventory_docs": [
{
"_id": 1.0,
"sku": "MON1003",
"type": "Monitor",
"instock": 120.0,
"size": "27 inch",
"resolution": "1920x1080"
}
]
}