How to find documents from multiple collection with similar field value - mongodb

Question

I have two collections:
Product
{ id: "1", model: "Light1", category: "Light"},
{ id: "2", model: "Light3", category: "Light"},
{ id: "3", model: "Lock1", category: "Lock"},
Item
{ id: "1", model: "Light1", category: "Light", color: "Blue"},
{ id: "2", model: "Light2", category: "Light", color: "Blue"},
{ id: "3", model: "Lock1", category: "Lock", color: "Blue"},
{ id: "4", model: "Light3", category: "Light", color: "Blue"}
{ id: "5", model: "Lock2", category: "Lock", color: "Blue"},
I want to find documents from the Item collection containing both model and category from the product collection.
From the example above, I want to get this so called new collection:
{ id: "1", model: "Light1", category: "Light", color: "Blue"},
{ id: "3", model: "Lock1", category: "Lock", color: "Blue"},
{ id: "4", model: "Light3", category: "Light", color: "Blue"}

Answer 1

You can try this aggregation query:
First $lookup from Item collection to join collections. This lookup uses a pipeline where you match the desired values: Local model is equal to foreign model and local category is equal to foreign category. This produces an array as output: if there is not any match the array will be empty.
So you can $match to not shown empty result array.
And use $project to output the values you want.
db.Item.aggregate([
{
"$lookup": {
"from": "Product",
"let": {
"model": "$model",
"category": "$category"
},
"pipeline": [
{
"$match": {
"$and": [
{
"$expr": {
"$eq": [
"$model",
"$$model"
]
}
},
{
"$expr": {
"$eq": [
"$category",
"$$category"
]
}
}
]
}
}
],
"as": "result"
}
},
{
"$match": {
"result": {
"$ne": []
}
}
},
{
"$project": {
"_id": 0,
"result": 0
}
}
])
Example here