MongoDB: Select from one collection based on another collection - json

Question

I'm new to MongoDB and I'm trying to find a category that meets the required params. I have two collections, categories (list of categories) and ref_categories (manages nested category relationships)
collection categories:
[
{
"id": "A1001",
"key": "3dmodels",
},
{
"id": "A1002",
"key": "animals",
},
{
"id": "A1003",
"key": "birds",
},
{
"id": "A1004",
"key": "reptiles",
},
{
"id": "A1005",
"key": "birds",
}
]
collection categories_ref:
[
{
"category_id": "A1001", // 3dmodels parented to
"p_category_id": "root", // root
},
{
"category_id": "A1002", // animals parented to
"p_category_id": "A1001", // 3dmodels
},
{
"category_id": "A1003", // birds parented to
"p_category_id": "A1002", // animals
},
{
"category_id": "A1004", // reptiles parented to
"p_category_id": "A1002", // animals
},
{
"category_id": "A1005", // birds parented to
"p_category_id": "A1004", // reptiles
}
]
You'll noticed in my Categories collection there are two entries for 'birds' however they each have a different parent category.
I'm trying to create a query that allows me to find the category by key and parent category key.
Pseudo example.... I want to find 'birds' but the entry who has a parent called 'animals'. Otherwise return null.
the expected output would be
// find category_key: "birds" parent_category_key: "animals"
{
"id": "A1003",
"key": "birds",
}

Answer 1

You can use a $lookup with a pipeline where you match two conditions:
Join based on id. Categories id is the same as reference category_id.
Also check p_category_id is the desired code.
And after that you can $match to not get elements where the join result is empty (i.e, keep elements where exists a value).
db.categories.aggregate([
{
"$match": {"key": "birds"}
},
{
"$lookup": {
"from": "categories_ref",
"let": {"id": "$id"},
"pipeline": [
{
"$match": {
"$expr": {
"$and": [
{
"$eq": ["$category_id","$$id"]
},
{
"$eq": ["$p_category_id","A1002"]
}
]
}
}
}
],
"as": "cat_ref"
}
},
{
"$match": {"cat_ref": {"$ne": []}}
},
{
"$project": {"cat_ref": 0}
}
])
Example here