"query" count posts for each category expressjs mongoose - node.js

Question

i am new at MERN stack
i tried to write this query but i can't and search a lot on google without any solution
i have to table one for posts and the second for categories
table 1 posts
--------------------------
|id | title | category |
--------------------------
| 1 | title1 | 1 |
| 2 | title2 | 2 |
| 3 | title3 | 1 |
| 4 | title4 | 1 |
=================
table 2 categories
---------------
|id | name |
---------------
| 1 | cat1 |
| 2 | cat2 |
====================
the results i want is this
---------------------------------
|id | name | number of posts |
---------------------------------
| 1 | cat1 | 3 |
| 2 | cat2 | 1 |
if it's can a help i write this query in mysql
SELECT categories.*,COUNT(posts.id) AS np FROM `categories` JOIN materials ON (categories.id = posts.category) GROUP BY categories.id
and thank you

Answer 1

You can try below aggregation in mongodb 3.6 and above
db.collection.aggregate([
{ "$lookup": {
"from": "posts",
"let": { "id", "$id" },
"pipeline": [
{ "$match": { "$expr": { "$eq": ["$category", "$$id"] }}},
{ "$count": "count" }
],
"as": "count"
}},
{ "$project": {
"name": 1,
"numberOfPosts": { "$arrayElemAt": ["$count.count", 0] }
}}
])