Query with join on mutiple collections with python in MongoDB - python-3.x

Question

I am newbie to MongoDB and Python (using pymongo 3.10.1). I can query one collection but I need to perform a join with two collections
collection1 {
code
some other fields
}
collection2 {
code
some other fields
}
I would like to achieve:
select * from collection2 left inner join collection1 on collection2.code = collection1.code
I found only basic examples for queries with Python to MongoDB.
How to achieve this with Python ? Could I use .aggregate and $lookup with Pythonn ?

Answer 1

Finally I get it working, here is the full code:
from pymongo import MongoClient
# connect to MongoDB, change the << MONGODB URL >> to reflect your own connection string
client = MongoClient(<< MONGODB URL >>)
db=client.mydb
docs = db.collection1.aggregate([{"$lookup":{
"from": "collection2", # other table name
"localField": "code", # key field in collection 2
"foreignField": "code", # key field in collection 1
"as": "linked_collections" # alias for resulting table
}},
{"$project": {"code": 1, "field_from_collection1": 1, "field_from_collection2": 1}}
])
#$project is to select fields we need, we could ommit it
for doc in docs:
print(doc)

Answer 2

So I feel like there are two parts to your question:
How do you do more complicated queriers with pymongo?
How do you do a join with mongo?
The first question is pretty simple, you can declare any type of query and just use find({<your query>}). Here's an example from W3
The answer to your main question is more complicated. Here's another stack article where it's talked about in more detail. But basically since 3.2 you can use $lookup to do joins.