Nested cursors with mongodb driver in phoenix/elixir - mongodb

Question

I am using the mongodb driver from https://github.com/ankhers/mongodb to query a mongodb database in an elixir/phoenix project. In another question, I asked about how to query nested jsons. Another issue would be how to query inserted documents. For example, I can do the following in python
date_=db['posts']['name'][name]['date'][date]
Here, 'posts' is the name of the collection, and the others are inserted documents. For example, one 'date' document was inserted through:
db['posts']['name'][name].insert_one({"date":date})
When I want to obtain all the inserted dates in python, I can do
date_list=[]
def get_date(db):
db_posts_name=db['posts']['name'][name]
for date_query in db_posts_name.find():
date_list.append(date_query["date"])
But I am at a loss in elixir/phoenix to do the same thing because if I do something like
list =
:mongo
|> Mongo.find("posts", %{})
|> Enum.fetch(4)
|> elem(1)
|> Map.fetch("name")
|> elem(1)
new_list =
:mongo
|> Mongo.find("posts", %{"name" => list})
another_list=new_list.find("date",%{})
I get error
Called with 3 arguments
%Mongo.Cursor{coll: "posts", conn: #PID<0.434.0>, opts: [slave_ok: true], query: %{"name" => name}, select: nil}
:find
[]
Is there a way to do this ?

Answer 1

Mongo.find returns always a cursor. A cursor is like a streaming api, so you have to
call some function like Enum.take() or Enum.to_list. If you processing very long collections
it is a good idea to use the Stream module instead.
If you want to fetch one document, then you can use Mongo.find_one.
I'm not understanding your example. I assume name is a parameter:
date_list=[]
def get_date(db):
db_posts_name=db['posts']['name'][name]
for date_query in db_posts_name.find():
date_list.append(date_query["date"])
The following code fetches in collection posts all documents which name is equal to the parameter name and returns only the date field:
date_list = :mongo
|> Mongo.find("posts", %{"name" => name}, %{"date" => 1})
|> Enum.map(fn %{"date" => date} -> date end)
By the way you can give elixir-mongodb-driver a try. This implementation supports the bulk api, change streams api and the transaction api as well.