[SOLVED] How to store aggregation pipeline in mongoDB?

How to store aggregation pipeline in mongoDB?

I am developing a rule engine using MongoDB and Python. I want to store Rules in MongoDB in a Rule database "myrulesdb" and want to use it to run aggregation pipeline on a different collection. Rule saved in MongoDB is

{
"_id" : ObjectId("57f46e843166d426a20d5e08"),
"Rule" : "[{\"$match\":{\"Name.First\":\"Sunil\"}},{\"$limit\":5}]",
"Description" : "You live long life"
}

Python Code:

import pymongo
from pymongo import MongoClient

client = MongoClient('localhost', 27017)

db = client['myDB']
coll = db['myColl']

ruledb = client['myrulesdb']
rulecoll = ruledb['myrules']

rule1 = rulecoll.find_one({}, {"Rule":1, "_id":0})

pipe = rule1['Rule']
print(pipe)

data = coll.aggregate(pipeline=pipe)

I am getting correct string when i print pipe.

[{"$match":{"Name.First":"Sunil"}},{"$limit":5}]

But when i pass this to aggregate() function, it gives me following error:

data = coll.aggregate(pipeline=pipe)
in aggregate raise TypeError("pipeline must be a list")
TypeError: pipeline must be a list

After retrieving pipeline from database, how can i pass it to aggregate() function?

Solution

I don't know why you are doing this but the culprit here is the value of the Rule field which is string. If you try a little debuging with the print() function your will see that type(pipe) yields <class 'str'>

You can fix this by loading the value using json.loads like this:

>>> import json
>>> r = {"Rule" : "[{\"$match\":{\"Name.First\":\"Sunil\"}},{\"$limit\":5}]"}
>>> pipe = json.loads(r['Rule'])
>>> pipe
[{'$match': {'Name.First': 'Sunil'}}, {'$limit': 5}]
>>> type(pipe)
<class 'list'>