How do I get projection $ to work in pymongo?

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How do I get projection $ to work in pymongo?



I have a query which runs in the mongo repl.


mongo



I have a collection of documents that look somewhat like this...


[
"_id" : ObjectId("5b6b6959828619572d48a9da"),
"files" : [

"original_filename" : "test_file1.csv",
"s3_key" : "intake/uploads/3edc78aa-2275-45ad-afcb-23bf4a765591/test_file1.csv"
,

"original_filename" : "test_file2.csv",
"s3_key" : "intake/uploads/3edc78aa-2275-45ad-afcb-23bf4a765591/test_file2.csv"

],
"created_at" : ISODate("2018-08-08T22:06:17.366Z")
]



In the mongo repl I can do:


mongo


db.uploads.find("_id": ObjectId("5b6b6959828619572d48a9da"), "files.original_filename": "test_file1.csv", "files.$": 1)



And I receive the data exactly as I want it:



"_id" : ObjectId("5b6b6959828619572d48a9da"),
"files" : [

"original_filename" : "test_file2.csv",
"s3_key" : "intake/uploads/3edc78aa-2275-45ad-afcb-23bf4a765591/test_file2.csv"

]



But in pymongo I do:


pymongo


query =
'_id': upload_id,
'files.original_filename': 'test_file1.csv'

projection = 'files.$': 1
result = list(self.uploads.find(query, projection))



And result is wrong:


['files': , '_id': ObjectId('5b6b6959828619572d48a9da')]



WHY?!?





Try with $elemMatch... db.uploads.find( "files.original_filename": "test_file1.csv" , "files": $elemMatch: original_filename: "test_file1.csv" )
– Anthony Winzlet
Aug 10 at 16:27



$elemMatch


db.uploads.find( "files.original_filename": "test_file1.csv" , "files": $elemMatch: original_filename: "test_file1.csv" )





I did try something similar. Thanks for responding! That does seem to work, but I'm still a little perplexed why there is a difference between the repl and python code.
– nackjicholson
Aug 10 at 16:31





Don't know why this works... But generally I use $elemMatch projection operator instead of $ positional operator and that's why I have suggested you
– Anthony Winzlet
Aug 10 at 16:38



$elemMatch


$




1 Answer
1


query =
'_id': upload_id

projection =
'files':
'$elemMatch':
'original_filename': original_filename



result = list(self.uploads.find(query, projection))



I used this projection instead and I seem to get the data I want. I would still like to know why, and if there are other better ways of doing this.






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