MongoDB API change
John Rose
Before I get into the reason for the post, I just want to say that I have been looking a several languages for a winter project, and Racket is definitely one of the best in terms of stability, documentation and tools. I'm old enough to have used "vi" before "vim" was even around, and over the intervening years (decades!), I have not been sure that graphical IDEs actually improved my efficiency, but I really like DrRacket! And, at least so far, "raco" has worked like a charm. Last winter's project language (to remain unnamed) had an equivalent to "raco" that made me want to scream. So ... kudos to the Racketeers!
Now on to MongoDB. I began playing with Racket MongoDB "Quickstart" example. It went well so I changed a few things, and I checked everything through the mongo shell. Then I began to play around with more Racket commands, and "mongo-db-collections" always returned an empty list, even when the mongo shell's "show collections" command returned a (correct) non-empty list. I finally tracked the problem down to a change in the MongoDB API since version 3.0 (my MongoDB is 3.4).
Instead of "<database>.system.namespaces" they now use "listCollections". So I first tried to change "~a.system.namespaces" to "listCollections" in (a copy of ) "... mongodb/basic/driver.rkt" without success. Then I noticed that the "mongo-list-databases" command used a "listDatabases" call to MongoDB. Thinking there might be a reason for the similar names, I tried modeling a new "mongo-db-collections" after "mongo-list-databases". After much wailing and gnashing of teeth (and learning some Racket in the process) I came up with what I hope is a suitable fix:
(provide/contract
[mongo-db-collections (mongo-db? . -> . (listof string?))])
(define (mongo-db-collections d)
(for/list ([c (in-vector (mongo-list-collections d))] #:when (hash-has-key? c 'idIndex))
(hash-ref c 'name)))
The "#:when" clause is because MongoDB slipped a "system.profile" collection into the response. It is distinguished from the ones for the database I was asking about by not having an 'idIndex field.
... and the helper function:
(define (mongo-list-collections d)
(hash-ref (hash-ref (mongo-db-execute-command! d `([listCollections . 1]))
'cursor) 'firstBatch))
The two "hash-ref"s are to peel the desired info out of the json (bson) returned by MongoDB.
I'm working on some routines to search the MongoDB and pretty-print the results of the search, but they are not very robust right now. If anyone is interested, or has more mature code for these tasks, let me know.
