@bytesnz

Jack Farley, Computer Engineer

For this new blog, I am using MongoDB for storing all the articles and notes. During development, when I initially copied my old articles over, the code wasn't set up to store the published dates as a native BSON date.

Once I had fixed the code to store the dates as native dates, I needed to change the dates stored as strings to native dates for the sorting to work correctly.

Digging into the MongoDB documentation, I found the $dateFromString aggregator that I could use with an update aggregation pipeline.

After testing in the very helpful MongoDB Web Shells playpen that are included in the documentation, I came up with the following command to run.

db.objects.updateMany({ published: { $type: "string" } }, [ { $set: { published: { $dateFromString: { dateString: "$published" } } } } ] );

The command:

1. filters the articles in the objects collection, leaving only those with a string type value for the published date
2. sets the published date to be a native date using the existing value as the string for the $dateFromString operator.

I prepared myself and adjusted the command for running it on my data. Command in hand, I jumped onto the mongo shell, ran the command and…

> db.objects.updateMany({ published: { $type: "string" } }, [ { $set: { published: { $dateFromString: { dateString: "$published" } } } } ] );
2020-08-05T08:38:49.463+0100 E QUERY    [thread1] Error: the update operation document must contain atomic operators :
DBCollection.prototype.updateMany@src/mongo/shell/crud_api.js:625:1
@(shell):1:1
> 

Uh oh. What?! Huh?! But…

I went back and made sure that I had the command right, which I did. Then I wondered… what version am I running…

Turns out it was the latest and greatest… available through the standard repos - v3.6, whereas the latest stable version was v4.4. Sad face.

Not wanting to deal with extra faff, I decided to use docker for running Mongo instead of installing it. Brimming with confidence, I jumped straight for the new version and tried to mount my data from v3.6

$ docker run --name mongodb -v /var/lib/mongodb:/data/db -p 27017:27017 mongo
...
{"t":{"$date":"2020-08-05T17:49:08.027+00:00"},"s":"I",  "c":"STORAGE",  "id":22315,   "ctx":"initandlisten","msg":"Opening WiredTiger","attr":{"config":"create,cache_size=5415M,session_max=33000,eviction=(threads_min=4,threads_max=4),config_base=false,statistics=(fast),log=(enabled=true,archive=true,path=journal,compressor=snappy),file_manager=(close_idle_time=100000,close_scan_interval=10,close_handle_minimum=250),statistics_log=(wait=0),verbose=[recovery_progress,checkpoint_progress,compact_progress],"}}
{"t":{"$date":"2020-08-05T17:49:08.753+00:00"},"s":"W",  "c":"STORAGE",  "id":22347,   "ctx":"initandlisten","msg":"Failed to start up WiredTiger under any compatibility version. This may be due to an unsupported upgrade or downgrade."}
{"t":{"$date":"2020-08-05T17:49:08.753+00:00"},"s":"F",  "c":"STORAGE",  "id":28595,   "ctx":"initandlisten","msg":"Terminating.","attr":{"reason":"95: Operation not supported"}}
{"t":{"$date":"2020-08-05T17:49:08.753+00:00"},"s":"F",  "c":"-",        "id":23091,   "ctx":"initandlisten","msg":"Fatal assertion","attr":{"msgid":28595,"file":"src/mongo/db/storage/wiredtiger/wiredtiger_kv_engine.cpp","line":1100}}
{"t":{"$date":"2020-08-05T17:49:08.753+00:00"},"s":"F",  "c":"-",        "id":23092,   "ctx":"initandlisten","msg":"\n\n***aborting after fassert() failure\n\n"}

Doh.

I would to upgrade from v3.6 to v4.0, to v4.2, to v4.4. Because I was using docker it was super easy.

## Start the mongodb docker container, ensuring it will delete itself when I stop it
$ docker run --rm -d --name mongodb -v /var/lib/mongodb:/data/db -p 27017:27017 mongo:4.0
xxxxxxxxxxxxxxxxxxxxx
## Run the magic upgrade command
$ docker exec -it mongodb mongo --eval 'db.adminCommand( { setFeatureCompatibilityVersion: "4.0" } )'
MongoDB shell version v4.0.19
connecting to: mongodb://127.0.0.1:27017/?gssapiServiceName=mongodb
Implicit session: session { "id" : UUID("xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx") }
MongoDB server version: 4.0.19
{ "ok" : 1 }
$ docker stop mongodb

then rinse and repeat for 4.2 and 4.4. Once upgraded, I was able to run the command. Yay!

> db.objects.updateMany({ published: { $type: "string" } }, [ { $set: { published: { $dateFromString: { dateString: "$published" } } } } ] );
{ "acknowledged" : true, "matchedCount" : 35, "modifiedCount" : 35 }