Using Watch command.

[paradoxewan]

Hi there, trying to use the watch command,

msg.collection="jobs"; msg.operation="watch"; var pipeline = null; var options ={ fullDocument:"default" }; msg.payload = [pipeline,options]; return msg;

im getting error "TypeError: Cannot create property 'readPreference' on number '1542372194371'"

Any ideas? Thanks!

[ozomer]

I've never tried the watch command. Are you able to run it directly with the native mongodb package? http://mongodb.github.io/node-mongodb-native/3.1/api/Db.html#watch

[RichardJohnn]

I saw that TypeError when I had an inject node with the default timestamp being sent. Are you using a function block with that code in it as the input to the watch, @paradoxewan ?

I haven't figured out how to use the watch command, in node-red yet. With the native package you keep a reference to the cursor and use a while loop on it https://docs.mongodb.com/manual/reference/method/db.collection.watch/

[littleyoda]

nodered Has anyone successfully used the watch command with nodered?

For your information my flow is attached, which I have used for testing. Unfortunately it does not work. flow.txt

node.js I was able to test it successfully in node.js: server_js.txt based on https://levelup.gitconnected.com/taking-advantage-of-mongodbs-change-streams-for-real-time-iot-applications-using-nodejs-a0a76ae1376d

[littleyoda]

I looked into the problem further. The underlying library returns a ChangeStream Object. Currently the return value is ignored.

I tried to change mongodb.js to take care of this ChangeStream Object. (see the attached diff) This change is still far from a productive solution. Someone with nodered experience has to check my work and certainly have to improve it (see all the TODOs).

diff.txt