# How to reference context based on topic (without using function node)?
**URL:** https://discourse.nodered.org/t/how-to-reference-context-based-on-topic-without-using-function-node/3384
**Category:** General
**Created:** [23 September 2018 15:44 UTC](https://discourse.nodered.org/t/how-to-reference-context-based-on-topic-without-using-function-node/3384 "2018-09-23T15:44:19Z")
**Posts on this page:** 6
**Page:** 1
### Author:  [@Riven5](https://discourse.nodered.org/u/Riven5)
#### Post date: [23 September 2018 15:44 UTC](https://discourse.nodered.org/t/how-to-reference-context-based-on-topic-without-using-function-node/3384/1 "2018-09-23T15:44:19Z")
Basically I'm wondering if there's a way to do the equivalent of `flow.set(msg.topic,"foo");` in a Change node and `switch(flow.get(msg.topic))` in a Switch. I've tried variations of $(x) , $.x , {{x}}, and [x] with no luck.
Note that I am not trying to save the topic to flow.foo. I'm trying to save "foo" to flow.[msg.topic]
Ultimately it doesn't matter since I can just use a function, but it seems like it would be a common-enough use case for the basic nodes to handle.
---
### Author:  [@TotallyInformation](https://discourse.nodered.org/u/TotallyInformation)
#### Post date: [23 September 2018 16:57 UTC](https://discourse.nodered.org/t/how-to-reference-context-based-on-topic-without-using-function-node/3384/2 "2018-09-23T16:57:39Z")
I don't think there is.
It would require some kind of "templating" in common input fields much like we currently get for environment variables.
---
### Author:  [@Andrei](https://discourse.nodered.org/u/Andrei)
#### Post date: [24 September 2018 05:14 UTC](https://discourse.nodered.org/t/how-to-reference-context-based-on-topic-without-using-function-node/3384/3 "2018-09-24T05:14:36Z")
> [@Riven5](#):
>
> I'm trying to save "foo" to flow.[msg.topic]
I only was able to workout a partial workaround solution.
In this example flow it is injected:
```auto
msg.payload = foo
msg.topic = alarm
```
The change node uses a jsonata expression to create the object `{alarm : msg.payload}` and have it stored in `flow.sta`
The function node will read to the object `msg` the object `flow.sta`
Flow:
```auto
[{"id":"35baa7cd.9b2f08","type":"tab","label":"Flow 4","disabled":false,"info":""},{"id":"5ae8597e.4bdc08","type":"debug","z":"35baa7cd.9b2f08","name":"","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"true","x":690,"y":120,"wires":[]},{"id":"42343f37.f095a","type":"inject","z":"35baa7cd.9b2f08","name":"","topic":"alarm","payload":"foo","payloadType":"str","repeat":"","crontab":"","once":false,"onceDelay":0.1,"x":160,"y":120,"wires":[["df81e676.991df8"]]},{"id":"df81e676.991df8","type":"change","z":"35baa7cd.9b2f08","name":"","rules":[{"t":"set","p":"#:(memoryOnly)::sta","pt":"flow","to":"$.{topic : payload}","tot":"jsonata"}],"action":"","property":"","from":"","to":"","reg":false,"x":350,"y":120,"wires":[["f7335a43.eb7498"]]},{"id":"f7335a43.eb7498","type":"function","z":"35baa7cd.9b2f08","name":"read flow.sta","func":"\nmsg = flow.get(\"sta\",\"memoryOnly\");\nreturn msg;","outputs":1,"noerr":0,"x":520,"y":120,"wires":[["5ae8597e.4bdc08"]]}]
```
---
### Author:  [@TotallyInformation](https://discourse.nodered.org/u/TotallyInformation)
#### Post date: [24 September 2018 06:17 UTC](https://discourse.nodered.org/t/how-to-reference-context-based-on-topic-without-using-function-node/3384/4 "2018-09-24T06:17:29Z")
> [@Andrei](#):
>
> The change node uses a jsonata expression to create the object
Haha, I wondered if someone would come up with that. I decided not to include it since the Riven5 was looking for something simpler than a function node and the danger with JSONata (though I like it) is that it can be hard to parse 6 months down the line. 🙂
---
### Author:  [@Riven5](https://discourse.nodered.org/u/Riven5)
#### Post date: [24 September 2018 12:10 UTC](https://discourse.nodered.org/t/how-to-reference-context-based-on-topic-without-using-function-node/3384/5 "2018-09-24T12:10:43Z")
That's an interesting solution I hadn't considered. I agree with @TotallyInformation that it could be confusing 6 months later, but it does the trick in a one-liner, seems to work fine with a variety of nodes, and I can retrieve it at the other end with $flowContext('sta.'&topic) . I'm going to go ahead and mark this solved. Thanks.
---
### Author:  [@Riven5](https://discourse.nodered.org/u/Riven5)
#### Post date: [24 September 2018 18:35 UTC](https://discourse.nodered.org/t/how-to-reference-context-based-on-topic-without-using-function-node/3384/6 "2018-09-24T18:35:49Z")
Slight revision. Since this is setting the whole object each time, it overwrites any other topics that have been previously saved. Need to do this instead:
```
$merge([$flowContext("sta"),{$.topic: payload}])
```
At which point I'm leaning towards just returning to the original function.