Can't get output data from nodes preceding node with no output data

nixikanius · June 10, 2023, 6:04am

Doing my own workflow I faced a problem with accessing output data from nodes preceding the node which doesn’t provide output data.

In my case, no output occurs in MongoDB find node when the searched document doesn’t exist yet. In such case, I want to continue processing (set “Always Output Data” in node settings) and then I want to use some data preceding the MongoDB node, but unfortunately the error occurs:

Missing pairedItem data (node ‘MongoDB’ probably didn’t supply it)

ExpressionError: Can’t get data for expression
    at createExpressionError (/usr/local/lib/node_modules/n8n/node_modules/n8n-workflow/dist/WorkflowDataProxy.js:453:20)
    at getPairedItem (/usr/local/lib/node_modules/n8n/node_modules/n8n-workflow/dist/WorkflowDataProxy.js:506:27)
    at pairedItemMethod (/usr/local/lib/node_modules/n8n/node_modules/n8n-workflow/dist/WorkflowDataProxy.js:714:40)
    at Object.get (/usr/local/lib/node_modules/n8n/node_modules/n8n-workflow/dist/WorkflowDataProxy.js:717:40)
    at Proxy.eval (eval at _create (/usr/local/lib/node_modules/n8n/node_modules/@n8n_io/riot-tmpl/dist/tmpl.js:377:14), <anonymous>:3:62)
    at Object._tmpl (/usr/local/lib/node_modules/n8n/node_modules/@n8n_io/riot-tmpl/dist/tmpl.js:335:60)
    at Expression.renderExpression (/usr/local/lib/node_modules/n8n/node_modules/n8n-workflow/dist/Expression.js:205:25)
    at Expression.resolveSimpleParameterValue (/usr/local/lib/node_modules/n8n/node_modules/n8n-workflow/dist/Expression.js:184:34)
    at resolveParameterValue (/usr/local/lib/node_modules/n8n/node_modules/n8n-workflow/dist/Expression.js:270:25)
    at Expression.getParameterValue (/usr/local/lib/node_modules/n8n/node_modules/n8n-workflow/dist/Expression.js:287:31)
    at resolveParameterValue (/usr/local/lib/node_modules/n8n/node_modules/n8n-workflow/dist/Expression.js:268:29)
    at /usr/local/lib/node_modules/n8n/node_modules/n8n-workflow/dist/Expression.js:276:61
    at Array.map (<anonymous>)
    at Expression.getParameterValue (/usr/local/lib/node_modules/n8n/node_modules/n8n-workflow/dist/Expression.js:276:47)
    at getNodeParameter (/usr/local/lib/node_modules/n8n/node_modules/n8n-core/dist/NodeExecuteFunctions.js:1227:42)
    at Object.getNodeParameter (/usr/local/lib/node_modules/n8n/node_modules/n8n-core/dist/NodeExecuteFunctions.js:1448:24)
    at Object.execute (/usr/local/lib/node_modules/n8n/node_modules/n8n-nodes-base/dist/nodes/Set/Set.node.js:173:18)
    at Workflow.runNode (/usr/local/lib/node_modules/n8n/node_modules/n8n-workflow/dist/Workflow.js:652:51)
    at /usr/local/lib/node_modules/n8n/node_modules/n8n-core/dist/WorkflowExecute.js:596:68
    at processTicksAndRejections (node:internal/process/task_queues:96:5)

Example workflow

Here are the synthetic workflow which transparently represents the problem:

Last node issues:

Can’t get data for expression (To fetch the data from other nodes that this expression needs, more information is needed from the node ‘MongoDB’ More info)
Execution Error.

n8n setup

n8n version: 0.230.3
Database: SQLite
n8n EXECUTIONS_PROCESS setting: own, main
Running n8n via: Docker
Operating system: MacOS

jan · June 10, 2023, 6:24am

Welcome to the community @nixikanius!

Yes, that is sadly a bug we also discovered a few days ago. There is already a PR to fix it which should be merged and released soon:

github.com/n8n-io/n8n

fix(core): Fix pairedItem for alwaysOutputData & multi identical resolve

n8n-io:master ← n8n-io:fix-paireditem-with-always-output-data

opened 02:44PM - 08 Jun 23 UTC

janober

+24 -0

Fixes pairedItem support for when: - always output data is activated on a node …(it now pairs all of them) - there are multiple possible items but they are all identical Tests are still missing. Github issue / Community forum post (link here to close automatically): Workflow to reproduce the issue it fixes: ``` { "meta": { "instanceId": "021d3c82ba2d3bc090cbf4fc81c9312668bcc34297e022bb3438c5c88a43a5ff" }, "nodes": [ { "parameters": { "values": { "string": [ { "name": "setting", "value": "hello" } ] }, "options": {} }, "name": "Set", "type": "n8n-nodes-base.set", "typeVersion": 1, "position": [ 480, -120 ], "id": "412a6f2b-5c3e-4521-bdc3-c9d00e30fc20" }, { "parameters": { "conditions": { "boolean": [ { "value1": true } ] } }, "id": "a3fb4c24-9608-490a-8ea0-9556b040a4b0", "name": "IF", "type": "n8n-nodes-base.if", "typeVersion": 1, "position": [ 1000, -120 ], "alwaysOutputData": true }, { "parameters": { "values": { "string": [ { "value": "={{ $('Set').item.json }}" } ] }, "options": {} }, "id": "b1ce1ea1-e695-4d3f-9788-5d4d2309910c", "name": "Set1", "type": "n8n-nodes-base.set", "typeVersion": 2, "position": [ 1320, -140 ] }, { "parameters": {}, "id": "8f04fbec-659b-4126-9f33-db2bdd29db1a", "name": "When clicking \"Execute Workflow\"", "type": "n8n-nodes-base.manualTrigger", "typeVersion": 1, "position": [ 220, -120 ] }, { "parameters": { "jsCode": "return [\n {\n 'thing': 1,\n 'letter': 'a'\n },\n {\n 'thing': 2,\n 'letter': 'b'\n },\n {\n 'thing': 3,\n 'letter': 'c'\n }\n]" }, "id": "1b6dfb33-5e75-452b-8e6d-8a45fdd3ab81", "name": "Code", "type": "n8n-nodes-base.code", "typeVersion": 1, "position": [ 760, -120 ] } ], "connections": { "Set": { "main": [ [ { "node": "Code", "type": "main", "index": 0 } ] ] }, "IF": { "main": [ [ { "node": "Set1", "type": "main", "index": 0 } ] ] }, "When clicking \"Execute Workflow\"": { "main": [ [ { "node": "Set", "type": "main", "index": 0 } ] ] }, "Code": { "main": [ [ { "node": "IF", "type": "main", "index": 0 } ] ] } } } ```

What you can do in the meantime is to use a different syntax. As the most time just one item gets passed through anyway can you directly target the first one (and in this case only one) with for example:

$('Set').first().json

I hope that is helpful!

nixikanius · June 10, 2023, 6:49am

@jan thank you very much for fast response. The workaround you suggested works as needed!

jan · June 10, 2023, 6:53am

Great to hear. Have fun!

system · September 8, 2023, 6:53am

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