Send User ID from Google Tag Manager to Google Analytics 4

user_7736744 0
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I have set up google tag manager with my GA4 property
(NOT legacy Universal Analytics)
according to the official guide:
https://support.google.com/tagmanager/answer/6103696?hl=en&ref_topic=3441530#zippy=%2Cweb-pages

and I have figured out how to send custom/recommended events to GA4 using
dataLayer.push({}) objects, using their "event" field in triggers and tags,
based on a Universal Analytics guide
(https://www.analyticsmania.com/post/track-logins-with-google-tag-manager/),
because this appears to be a black hole in the official guide:
https://support.google.com/tagmanager/answer/9442095
only describes, briefly, the GTM-side of the story.

Same goes for sending User ID: the docs only tell
when creating a Google Analytics 4 Configuration tag:

To set a user ID, add a row to Fields to Set. 
Set the Field Name to user_id, and the Value to 
a Tag Manager Variable that returns the user ID.

and I only take the knowledge from the external UA guides
to use a Data Layer Variable that reads the user_id field from the dataLayer,
and also create a corresponding user_id User Property on Google Analytics, which in Universal Analytics used to
have related settings that don't exist anymore (https://www.analyticsmania.com/post/google-analytics-user-id-with-google-tag-manager/).

So I have deployed my GA4 Events with the user_id data layer variable
referenced in their Configuration (and it is visible on the datalayer in Debug View), created the corresponding User Property in GA, and enabled User-ID reporting.
Yet, in my real-time view, the user_id fields always get some weird "gtm.js" value
(as if my data layer variable value was replaced by the event name
from the default dataLayer.push({event:"gtm.js",user_id:"ignored value")).
What in the world is missing for my user_id field to be recognised? user_id in debug view

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Thanks!

google-tag-manager google-analytics-4 google-analytics
2 Answers

Thanks @bpstrngr! Was going around in circles for a while before finding your answer.

To clarify the step by step in case anyone else bumps into this;

  1. When sending userId in dataLayer add an event name:
window.dataLayer.push({
   'userId' : '{{ myUserId }}',
   'event': 'userIdSet',
})

  1. In GTM, create a Custom Event trigger that listens to that event (set event name equal to whatever event you wrote in step 1): GTM Trigger configuration print screen

  2. Add this event as a firing trigger into your tag: GTM Tag configuration print screen

user_39803030

So at long last it turned out, that since the configuration tag was fired by the Page View event, that happened before I pushed the user_id to the dataLayer, so it filled the non-existing value with the event name instead.

I gave the user_id event a name to create a custom event trigger, that re-triggers the configuration tag. This way the user_id-s are finally visible.

user_77367440
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