Hello All!
New to HubSpot and was looking to associate Contacts to ‘Custom_Object_x’ in our instance. Would I use the instructions here (Accounts Dashboard | HubSpot) to ‘Associate Custom Objects’? How would I define cardinality? (Do I need to define it?) It would be a one to many in this case.
Is there an example call you used? I have just started working with HubSpots APIs and would greatly appreciate any guidance.
Hi everyone! 
Just adding a quick update for anyone coming across this thread.
HubSpot now offers more flexibility for managing associations between standard and custom objects, including association labels and limits.
To answer @AMorales7’s follow-up question: Yes, you would use a GET request to retrieve existing association definitions and their IDs. POST requests are used when creating new association definitions.
For one-to-many relationships, I’d recommend reviewing HubSpot’s current association limits and configuration options rather than relying on the older cardinality example above.
You can explore the latest options in the HubSpot developer documentation.
Hope this helps!
Hello @AMorales7 , associations for custom objects are created automatically for some standard objects as per the documentation in this link. This means that the moment you create a custom object there are association definitions automatically created between the custom object and email, meeting_event, note, task, call, and conversation_session.
To check existing association definitions, use a GET request to retrieve the custom object schema or the Associations API. POST is used to create association definitions, not retrieve them.
HubSpot now supports configuring association limits, including restrictions on how many records can be associated. These can be managed through association settings or the Associations API, depending on the object and subscription.
So with a POST to https://api.hubapi.com/crm/v3/schemas/{CUSTOM_OBJECT_ID}/associations
{
"fromObjectTypeId": "<CUSTOM_OBJECT_ID>",
"toObjectTypeId": "<TARGET_OBJECT_ID>",
"name": "<ASSOCIATION_NAME>",
"cardinality": "ONE_TO_ONE",
"inverseCardinality": "ONE_TO_MANY",
"id": "<ASSOCIATION_ID>"
},
{
"fromObjectTypeId": "<TARGET_OBJECT_ID>",
"toObjectTypeId": "<CUSTOM_OBJECT_ID>",
"name": "<INVERSE_ASSOCIATION_NAME>",
"cardinality": "ONE_TO_MANY",
"inverseCardinality": "ONE_TO_ONE",
"id": "<INVERSE_ASSOCIATION_ID>"
}
[For current implementations, use HubSpot’s supported association definitions and limits rather than relying on this 2021 cardinality example.]
At last I’m attatching this link here " One-to-many custom object associations " where there is some discussion related to this topic.
If you need further assistance don’t hesitate to write here.
There is also this great resource in this link right here that might help you get started " Custom object records API guide - HubSpot docs "
If my answer was helpful please mark it as a solution.
@dennisedson see I’m into API’s too!!! Please tag other people who you think might be of help, I haven’t made any friends here yet (except of you ofcourse) 
Moderator Note: this post was reviewed for relevancy and optimized for clarity on October 23, 2026. Thank you for your contributions to the HubSpot Community!
Incredibly helpful miljkovicmisa!! Many thanks for all the great detail. I dug into the docs you sent over the weekend. Quick question. If I wanted to check associations and get their ID’s as you mentioned wouldnt I use a GET call vs a POST?
Hi @AMorales7 ,
You can use this documentation for Custom_Object_x associate to Custom_Object_y
Link : Introducing Custom Objects for HubSpot
Hope this helps!
If we were able to answer your query, kindly help the community by marking it as a solution.
Thanks and Regards.
@miljkovicmisa Purhaps you have an answer to my problem with Custom Objects as well.
I need to include custom object (CO) tokens in automated mails with standard objects:
1. Contact properties
2. Associated CO properties
3. Deal properties.
I’ve made the custom object (called Recipients) through API, and the object have a one-to-many relation with Deal object. In other words the Recipent can have several Deals, but the Deal can only have one Recipient.
I want to be able to include a contacts name, as well as the Recipient name, and the Deal name, but the Deal-centeret flow does not seem to pull the associated object property. The contact name is no problem.
I hope someone can help.
Thank you
Andreas