Possibility of 1:1 or 1:M associations between all objects, including custom objects

[ManuelI]

It is possible to set one-to-many associations between standard objects (as described here), however that does not extend to custom objects. Currently it is only possible to create many-to-many relationships, which significantly reduces the usability and functionality of custom objects. 

As an example, we have added a custom object called Partner Agreements. We would like to associate that object to one company we share the agreement with, but a company should only be allowed to have one of these associations. 

[eliot_martin]

This issue needs more visibility.  

The inability to control custom object association is a significant issue when building solutions in HubSpot.  The thread linked below hints at an API based workaround for forcing the cardinality of custom objects but the HubSpot UI doesn't fully support this causing the Add buttons still appear on the one_to_one side when a record is already set.

HubSpot Community - One-to-many custom object associations - HubSpot Community

Did HubSpot never think that there would be scenarios where its users would need to avoid M:M relationships?

[NRosenstock]

Definitely - I believe Salesforce can do this.  Hubspot needs to as well.

[JKazarian]

The HubSpot UI needs to support 1:1 associations between custom objects and from custom objects to standard objects. 

[KRhoads]

Hello, we have the same exact need here. When trying to record contracts with customers there should be a 1:1 relationship for contract to companies, but a 1:M relationship for companies to contracts. 

[asmitj]

I'd like to create a 1:1 Deal-to-Contact association. Right now, if a deal is associated to a company that has multiple contacts, all these contacts get automatically associated to said deal. This in turn messes up our reporting with the same deal amount being counted as many times as there are contacts, when it should only be counted once.