How To Fix USMD1090 - Entity type &1, relationship &2: Leading entity type &3 is not unique

SAP Error Message - Details

• Message type: E = Error

• Message class: USMD1 - Datenmodel

• Message number: 090

• Message text: Entity type &1, relationship &2: Leading entity type &3 is not unique

What causes this issue?

The <LS>From-entity type </>&V4& has the leading entity type &V3&, which
needs to be unique in the context of the <LS>To-entity type </>&V1&:
&V3& must be either a part of the key of &V1& or must be assigned
through exactly one relationship of the type <LS>Referencing</> to the
entity type &V1&.

System Response

The data model cannot be activated.

How to fix this error?

Adjust the data model so that the condition described above is
fulfilled.

Procedure for System Administrators

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

Error message extract from SAP system. Copyright SAP SE.