Problem with not to PK relationships was found.
- create relationship with join not to PK (target or source attribute isn't PK) in modeler
- create entities
- create relationship between entities
- commit changes
In database objects will be created, but relationship won't be created. So you can't be able to get joined objects.
This happens because in you can't create arcDiff.
Look DataDomainDBDiffBuilder arcCreated method and appendForeignKeys method.
In current logic you can get only PK attribute value from ObjectId.
First step is to add validation of this case to prevent creation of this relationship.