E-R Diagram function is too simple.
It' better to add more icons like a weak entity, weak relationship to represent the relationship among entities. For now, I cannot even draw a diamond to show relationship.
Jack Myers commented
All relationships should not be marked as identifying (sadly, the default). Then when you have a weak entity, the relationship which will define its true primary key will be the identifying relationship.
However, there is no workaround for a weak entity. So this is definitely needed.