entity-relationship-model

Someone told me that it was bad design to have loops in the datamodel. I have heard this before a couple of times but didn't pay much attention. For example you have entities User, Project, Activity. A project is owned by a User, so we have a one-to-many relationship from user to Project. An activity can be assigned to a single User, another one-to-many relationship from User to Activity. Of course a project is defined by a set of activities, another one-to-many relationship from Project to Activity. Thus a loop is formed. I asked this guy why is it bad design but he told me he didn't know

I was only able to find the following two differences: The relationships in an E-R model are explicitly defined, while they are implicit in a relational model. Relational models require an intermediate table (often called a "junction table") to hold two foreign keys that implement the many-to-many relationship. And why do we use the relational model, when we have an E-R diagram ? You have it backwards. The relationships in an E-R model are explicitly defined, while they are implicit in a relational model. No. Each Relational Model (RM) database base table and query result represents an