Question

A user may have several labels, and links. Then, a user associates a label (or more) to a link. How does one represent the later relationship?

A solution could be a many-to-many relationship btw user and link with the optional attribute label. http://en.wikibooks.org/wiki/Java_Persistence/ManyToMany#Mapping_a_Join_Table_with_Additional_Columns
In this case the relationship btw user and label may better be ‘virtual’.

Any alternative I’m not seeing?

PS: I’ve used google bookmarks terminology, as it matches my case quite well.

Answer 1

If my understanding is correct, then you are in the following case:

Replacing ternary relationships

When ternary relationships occurs in
an ER model they should always be
removed before finishing the model.
Sometimes the relationships can be
replaced by a series of binary
relationships that link pairs of the
original ternary relationship.

                            
_{(source: grussell.org)}

                                                   Figure

: A ternary relationship example

• This can result in the loss of some information – It is no longer clear
  which sales assistant sold a customer
  a particular product.
• Try replacing the ternary relationship with an entity type and a
  set of binary relationships.

Relationships are usually verbs, so
name the new entity type by the
relationship verb rewritten as a noun.

• The relationship sells can become the entity type sale.

                            
_{(source: grussell.org)}

                                                  Figure
: Replacing a ternary relationship

• So a sales assistant can be linked to a specific customer and both of
  them to the sale of a particular
  product.
• This process also works for higher order relationships.

And this would be my suggestion: introduce a new entity.