Cardinality

Cardinality annotates one endpoint of a relationship. It expresses the minimum and maximum number of instances of the other participating entity that are associated with each instance of this entity.

Convention

remodel-core uses the look-here convention (Heuser's textbook, widely used in Brazil's academic ER modeling tradition): the cardinality written next to entity E describes how many tuples of the other entity in the relationship participate per tuple of E.

For example:

Author ──── [wrote] ──── Book
 1..N                    0..N

• Author.cardinality = OneToMany → "each Author wrote 1..N books" (many books per author)
• Book.cardinality = ZeroToMany → "each Book has 0..N authors" (many authors per book, or none)

Variants

Usage

m.relate("works_for", employee, Cardinality::ZeroToMany)
 .with(department, Cardinality::OneToOne);

This reads: each Employee works for 0..N departments (employee side), and each Department has exactly one role per employee (department side). The converter uses these cardinalities to decide whether to place a foreign key on the employee table, the department table, or emit a junction table.

Helper methods

let c = Cardinality::OneToMany;

c.is_many();       // true  — max is unbounded (ZeroToMany | OneToMany)
c.is_mandatory();  // true  — min ≥ 1 (OneToOne | OneToMany) → FK is NOT NULL

c.notation();      // "1..N"
c.code();          // 3  — brModelo wire integer (0..=3)

Cardinality::from_code(2);  // Some(ZeroToMany)
Cardinality::from_code(9);  // None — unknown code

Impact on conversion

The converter uses cardinalities to resolve how a relationship becomes foreign keys or a junction table:

You can override this per-relationship with RelationshipResolution. See Conversion for details.