Consider the entities 'Hotel Room' and 'Person' with a many-to-many…

Concept

In an ER model, an attribute is placed on the construct whose identity functionally determines that fact. Entity attributes describe one entity instance; relationship attributes describe a particular association among participating entity instances.

Application

1. Take a person P and two rooms R1 and R2. P may have separate lodging occurrences with R1 and R2, and the payment can differ between those occurrences.

2. A value stored only on Person would give P one person-level payment and could not distinguish the room-specific lodging occurrences.

3. A value stored only on Hotel Room would give each room one room-level payment and could not distinguish which person or lodging occurrence made it.

4. The pair (P, R1) identifies one Lodging relationship instance, while (P, R2) identifies another, so occurrence-specific payment data is determined by the Lodging association.

Cross-check and contrast

• Person is appropriate for person-specific facts that do not vary by room.

• Hotel Room is appropriate for room-specific facts that do not vary by occupant.

• Lodging is appropriate for facts that vary with the person–room association; in a relational design, an associative table can store the two foreign keys and the payment.

• None of these is unnecessary because ER relationships are allowed to carry attributes.

Therefore, the rent payment should appear as an attribute of Lodging.