What would be the correct cardinality and modality for the following business rules?

Each Vehicle has only one owner.

But each owner can have more than one vehicle.

Question

What would be the correct cardinality and modality for the following business rules?

Each Vehicle has only one owner.

But each owner can have more than one vehicle.

BlackTom AI · Accepted Answer

Let’s break down the business rules into relationships between Owner and Vehicle. The rule states that each Vehicle has only one owner. This means for every row in the Vehicle table, there is exactly one corresponding Owner. Conversely, each owner can have more than one vehicle, so an Owner can be linked to many Vehicle records.

Option A: "Owner table - cardinality one, modality one; Vehicle table - cardinality many, modality one". This aligns with the idea that from the Owner perspective, an owner can be linked to multiple vehicles (Vehicle side is many), and each Vehicle is owned by exactly one Owner (Owner side is one). The modality being one suggests the ownership is mandatory on both sides (a Vehicle must have an Owner, and an Owner must have at least one Vehicle) depending on how modality is interpreted in the diagram.

Option B: "Owner table - cardinality one, modality many; Vehicle table - cardinality many, modality one". Here the Owner side would imply that an Owner can be linked to many relationships, which contradicts the Sponsor: an Owner can have multiple vehicles, but the ownership relationship from the Owner’s point of view is not typically described as a one-to-many with modality many on the Owner side. This creates inconsistency, as it suggests Owners participate in many relationships but doesn’t clearly reflect that an Owner must own at least one Vehicle.

Option C: "Owner table - cardinality one, modality one; Vehicle table - cardinality one, modality many". This would imply each Vehicle can have many owners (Vehicle side modality many) or a one-to-many in the opposite direction, which contradicts the rule that a Vehicle has only one owner. The "modality many" on the Vehicle side would imply optional or multiple owners per vehicle, which is not permitted by the rule.

In summary, the rule set describes a one-to-many relationship from Owner to Vehicle: one Owner can own many Vehicles, but each Vehicle has exactly one Owner. The first option reflects that interpretation by setting the Owner side to one and the Vehicle side to many, with the suitable modality indicating the mandatory ownership at the Vehicle side. The other options either allow multiple owners per vehicle or imply optionality that conflicts with the stated constraints.

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What would be the correct cardinality and modality for the following business rules? Each Vehicle has only one owner. But each owner can have more than one vehicle.

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