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University of California, Davis

Application Development Methodology

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Development Technique Concepts

Entity Type Horizon

A context within which an entity type is identifiable by a specific identifier, but beyond which the entity type is not uniquely identifiable.

Each entity type has a logical horizon. This is the collection of other entity types (possibly none) that can be uniquely selected by following chains of many-to-one relationships away from it.

Example of a Logical Horizon

Each entity type has a logical horizon. This is the collection of other entity types [possibly none] that can be uniquely selected by following chains of many to one relationships away from it. For example, the logical horizon for an ATTENDANCE is:

COURSE, SESSION and ITEM will form an initial grouping, as will STUDENT and BOOKING, because of their mandatory association with the COURSE grouping, both directly to SESSION and indirectly through COURSE PRESENTATION, it is more strongly associated with the COURSE grouping than with the STUDENT grouping, to which it has but a single association. At a further level of summarization, it is therefore merged into the COURSE grouping.

Entity Type Horizon for Unique Identification

For an ORDER ITEM, there is a "horizon of identifiability" which is called the entity type horizon. Within this horizon, the ORDER ITEM can uniquely identify singly related entity types, but anything outside of the horizon cannot be identified. For example, if a CUSTOMER makes many PAYMENTs, there is no way of distinguishing individual PAYMENTs from the viewpoint of an ORDER ITEM or even an ORDER. For each PAYMENT entity, there is a horizon that encompasses its CUSTOMER but that does not include any ORDERs.

You will find horizons useful when you analyze the structure of entity relationship models. For example, you may know that you want to be able to determine, uniquely, which ORDER a DELIVERY was for. So you know that ORDER must be within the horizon of a DELIVERY.

You can also consider the optionality of relationships when analyzing horizons. For example, if the plural end of a one-to-many relationship is optional, you cannot assume that the single entity type is identifiable, because there may be no pairing.