Tuesday, 12 October 2021

Connection traps in ER Modeling

Connection trap is the misinterpretation of the meaning of certain relationships. This connection traps can be broadly classified into fan and chasm trap.

Any conceptual model will contain potential connection traps. An error in the interpretation of the meaning of the relationship may cause the database to be incapable of storing certain information. Both the fan and chasm trap arise when the relationships appear to exist between entity types, but the links between occurrences may be ambiguous or not exist.

a) Fan Trap

Fan trap occurs when the model represents a relationship between entity types but the pathway between certain entity occurrences is ambiguous. Fan trap occurs when 1–M relationships fan out from a single entity.

In order to understand the concept of Fan trap, consider the following example

Contractor works in a team. . . . . . . . . Statement (1)
Team develops projects. . . . . . . . . . . . Statement (2)

Statement (1) represents M–1 relationship. Statement (2) represents 1–M relationship. But the information about which contractors are involved in developing which projects is not clear. Consider another example of Fan trap.

Department is on Site. . . . . . . . . Statement (1)
Site employs Staff. . . . . . . . . . . . . . . Statement (2)

Statement (1) represents M–1 relationship, because many departments may be in a single site. Statement (2) represents 1–M relationships. However which staff works in a particular department is ambiguous.

The fan trap is resolved by reconstructing the original ER model to represent the correct association.

2. Chasm Trap

A chasm trap occurs when a model suggests the existence of a relationship between entity types, but the pathway does not exist between certain entity occurrences. It occurs where there is a relationship with partial participation, which forms part of the pathway between entities that are related.


Consider the relationship

A single branch may be allocated to many staff who oversees the management of properties for rent. It should be noted that not all staff oversee property and not all property is managed by a member of staff. Hence there exist a partial participation of Staff and Property in the relation “oversees,” which means that some properties cannot be associated with a branch office through a member of staff. Hence the model has to modified as shown later.


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