Analyzing Data › Defining Entity Types, Relationships, and Attributes › Defining Relationships › Relationship Transferability

Relationship Transferability

In some cases, it must be possible to move a pairing from one entity to another entity of the same entity type.

For example, two relationship memberships:

• Each PROJECT MANAGER sometimes manages exactly one PROJECT
• Each PROJECT always is managed by exactly one PROJECT MANAGER

This model represents a changeable situation. A PROJECT is always managed by a PROJECT MANAGER, but the identity of the PROJECT MANAGER change.

For example, Lisa Trueheart is been assigned the management responsibility instead of Ernest R. Steadman. As a result, the pairing “is managed by Ernest R. Steadman” must be transferred to Lisa Trueheart (another entity of type PROJECT MANAGER).

This illustrates a characteristic of relationship memberships that are called permanence of membership.

A relationship membership can be one of the following options:

• Permanent
  • Once a pairing is established, it relates the same two entities until the entities are disassociated.
  • A relationship membership whose pairings are permanent is named a fixed membership.
• Temporary
  • The pairing can move from entity to entity as needed.
  • A relationship membership whose pairings are temporary is named a transferable membership.

In the CUSTOMER/ORDER example, it is stated that after a customer places an order, the order at the customer’s request be transferred to a different customer. Therefore, the membership ORDER is placed by CUSTOMER is transferable.

The enterprise never performs the converse (transferring a customer between orders).

To complete the sentences, these facts are appended:

• Each CUSTOMER sometimes (95 percent) places at least two (estimated), at most 10 (estimated) and on average three ORDERS and is not transferable
• Each ORDER always is placed by exactly one CUSTOMER and is transferable

Transferring a relationship causes knowledge of previous pairings to be lost. Ask whether the business needs to know which entities were formerly associated. If so, the membership should not be transferable, and the relationship cardinalities must also be reviewed.

For example, if the former managers of a project remain of interest, then a project is managed (over time) by many project managers.

More information:

Building the Analysis Model