2.3.4 Relationship

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2.3.4 Relationship

A relation is attached to an entity, and describes how an entity is related to another one.

Two kinds of relationships can be defined: simple and flattened relationship. However flattened relationshipsare not supported/tested yet.

Simple relationships join their (source) entity to a destination entity.

A relation may have the following properties:

name:

the name of the relationship, unique within the entity.

deleteRule:

this specifies how the object should react when it is deleted. Four options are possible options: NULLIFY, DELETE_DENY, DELETE_CASCADE and DELETE_NOACTION. We will use the model AuthorBooks defined in section 2.1.1, with entities Writer and Books, to highlight the differences between those 4 values:

if Book's relationship toAuthor has the delete rule DELETE_NULLIFY, then when a book is deleted it is automatically removed from the corresponding author's toBooks relationship.
If Author's relationship toBooks has the delete rule DELETE_DENY, then if an author is marked for deletion while it is still in relation with books, the deletion will be denied when you try to make it persistent.
If Author's relationship toBooks has the delete rule DELETE_CASCADE, then the deletion of an author will automatically delete the related books as well.
You can also instructs the framework not to take any actions when an object is deleted, with respect to its relationships. In this case, you use the delete rule DELETE_NOACTION. This is however quite an advanced setting and should be used with great care, since inappropriate use of this setting can result in dangling references in your databases.

This will be enforced when an object is inserted/updated or when it is deleted. See sections 4.4 and 4.6 for further details.

isClassProperty:

tells whether the Relationship's key (i.e. its name, for example: toBooks) is part of the class' API. It should be set in most cases.

multiplicityLowerBound, multiplicityUpperBound:

the multiplicty lower bound of a relationship defines the minimum number of objects that should be in relation. If the lower bound is zero, none are required: this is an optional relationship; if the lower bound is one or more, this is a mandatory relationship.

The multiplicity upper bound defines the maximum allowed number of objects in relations. It can be any positive number, given that it is always greater or equal than the lower bound. As a special exception, values -1 (minus one) or '*' serve to indicate an unconstrained upper bound.

The exact definition of to-one and to-many relationships (see 2.2) are then: to-one relationship: multiplicityUpperBound<=1, to-many relationship: multiplicityUpperBound>1.

destinationEntity:

designates the name of the destination entity.

joinSemantic:

the join's sematic can be either:

(default) Inner join
Full outer join
Left outer join
Right outer join

displayLabel:

same as for Attributes, above.

comment:

a comment, for maintainers of the model.

The full definition of a relationship implies that it defines at least one join. However, this will not appear in PyModels, just in xml models. It's not really part of Entity-Relationship Modelling, it is just a mean to identify the correspondance between primary and foreign keys that defines a relationship. For this reasons, we will wait until section 2.5.5 to define them.