PATH
WebObjects 4.0 Documentation >
WebObjects Programming Topics
Propagating Deletions Across Relationships
Synopsis
Describes the different delete rules for relationships specified in the EOModel.
Description
EOModeler allows the user to set the delete rules for a relationship in the advanced relationship inspector. The different delete rules are discussed here.
Nullify Delete Rule
When the object is deleted and one of the relationships has a nullify delete rule, EOF traverses through the object graph to the relationship object(s) and sets any back pointer to the deleted object to nil. For example, if a department has employees, the employees relationship in the department has a nullify delete rule, and the department is deleted, EOF removes any back pointer from the employee(s) to the department.
Disadvantages
· EOF performs expensive round trips to the database to get the objects at the other side of the relationship and reset their back pointers.
· EOF must resolve faults for the objects on the other side of the relationship.
Cascade Delete Rule
When the parent object is deleted, EOF deletes all the objects at the end of a relationship with the cascade delete rule. For example, a department has employees. If the department's employee relationship's delete rule is set as a cascade delete and the department object is deleted, it also deletes the employee objects.
Advantages
· Maintains database consistency; back pointers are eliminated along with the objects that contain them.
Disadvantages
· EOF performs expensive round trips to the database to access and delete the objects on the other side of the relationship.
· EOF must resolve faults for the objects on the other side of the relationship.
Deny Delete Rule
EOF will refuse to delete an object, that has an object at the other end of a relationship with the deny delete rule. For example, the department cannot be deleted if there are employees in the department.
Advantages
· Maintains database consistency; prevents rows that are destinations of back pointers from being deleted.
Disadvantages
· EOF performs expensive round trips to the database to determine if there are objects on the other side of the relationship.
· EOF must resolve faults for the objects on the other side of the relationship.
No Action Delete Rule
When the parent object is deleted, it does not do any checks to the objects at the other end of a relationship with the no action delete rule. Referring to the department/employee example, when the department is deleted, the employees are left with back pointers pointing to the nonexistent department.
Questions
· How do I prevent the deletion or an EO which has other EOs in its relationships?
· How do I prevent EOF from doing anything when an EO is deleted?
· How do I delete all the EOs associated with the parent EO when the parent is deleted?
Keywords
· Delete Rules
· Cascade
· Nullify
· Deny
· No Action
Revision History
22 July, 1998. Seejo Pylappan. First Draft.
19 November, 1998. Clif Liu. Second Draft.
