Mapping Objects to Relational Databases: O/R Mapping In Detail
One-to-Many Relationship Definition - In relational databases, a one-to-many relationship occurs when a parent record in one table can potentially. Examples of poor database design are all too common. This effectively creates two one-to-many relationships. This concept has been used in paper-based ( published) databases, each inscribed stone catalogued in R. A. S. Macalister's. In a one-to-many relationship, one record in a table can be associated with one or more records in another table. For example, each customer can have many.
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The value held by the Employee. EmployeePOID column is used to identify the single position that needs to be read into memory. The Position object is read in and instantiated due to the automatic read indicated in the holds row.
Because the relationship is to be automatically traversed, and to maintain referential integrity, a transaction is created. The next step is to add update statements for each object to the transaction. Each update statement includes both the business attributes and the key values mapped in Figure Because relationships are implemented via foreign keys, and because those values are being updated, the relationship is effectively being persisted.
The transaction is submitted to the database and run see Introduction to Transaction Control for details. There is one annoyance with the way the holds relationship has been mapped into the database.
Had there been a potential requirement for the holds relationship to turn into a one-to-many relationship, something that a change case would indicate, then you would be motivated to implement the foreign key to reflect this potential requirement. For example, the existing data model would support an employee holding many positions. However, had the object schema been taken into account, and if there were no future requirements motivating you to model it other wise, it would have been cleaner to implement the foreign key in the Employee table instead.
This is a one-to-many relationship — an employee works in one division and a single division has many employees working in it. As you can see in Figure 13 an interesting thing about this relationship is that it should be automatically traversed from Employee to Division, something often referred to as a cascading read, but not in the other direction.One to Many Relationship - Database Management System
Cascading saves and cascading deletes are also possible, something covered in the discussion of referential integrity. When an employee is read into memory the relationship is automatically traversed to read in the division that they work in.
If the application needs to it will read the Division object into memory, then it will set the value of Employee. Saving the relationship works in the same way as it does for one-to-one relationships — when the objects are saved so are their primary and foreign key values so therefore the relationship is automatically saved.
Every example in this article uses foreign keys, such as Employee. For example, if the Employee table of Figure 12 were to include a SocialSecurityNumber column then that would be an alternate key for that table assuming all employees are American citizens. If this where the case you would have the option to replace the Position.
In Figure 11 there is a many-to-many relationship between Employee and Task.
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In the data schema of Figure 12 I needed to introduce the associative table EmployeeTask to implement a many-to-many relationship the Employee and Task tables. In relational databases the attributes contained in an associative table are traditionally the combination of the keys in the tables involved in the relationship, in the case EmployeePOID and TaskPOID.
The name of an associative table is typically either the combination of the names of the tables that it associates or the name of the association that it implements. In this case I chose EmployeeTask over Assigned. Notice the multiplicities in Figure The rule is that the multiplicities "cross over" once the associative table is introduced, as indicated in Figure A multiplicity of 1 is always introduced on the outside edges of the relationship within the data schema to preserve overall multiplicity of the original relationship.
The original relationship indicated that an employee is assigned to one or more tasks and that a task has zero or more employees assigned to it.
In the data schema you see that this is still true even with the associative table in place to maintain the relationship. Assume that an employee object is in memory and we need a list of all the tasks they have been assigned. The steps that the application would need to go through are: The Select statement is run against the database.
The data records representing these tasks are marshaled into Task objects. Part of this effort includes checking to see if the Task object is already in memory. If it is then we may choose to refresh the object with the new data values this is a concurrency issue. A similar process would have been followed to read in the employees involved in a given task. To save the relationship, still from the point of view of the Employee object, the steps would be: Add Update statements for any task objects that have changed.
Add Insert statements for the Task table for any new tasks that you have created.