How to cascade creation

# How to cascade object creation

## Overview

Cascading creation allows you to transfer some of the responsibilities of creating objects to Castor JDO. To be more precise: if you enable cascading creation on a relation between two classes, all objects on one end of that relation that have not yet been created will be created when the other end gets persisted. This saves you from manually creating every single object, which is especially useful when dealing with large object graphs that have 1:M (one to many) relations or many objects in a single relationship.

## Enabling cascading creation

To enable cascading creation on a relation, you simply set the cascading attribute of the `<sql>` field describing the relation to "`create`" (or "`all`"):

In other words, the field mapping for the Java property book ...

    <field name="book" type="myapp.Book" >
      <sql name="book_id" />
    </field>

becomes

    <field name="book" type="myapp.Book" >
      <sql name="book_id" cascading="create" />
    </field>

In case of bidirectional relations, it does matters on which end you enable cascading creation. It is also possible to enable it on both ends.

## Scenarios

### db.create()

The most intuitive case is when you explicitly call db.create() on an object that has cascading creation enabled on one or more of his relations. If the objects in those relationships have not yet been created, they will be as part of the create() execution.

Here is a simple example, where the objects Author and Book are in a one-to-one relation (i.e. every Book has exactly one Author):

    db.begin();

    Author author = new Author();
    author.setId(1);
    author.setName("John Jackson");

    Book book = new Book();
    book.setId(1);
    book.setTitle("My Life");
    book.setAuthor(author);

    db.create(book);

    db.commit();

Once the commit operation has successfully completed, both the Author and the Book instance will have been persisted to your data store. To highlight this, lets's have a look at the corresponding database tables before and after the execution of above code fragment.

Before

Author

id | name
--- | ---
(empty table)

Book

id | title | author_id
--- | --- | ---
(empty table)

After

Author

id | name
--- | ---
1 | "John Jackson"

Book

id | title | author_id
--- | --- | ---
1 | "My Life" | 1

### db.commit()

Cascading creation also works implicitly: any objects that are on the receiving end of a cascaded relation will be created upon transaction commit, provided they do not exist yet and that the object on the primary end of that relation does. In other words: if you modify a relation property of a loaded object, any new objects that now need to be created will be created.

To demonstrate, let's continue the example from the previous section. We, again, have a Book and an Author, in a one-to-one relation, both already persisted. If we now change the book's author to someone new, any object that is not yet in the database will be persisted automatically. Just call db.commit() after setting the new author, and the new author will be persisted as well.

    db.begin();

    Author author = new Author();
    author.setId(2);
    author.setName("Bruce Willis");

    Book book = db.load(Book.class, 1);
    book.setAuthor(author);

    db.commit();

In terms of unit test assertions, the current state of the author and book instances can be expressed as follows:

    db.begin();

    Book book = db.load(Book.class, 1);
    assertNotNull(book);
    assertEquals(1, book.getId());

    Author author = book.getAuthor();
    assertNotNull(author);
    assertEquals(2, book.getId());

    db.commit();

As above, let's have a look at the corresponding database tables for the entities Author and Book:

Before

Author

id | name
--- | ---
1 | "John Jackson"

Book

id | title | author_id
--- | --- | ---
1 | "My Life" | 1

After

Author

id | name
--- | ---
1 | "John Jackson"
2 | "Bruce Willis"

Book

id | title | author_id
--- | --- | ---
1 | "My Life" | 2

Please note that we now have two authors stored, and that the book with an id value of '1' now has a foreign key relationship to the author with the id value '2'.

### Cascading create and collections

The real benefit of using cascading for object creation shows when dealing with 1:M relations, usually expressed through Java collections in your entity classes.

For the remainder of this secction, we will use the Java classes Department and Employee, which have a 1:M relationship (in other words, every department has one or more employees). On the Java side, this is expressed as the Department having a collection of Employee objects in form of a Java collection. In the database, this will obviously be the other way around, with the emp table referencing the dept table. Every example in this section will use the same database state as a starting point, as shown here:

dept

id | name
--- | ---
23 | "Accounting"

emp

id | name | dept_id
--- | --- | ---
1 | "John" | 23
2 | "Paul" | 23
3 | "Ringo" | 23

Example 1: Adding objects

    db.begin();

    Employee employee = new Employee();
    employee.setId(4);
    employee.setName("George");

    Department department = db.load(Department.class, 23);
    department.getEmployees().add(employee);

    db.commit();

After

dept

id | name
--- | ---
23 | "Accounting"

emp

id | name | dept_id
--- | --- | ---
1 | "John" | 23
2 | "Paul" | 23
3 | "Ringo" | 23
4 | "George" | 23

Example 2: Removing objects

    db.begin();

    Department department = db.load(Department.class, 23);
    department.getEmployees().remove(2);

    db.commit();

After

dept

id | name
--- | ---
23 | "Accounting"

emp

id | name | dept_id
--- | --- | ---
1 | "John" | 23
2 | "Paul" | 23
3 | "Ringo" | NULL

this of course only works if you allow the employee's foreign key dept_id to be `NULL` or, alternatively, also delete the Employee when you remove the relationship (either by manually calling db.remove() or TODO)

Example 3: Adding & removing objects

    db.begin();

    Employee e4 = new Employee();
    e4.setId(4);
    e4.setName("George");

    Employee e5 = new Employee();
    e5.setId(5);
    e5.setName("Joe");

    Employee e6 = new Employee();
    e6.setId(6);
    e6.setName("Jack");

    Department dep = db.load(Department.class, 23);
    dep.setEmployees(Arrays.asList(e4, e5, e6));

    db.commit();

Database after:

dept

id | name
--- | ---
23 | "Accounting"

emp

id | name | dept_id
--- | --- | ---
1 | "John" | NULL
2 | "Paul" | NULL
3 | "Ringo" | NULL
4 | "George" | 23
5 | "Joe" | 23
6 | "Jack" | 23

The note to example 2 also applies here.

## See also

- How to use cascading operations - overview