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Container-Managed Persistence in Borland Enterprise Server
Mapping CMP fields to multiple tables
You may have an entity that contains information persisted in multiple tables. These
tables must be linked by at least one column representing a foreign key in the linked
table. For example, you might have a LineItem entity bean mapping to a table LINE_ITEM
with a primary key LINE that is a foreign key in a table called QUANTITY. The LineItem
entity also contains some fields from the QUANTITY table that correspond to LINE entries
in LINE_ITEM. Here's what our LINE_ITEM table might look like:
LINE ORDER_NO ITEM QUANTITY COLOR SIZE
001 XXXXXXX01 Kitty Sweater 2 red XL
QUANTITY, COLOR, and SIZE are all values that are also stored in the QUANITY
table, shown here. Note the identical values for some of the fields. This is because the
LINE_ITEM table itself stores information in the QUANTITY table, using the LineItem
entity to provide composite information.
LINE QUANTITY COLOR SIZE
001 2 red XL
Again, we can describe these relationships using a combination of
elements and a element. The elements define the fields found
in LineItem. Since there are some fields that require information from QUANTITY, we'll
specify that generically by using a TABLE_NAME.COLUMN_NAME syntax. For instance, we'd
define LINE_ITEM's COLOR column as QUANITY.COLOR. Finally, we'll specify the linking
column, LINE, that makes up our primary key/foreign key relationship. We'll do this
using the element.
Now let's look at the XML. First we define the CMP fields for the LineItem entity bean:
LineItem
.
.
orderNumber
ORDER_NO
line
LINE
item
ITEM
quantity
QUANTITY.QUANTITY
color
QUANTITY.COLOR
size
QUANTITY.SIZE
Chapter 15: Entity Beans and Table Mapping for CMP 2.0 135