Java Reference
In-Depth Information
of the relationship between the entities. In relational terms, this means that the
BIDS
table
holds foreign-key references to the tables holding the
Item
and
Bidder
entities. The
@JoinColumn
annotation specifies what these foreign-key references are. In the case of
the bid-item relationship, the foreign key stored in the
BIDS
table is
ITEM_ID
, which ref-
erences a key named
ITEM_ID
that's likely the primary key of the
ITEMS
table. In the
case of the bid-bidder relationship, the foreign key is
BIDDER_ID
and it references the
USER_ID
column. Having looked at the
Bid
entity, let's now turn our attention to how the
entity winds up in the database through the
BidDao
bean.
2.4.2. Using the EntityManager
You've probably noticed that the
Bid
entity doesn't have a method of saving itself into the
database. The JPA
EntityManager
performs this bit of heavy lifting by reading the O/R
mapping configuration and providing entity persistence operations through an API-based
interface. The
EntityManager
knows how to store a POJO entity into the database as
a relational record, read relational data from a database and turn it into an entity, update
entity data stored in the database, and delete data mapped to an entity instance from the
database. The
EntityManager
has methods corresponding to each of these CRUD (cre-
ate, read, update, delete) operations, in addition to support for the robust Java Persistence
Query Language (JPQL).
Let's take a more detailed look at the
BidDao
in the next listing to see how some of these
EntityManager
operations work.
