data matrix code java generator Object reference for one-to-one relationship in Java

Creator DataMatrix in Java Object reference for one-to-one relationship

Object reference for one-to-one relationship
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public class BillingInfo { protected String creditCardType; protected String creditCardNumber; protected String nameOnCreditCard; protected Date creditCardExpiration; protected String bankAccountNumber; protected String bankName; protected String routingNumber; }
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From an OO perspective, it would make sense for the database tables storing this data to mirror the Java implementation in listing 8.1. In this scheme, two different tables, USERS and BILLING_INFO, would have to be created, with the billing object reference in the User object b translated into a foreign key to the BILLING_ INFO table s key in the USERS table (perhaps called BILLING_ID). The problem is
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A unidirectional one-to-one (optional) relationship between User and BillingInfo
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Object-relational mapping
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that this scheme does not make complete sense in the relational world. As a matter of fact, since the objects are merely expressing a one-to-one relationship, normalization would dictate that the USERS and BILLING_INFO tables be merged into one. This would eliminate the almost pointless BILLING_INFO table and the redundant foreign key in the USERS table. The extended USERS table could look like this:
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USER_ID EMAIL CREDIT_CARD_TYPE CREDIT_CARD_NUMBER NAME_ON_CREDIT_CARD CREDIT_CARD_EXPIRATION BANK_ACCOUNT_NUMBER BANK_NAME ROUTING_NUMBER NOT NULL, PRIMARY KEY NOT NULL NUMBER VARCHAR2(255) VARCHAR2(255) VARCHAR2(255) VARCHAR2(255) DATE VARCHAR2(255) VARCHAR2(255) VARCHAR2(255)
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In effect, our persistence layer mapping code would have to resolve this difference by pulling field data out of both the USERS and related BILLING_INFO tables and storing it into the columns of the combined USERS table. A bad approach, but an all-too-common one, would be to compromise your domain model to make it fit the relational data model (get rid of the separate BillingInfo object). While this would certainly make the mapping code simpler, you would lose out on a sensible domain model. In addition, you would write awkward code for the parts of your application that deal only with the BillingInfo object and not the User object. If you remember our discussion in chapter 7, then you probably realize that BillingInfo may make sense as an embedded object since you do not want to have a separate identity, and want to store the data in the USERS table. One-to-many relationships The relational primary-key/foreign-key mechanism is ideally suited for a parentchild one-to-many relationship between tables. Let s take the probable relationship between the ITEMS and BIDS tables, for example. The tables will probably look like those shown in listing 8.2.
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Listing 8.2 One-to-many relationship between ITEMS and BIDS tables
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ITEMS ITEM_ID TITLE DESCRIPTION INITIAL_PRICE BID_START_DATE BID_END_DATE NOT NOT NOT NOT NOT NOT NULL, PRIMARY KEY NULL NULL NULL NULL NULL NUMBER VARCHAR2(255) CLOB NUMBER TIMESTAMP TIMESTAMP
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The impedance mismatch
ITEM_SELLER_ID
NOT NULL, NUMBER FOREIGN KEY (USERS(USER_ID))
BIDS BID_ID AMOUNT BID_DATE BID_BIDDER_ID BID_ITEM_ID
NOT NULL, PRIMARY KEY NUMBER NOT NULL NUMBER NOT NULL TIMESTAMP NOT NULL, NUMBER FOREIGN KEY (USER(USER_ID)) NOT NULL, NUMBER FOREIGN KEY (ITEMS(ITEM_ID))
Foreign key signifying one-to-many relationship
The ITEM_ID foreign key into the ITEMS table from the BIDS table means that multiple BIDS table rows can refer to the same record in the ITEMS table. This implements a many-to-one relationship going from the BIDS table to the ITEMS table, and it is simple to retrieve an item given a bid record. On the other hand, retrieval from ITEMS to BIDS will require a little more effort in looking for BIDS rows that match a given ITEM_ID key. As we mentioned in the previous chapter, however, the relationship between the Item and Bid domain objects is one-many bidirectional. This means that the Item object has a reference to a set of Bid objects while the Bid object holds a reference to an Item object. As a Java developer, you might have expected the ITEMS table to contain some kind of reference to the BIDS table in addition to the ITEM_ID foreign key in the BIDS table. The problem is that such a table structure simply does not make sense in the relational world. Instead, our ORM layer must translate the parent-child unidirectional database relationship into a bidirectional one-to-many relationship in the OO world by using a lookup scheme instead of simple, directional references. Many-to-many Many-to-many relationships are common in enterprise development. In our ActionBazaar domain model presented in chapter 7, the relationship between the Item and Category domain objects is many-to-many. That is, an item can belong in multiple categories while a category can contain more than one item. This is fairly easy to implement in the OO world with a set of references on either side of the relationship. In the database world, on the other hand, the only way to implement a relationship is through a foreign key, which is inherently one-to-many. As a result, the only real way to implement many-to-many relationships is by breaking them down into two one-to-many relationships. Let s see how this works by taking a look at the database table representation of the item-category relationship in listing 8.3.
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