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4. generate, create gs1 datamatrix barcode none for java projects ASP.NET Web Application Recall that when we p applet datamatrix 2d barcode rinted out the Customer entity in the previous section, the associated Account values were null. This is because the default is to lazily load associated entities in a one-to-many relationship. By this we mean that when we load a Customer object from the database we do not automatically load associated Account objects.

This has obvious performance benefits at the loading stage. If the application then requires an associated Account object then this is retrieved when the application makes an explicit reference to that object. This is reasonable if, in our application, a Customer makes only occasional reference to an Account object.

If, in our application, the Customer object frequently references an Account, then it makes sense to override the default and initialize associated Account objects at load time. This is called eager loading; we use the fetch element in the @OneToMany annotation for this:. @OneToMany(mappedBy=" customer", fetch=EAGER) public Collection<Account> getAccounts() { return accounts; }. In some circumstances gs1 datamatrix barcode for Java the default is for eager loading to take place, for example with one-to-one mappings or with basic mappings. We can override this default. For example, if we had a large column which is infrequently referenced:.

@Basic(fetch=LAZY). However, a lazy loadi barcode data matrix for Java ng override is taken as a hint to the persistence provider. The persistence provider may eagerly load the field in any case if it calculates negligible performance overhead. Typically an entire database row will occupy contiguous blocks on a disk, so there is no gain in retrieving just some of the columns of the database row.

The reverse is not true. The persistence provider will always perform an eager fetch if this is explicitly specified. We will now take a look at the @ManyToMany annotation options.

Recall that for many-to-many relationships it is arbitrary which is the owning side; we have chosen Customer to be the owning side and Address the inverse side. The default is to map to a join table. To override the default, or just to make mappings explicit, we use the @JoinTable annotation as follows:.

@ManyToMany(fetch=EAG ER) @JoinTable( name="CUSTOMER_ADDRESS", joinColumns=@JoinColumn( name="CUST_ID", referencedColumnName="CUSTOMER_ID"), inverseJoinColumns=@JoinColumn( name="ADD_ID", [ 77 ]. Object/Relational Map Data Matrix barcode for Java ping referencedColumnName="ADDRESS_ID") ) public Collection<Address> getAddresses() { return addresses; }. The name element spec ifies the join table name; in our example CUSTOMER_ADDRESS happens to be the same as the default. The joinColumns element specifies the join table foreign key column or columns which references the owning table. As our foreign key consists of a single column, we use an embedded @JoinColumn annotation.

In the expression,. @JoinColumn( name="CU Java ECC200 ST_ID", referencedColumnName="CUSTOMER_ID"). the name element is t he name of the foreign key column in the join table. In our case we choose to name this column CUST_ID, rather than accept the default CUSTOMER_ID. We would need to do this if we were mapping onto an existing join table with the foreign key column named CUST_ID.

The referencedColumnName is the name of the column referenced by this foreign key. In our example this is the primary key column of the owning, CUSTOMER, table which we earlier specified as CUSTOMER_ID. In some cases we may have a composite foreign key, consisting of two or more columns, although it is rare to have more than two columns.

In such situations we would use the @JoinColumns annotation. Composite foreign keys usually occur when we are mapping our object model onto a legacy database. We shall see an example of this later in this chapter.

We return to the final element in our @JoinTable annotation example, namely inverseJoinColumns. This specifies the join table foreign key column or columns which reference the inverse table. Again we use an embedded @JoinColumn annotation:.

@JoinColumn( name="AD j2se Data Matrix 2d barcode D_ID", referencedColumnName="ADDRESS_ID"). Here we have overridd en the default ADDRESS_ID, and named our foreign key column as ADD_ID. This references the primary key column, ADDRESS_ID, of the inverse ADDRESS table. Again inverseJoinColumns may refer to composite foreign columns, in which case these will be specified using the @JoinColumns annotation.

Note that we have added a fetch=EAGER clause to the @ManyToMany annotation. For many-to-many relationships lazy loading is the default behavior..

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