How to set up @Id field properly when database table is already populated JPA

问题

I am having a single table in my database and I have added the @Id attribute over the field. As strategy I use GenerationType.IDENTITY. This works fine WHEN the database table is not already populated by rows from a SQL script.

How can I manage to get it to work when the table already has some rows in it? Because it doesn't work when I am trying to insert entities from my application when it is pre-populated.

I am using Derby database for this and eclipselink as implementation.

回答1:

Use sequence generator that gets its ids from a sequence table. Something like this:

@Id
@GeneratedValue(generator = "yourTableIdGenerator")
@GenericGenerator(name = "yourTableIdGenerator", strategy = "org.hibernate.id.enhanced.SequenceStyleGenerator", parameters = {
                    @Parameter(name = "sequence_name", value = "your_table_id_seq"),
                    @Parameter(name = "optimizer", value = "hilo"),
                    @Parameter(name = "initial_value", value = "1000"),
                    @Parameter(name = "increment_size", value = "10") }
                )
@Column(name = "your_table_id", length = 15)
public Long getId() {
    return id;
}

Set the initial_value to a value larger than the maximum id of the rows populated from the script.

From the java 6 ee api: http://download.oracle.com/javaee/6/api/javax/persistence/TableGenerator.html

@TableGenerator(
   name="empGen",
   table="ID_GEN",
   pkColumnName="GEN_KEY",
   valueColumnName="GEN_VALUE",
   pkColumnValue="EMP_ID",
   initialValue = 1000,
   allocationSize=1)
@Id
@GeneratedValue(strategy=TABLE, generator="empGen")
int id;

回答2:

I will assume that your error is because the IDENTITY that you has created for you table starts (by example) in "1" and you have already populated the table with a row with the id "1" . So sure exist a PK Constraint violation. If this is the scenario I can suggest:

Check the number of rows for prepopulate the data. By example 3500.
Create the identity column in a superior number for avoid PK constraint violation, by example the following table starts the identity in 4000:

This should work in Derby DB

CREATE TABLE MAPS    (
    MAP_ID INTEGER NOT NULL GENERATED ALWAYS AS IDENTITY (START WITH 4000, INCREMENT BY 1),
    MAP_NAME VARCHAR(24) NOT NULL,
    REGION VARCHAR(26),
    AREA DECIMAL(8,4) NOT NULL,
    PHOTO_FORMAT VARCHAR(26) NOT NULL,
    PICTURE BLOB(102400),
    UNIQUE (MAP_ID, MAP_NAME)
)

来源：https://stackoverflow.com/questions/8122194/how-to-set-up-id-field-properly-when-database-table-is-already-populated-jpa

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java

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