## Database & Table Versioning

Iciql supports an optional, simple versioning mechanism.  There are two parts to the mechanism.

1. You must supply an implementation of `com.iciql.DbUpgrader` to your `com.iciql.Db` instance.
2. One or more of your table model classes must specify the `IQVersion(version)` annotation<br/>
AND/OR<br/>
Your `com.iciql.DbUpgrader` implementation must specify the `IQVersion(version)` annotation

### How does it work?
If you choose to use versioning, iciql will maintain a table within your database named *iq_versions* which is defined as:

    CREATE TABLE IQ_VERSIONS(SCHEMANAME VARCHAR(255) NOT NULL, TABLENAME VARCHAR(255) NOT NULL, VERSION INT NOT NULL)
    
This database table is automatically created if and only if at least one of your model classes specifies a *version* > 0.

When you generate a statement, iciql will compare the annotated version field of your model class to its last known value in the *iq_versions* table.  If *iq_versions* lags behind the model annotation, iciql will immediately call the registered `com.iciql.DbUpgrader` implementation before generating and executing the current statement.

When an upgrade scenario is identified, the current version and the annotated version information is passed to either:

- `DbUpgrader.upgradeDatabase(db, fromVersion, toVersion)`
- `DbUpgrader.upgradeTable(db, schema, table, fromVersion, toVersion)`

both of which allow for non-linear upgrades.  If the upgrade method call is successful and returns *true*, iciql will update the *iq_versions* table with the annotated version number.

The actual upgrade procedure is beyond the scope of iciql and is your responsibility to implement.  This is simply a mechanism to automatically identify when an upgrade is necessary.

**NOTE:**<br/>
The database entry of the *iq_versions* table is specified as SCHEMANAME='' and TABLENAME=''.

### Effective use of Versioning with a DAO

When Iciql identifies that a version upgrade is necessary it will call the appropriate method and give you a `Db` instance.  With the `Db` instance you may open a version-specific [DAO](dao.html) instance that could give you a clean way to define all your upgrade commands.

---JAVA---
public interface V2Upgrade extends Dao {

    @SqlStatement("ALTER TABLE PRODUCT ADD COLUMN TEST INT DEFAULT 0")
    void updateProductTable();
    
    @SqlStatement("UPDATE PRODUCT SET CATEGORY = :new WHERE CATEGORY = :old"")
    void renameCategory(@Bind("old") String oldName, @Bind("new") String newName);
}

public class MyUpgrader implements DbUpgrader {

    public boolean upgradeDatabase(Db db, int fromVersion, int toVersion) {
    
        if (2 == toVersion) {
            V2Upgrade dao = db.open(V2Upgrade.class);
            dao.updateProductTable();
            dao.renameCategory("Condiments", "Dressings");
            return true;
        }
        
        return false;
    }

---JAVA---