Column Expressions Create Information
Expressions can also be used to create new values from the database values automagically, allowing you to change the data type of column, retrieve dynamic values, or transform the data before it reaches your java code
For instance we had a database that used integers to store dates in a non-standard way. To get the correct dates automatically we created the field normally then used an expression column to get the value as a date. To simplify the process of transforming dozens of dates, we placed the expression in a custom QDT
Here's what it looked like in the DBRow:
@DBColumn
public DBInteger dateReconciled = new DBInteger();
@DBColumn
public DBDateOnlyKappa dateReconciledAsDate =
new DBDateOnlyKappa(this.column(this.dateReconciled));
And the custom QDT was:
public class DBDateOnlyKappa extends DBDateOnly{
public DBDateOnlyKappa(NumberExpression originalColumn) {
super(DateExpression.value(new Date(-1, 11, 31)).addDays(originalColumn.minus(693901)));
}
}
This is a real example of the power provided by QueryableDatatype's expression column functionality
Expression columns create a column from the expression when the DBQuery is run. since it is a DBExpression everything that is available to an expression is available as a data column
A particularly good example of this is accessing dynamic values from the database like current user, database system time, or the next value in a sequence
@DBColumn
public DBDate systemDate = new DBDate(DateExpression.currentDate());
@DBColumn
public DBString systemUser = new DBString(StringExpression.currentUser());
@DBColumn
public DBNumber nextValue = new DBNumber(NumberExpression.getNextSequenceValue("MySequence"));
This is also highly valuable to provide Map/Reduce-like functionality where the query examples provide the Reduce behaviour and the expression columns provide the Mapping
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