Query By Example

When using Query By Example(QBE) you provide db4o with a template object. db4o will return all of the objects which match all non-default field values. This is done via reflecting all of the fields and building a query expression where all non-default-value fields are combined with AND expressions. Here's a simple example:

1public static void RetrievePilotByName(IObjectContainer db)
2    ...{
3      Pilot proto = new Pilot("Michael Schumacher", 0);
4      IObjectSet result = db.Get(proto);
5      ListResult(result);
6    }

1Private Shared Sub RetrievePilotByName()Sub RetrievePilotByName(ByVal db As IObjectContainer)
2            Dim proto As Pilot = New Pilot("Michael Schumacher", 0)
3            Dim result As IObjectSet = db.Get(proto)
4            ListResult(result)
5        End Sub

Querying this way has some obvious limitations:

• db4o must reflect all members of your example object.
• You cannot perform advanced query expressions. (AND, OR, NOT, etc.)
• You cannot constrain on values like 0 (integers), "" (empty strings), or nulls (reference types) because they would be interpreted as unconstrained.
• You need to be able to create objects without initialized fields. That means you can not initialize fields where they are declared.
  
• You need a constructor to create objects without initialized fields.

For more information see QBE Limitations. To get around all of these constraints, db4o provides the Native Query (NQ) system.