gistfile1.py

class OccurrencePolygonSet(db.Model):
    # Normal properties...

class Ecoregion(db.Model):
    cluster = db.ReferenceProperty(OccurrencePolygonSet)
    terms = db.StringListProperty() 
    # Other properties here...

# Find all ecoregions for a cluster where the ecoregion is introduced and g200Num is 2:
cluster = OccurrencePolygonSet.get(some_key)
Ecoregion.all().filter('cluster =', cluster).filter('terms = introduced').filter('g200Num =', 2).fetch()