Add "db_select" hooks for database Fields

[Adrian Holovaty]

I'm proposing a "db_select" method on the database Field class. This would specify the raw SQL to use when listing the field in the SELECT clause of an SQL statement. This will allow for more useful SELECTs in the case of opaque data types such as PostGIS "geometry" fields, which are returned by default as an encoded series of bytes.

It could also allow certain fields, such as BLOBs, to *never* be returned in a SELECT statement. (I'm 50/50 on this particular idea.)

class Field:
    def db_select(self, table):
        # Default behavior is to select the column name.
        return '%s.%s' % (table, self.db_column)

class GeometryField(Field):
    def db_select(self, table):
        # For a PostGIS geometry field, select the textual version of the geometry.
        return 'AsText(%s.%s)' % (table, self.db_column)

class BlobField(Field):
    def db_select(self, table):
        # None means "Don't ever include this field in a SELECT list."
        return None

[Alex Gaynor]

There are other internals for doing this now.