from django.db.models.sql.where import Constraint, WhereNode

class GeoConstraint(Constraint):

    """

    This subclass overrides `process` to better handle geographic SQL

    construction.

    def __init__(self, init_constraint):

        self.alias = init_constraint.alias

        self.col = init_constraint.col

        self.field = init_constraint.field

    def process(self, lookup_type, value, connection):

        if isinstance(value, SQLEvaluator):

            # Make sure the F Expression destination field exists, and

            # set an `srid` attribute with the same as that of the

            # destination.

            geo_fld = GeoWhereNode._check_geo_field(value.opts, value.expression.name)

            if not geo_fld:

                raise ValueError('No geographic field found in expression.')

            value.srid = geo_fld.srid

        db_type = self.field.db_type(connection=connection)

        params = self.field.get_db_prep_lookup(lookup_type, value, connection=connection)

        return (self.alias, self.col, db_type), params

        if isinstance(data, (list, tuple)):

            obj, lookup_type, value = data

            if ( isinstance(obj, Constraint) and

                 isinstance(obj.field, GeometryField) ):

                data = (GeoConstraint(obj), lookup_type, value)

        super(GeoWhereNode, self).add(data, connector)

    def make_atom(self, child, qn, connection):

        lvalue, lookup_type, value_annot, params_or_value = child

        if isinstance(lvalue, GeoConstraint):

            data, params = lvalue.process(lookup_type, params_or_value, connection)

            spatial_sql = connection.ops.spatial_lookup_sql(data, lookup_type, params_or_value, lvalue.field, qn)

            return spatial_sql, params

            return super(GeoWhereNode, self).make_atom(child, qn, connection)

        Utility for checking the given lookup with the given model options.

            # model field associated with the next field in the list