from django.core.exceptions import FieldError

from django.db.models.fields import FieldDoesNotExist

from django.db.models.sql.constants import LOOKUP_SEP

class SQLEvaluator(object):

    def __init__(self, expression, query, allow_joins=True):

        self.expression = expression

        self.opts = query.get_meta()

        self.cols = {}

        self.contains_aggregate = False

        self.expression.prepare(self, query, allow_joins)

    def relabel_aliases(self, change_map):

        for node, col in self.cols.items():

            self.cols[node] = (change_map.get(col[0], col[0]), col[1])

    #####################################################

    # Vistor methods for initial expression preparation #

    #####################################################

    def prepare_node(self, node, query, allow_joins):

        for child in node.children:

            if hasattr(child, 'prepare'):

                child.prepare(self, query, allow_joins)

    def prepare_leaf(self, node, query, allow_joins):

        if not allow_joins and LOOKUP_SEP in node.name:

            raise FieldError("Joined field references are not permitted in this query")

        field_list = node.name.split(LOOKUP_SEP)

        if (len(field_list) == 1 and

            node.name in query.aggregate_select.keys()):

            self.contains_aggregate = True

            self.cols[node] = query.aggregate_select[node.name]

        else:

            try:

                field, source, opts, join_list, last, _ = query.setup_joins(

                    field_list, query.get_meta(),

                    query.get_initial_alias(), False)

                col, _, join_list = query.trim_joins(source, join_list, last, False)

                self.cols[node] = (join_list[-1], col)

            except FieldDoesNotExist:

                raise FieldError("Cannot resolve keyword %r into field. "

                                 "Choices are: %s" % (self.name,

                                                      [f.name for f in self.opts.fields]))

    ##################################################

    # Vistor methods for final expression evaluation #

    ##################################################

        expressions = []

        expression_params = []

        for child in node.children:

            if hasattr(child, 'evaluate'):

            else:

                sql, params = '%s', (child,)

            if len(getattr(child, 'children', [])) > 1:

                format = '(%s)'

            else:

                format = '%s'

            if sql:

                expressions.append(format % sql)

                expression_params.extend(params)

        return connection.ops.combine_expression(node.connector, expressions), expression_params

        col = self.cols[node]

        if hasattr(col, 'as_sql'):

        else:

            return '%s.%s' % (qn(col[0]), qn(col[1])), ()