from django.db.models.sql import compiler
from django.utils.six.moves import zip_longest


class SQLCompiler(compiler.SQLCompiler):
    def resolve_columns(self, row, fields=()):
        # If this query has limit/offset information, then we expect the
        # first column to be an extra "_RN" column that we need to throw
        # away.
        if self.query.high_mark is not None or self.query.low_mark:
            rn_offset = 1
        else:
            rn_offset = 0
        index_start = rn_offset + len(self.query.extra_select)
        values = [self.query.convert_values(v, None, connection=self.connection)
                  for v in row[rn_offset:index_start]]
        for value, field in zip_longest(row[index_start:], fields):
            values.append(self.query.convert_values(value, field, connection=self.connection))
        return tuple(values)

    def as_sql(self, with_limits=True, with_col_aliases=False):
        """
        Creates the SQL for this query. Returns the SQL string and list
        of parameters.  This is overriden from the original Query class
        to handle the additional SQL Oracle requires to emulate LIMIT
        and OFFSET.

        If 'with_limits' is False, any limit/offset information is not
        included in the query.
        """
        if with_limits and self.query.low_mark == self.query.high_mark:
            return '', ()

        # The `do_offset` flag indicates whether we need to construct
        # the SQL needed to use limit/offset with Oracle.
        do_offset = with_limits and (self.query.high_mark is not None
                                     or self.query.low_mark)
        if not do_offset:
            sql, params = super(SQLCompiler, self).as_sql(with_limits=False,
                    with_col_aliases=with_col_aliases)
        else:
            sql, params = super(SQLCompiler, self).as_sql(with_limits=False,
                                                    with_col_aliases=True)

            # Wrap the base query in an outer SELECT * with boundaries on
            # the "_RN" column.  This is the canonical way to emulate LIMIT
            # and OFFSET on Oracle.
            high_where = ''
            if self.query.high_mark is not None:
                high_where = 'WHERE ROWNUM <= %d' % (self.query.high_mark,)
            sql = 'SELECT * FROM (SELECT ROWNUM AS "_RN", "_SUB".* FROM (%s) "_SUB" %s) WHERE "_RN" > %d' % (sql, high_where, self.query.low_mark)

        return sql, params


class SQLInsertCompiler(compiler.SQLInsertCompiler, SQLCompiler):
    pass

class SQLDeleteCompiler(compiler.SQLDeleteCompiler, SQLCompiler):
    pass

class SQLUpdateCompiler(compiler.SQLUpdateCompiler, SQLCompiler):
    pass

class SQLAggregateCompiler(compiler.SQLAggregateCompiler, SQLCompiler):
    pass

class SQLDateCompiler(compiler.SQLDateCompiler, SQLCompiler):
    pass

class SQLDateTimeCompiler(compiler.SQLDateTimeCompiler, SQLCompiler):
    pass