app/django/db/backends/oracle/query.py
author Madhusudan.C.S <madhusudancs@gmail.com>
Wed, 03 Jun 2009 23:38:02 +0530
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"""
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Custom Query class for Oracle.
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Derives from: django.db.models.sql.query.Query
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"""
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import datetime
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from django.db.backends import util
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# Cache. Maps default query class to new Oracle query class.
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_classes = {}
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def query_class(QueryClass, Database):
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    """
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    Returns a custom django.db.models.sql.query.Query subclass that is
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    appropriate for Oracle.
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    The 'Database' module (cx_Oracle) is passed in here so that all the setup
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    required to import it only needs to be done by the calling module.
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    """
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    global _classes
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    try:
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        return _classes[QueryClass]
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    except KeyError:
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        pass
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    class OracleQuery(QueryClass):
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        def resolve_columns(self, row, fields=()):
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            # If this query has limit/offset information, then we expect the
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            # first column to be an extra "_RN" column that we need to throw
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            # away.
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            if self.high_mark is not None or self.low_mark:
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                rn_offset = 1
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            else:
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                rn_offset = 0
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            index_start = rn_offset + len(self.extra_select.keys())
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            values = [self.convert_values(v, None)
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                      for v in row[rn_offset:index_start]]
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            for value, field in map(None, row[index_start:], fields):
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                values.append(self.convert_values(value, field))
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            return values
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        def convert_values(self, value, field):
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            from django.db.models.fields import DateField, DateTimeField, \
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                 TimeField, BooleanField, NullBooleanField, DecimalField, Field
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            if isinstance(value, Database.LOB):
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                value = value.read()
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            # Oracle stores empty strings as null. We need to undo this in
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            # order to adhere to the Django convention of using the empty
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            # string instead of null, but only if the field accepts the
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            # empty string.
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            if value is None and isinstance(field, Field) and field.empty_strings_allowed:
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                value = u''
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            # Convert 1 or 0 to True or False
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            elif value in (1, 0) and isinstance(field, (BooleanField, NullBooleanField)):
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                value = bool(value)
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            # Convert floats to decimals
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            elif value is not None and isinstance(field, DecimalField):
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                value = util.typecast_decimal(field.format_number(value))
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            # cx_Oracle always returns datetime.datetime objects for
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            # DATE and TIMESTAMP columns, but Django wants to see a
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            # python datetime.date, .time, or .datetime.  We use the type
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            # of the Field to determine which to cast to, but it's not
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            # always available.
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            # As a workaround, we cast to date if all the time-related
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            # values are 0, or to time if the date is 1/1/1900.
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            # This could be cleaned a bit by adding a method to the Field
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            # classes to normalize values from the database (the to_python
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            # method is used for validation and isn't what we want here).
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            elif isinstance(value, Database.Timestamp):
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                # In Python 2.3, the cx_Oracle driver returns its own
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                # Timestamp object that we must convert to a datetime class.
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                if not isinstance(value, datetime.datetime):
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                    value = datetime.datetime(value.year, value.month,
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                            value.day, value.hour, value.minute, value.second,
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                            value.fsecond)
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                if isinstance(field, DateTimeField):
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                    # DateTimeField subclasses DateField so must be checked
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                    # first.
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                    pass
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                elif isinstance(field, DateField):
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                    value = value.date()
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                elif isinstance(field, TimeField) or (value.year == 1900 and value.month == value.day == 1):
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                    value = value.time()
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                elif value.hour == value.minute == value.second == value.microsecond == 0:
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                    value = value.date()
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            return value
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        def as_sql(self, with_limits=True, with_col_aliases=False):
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            """
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            Creates the SQL for this query. Returns the SQL string and list
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            of parameters.  This is overriden from the original Query class
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            to handle the additional SQL Oracle requires to emulate LIMIT
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            and OFFSET.
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            If 'with_limits' is False, any limit/offset information is not
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            included in the query.
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            """
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            # The `do_offset` flag indicates whether we need to construct
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            # the SQL needed to use limit/offset with Oracle.
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            do_offset = with_limits and (self.high_mark is not None
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                                         or self.low_mark)
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            if not do_offset:
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                sql, params = super(OracleQuery, self).as_sql(with_limits=False,
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                        with_col_aliases=with_col_aliases)
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            else:
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                sql, params = super(OracleQuery, self).as_sql(with_limits=False,
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                                                        with_col_aliases=True)
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                # Wrap the base query in an outer SELECT * with boundaries on
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                # the "_RN" column.  This is the canonical way to emulate LIMIT
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                # and OFFSET on Oracle.
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                sql = 'SELECT * FROM (SELECT ROW_NUMBER() OVER (ORDER BY 1) AS "_RN", "_SUB".* FROM (%s) "_SUB") WHERE "_RN" > %d' % (sql, self.low_mark)
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                if self.high_mark is not None:
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                    sql = '%s AND "_RN" <= %d' % (sql, self.high_mark)
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            return sql, params
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    _classes[QueryClass] = OracleQuery
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    return OracleQuery