columns.py

# Copyright DataStax, Inc.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
# http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.

from copy import deepcopy, copy
from datetime import date, datetime, timedelta
import logging
from uuid import UUID as _UUID

from cassandra import util
from cassandra.cqltypes import SimpleDateType, _cqltypes, UserType
from cassandra.cqlengine import ValidationError
from cassandra.cqlengine.functions import get_total_seconds
from cassandra.util import Duration as _Duration

log = logging.getLogger(__name__)


class BaseValueManager(object):

    def __init__(self, instance, column, value):
        self.instance = instance
        self.column = column
        self.value = value
        self.previous_value = None
        self.explicit = False

    @property
    def deleted(self):
        return self.column._val_is_null(self.value) and (self.explicit or not self.column._val_is_null(self.previous_value))

    @property
    def changed(self):
        """
        Indicates whether or not this value has changed.

        :rtype: boolean

        """
        if self.explicit:
            return self.value != self.previous_value

        if isinstance(self.column, BaseContainerColumn):
            default_value = self.column.get_default()
            if self.column._val_is_null(default_value):
                return not self.column._val_is_null(self.value) and self.value != self.previous_value
            elif self.previous_value is None:
                return self.value != default_value

            return self.value != self.previous_value

        return False

    def reset_previous_value(self):
        self.previous_value = deepcopy(self.value)

    def getval(self):
        return self.value

    def setval(self, val):
        self.value = val
        self.explicit = True

    def delval(self):
        self.value = None

    def get_property(self):
        _get = lambda slf: self.getval()
        _set = lambda slf, val: self.setval(val)
        _del = lambda slf: self.delval()

        if self.column.can_delete:
            return property(_get, _set, _del)
        else:
            return property(_get, _set)


class Column(object):

    # the cassandra type this column maps to
    db_type = None
    value_manager = BaseValueManager

    instance_counter = 0

    _python_type_hashable = True

    primary_key = False
    """
    bool flag, indicates this column is a primary key. The first primary key defined
    on a model is the partition key (unless partition keys are set), all others are cluster keys
    """

    partition_key = False

    """
    indicates that this column should be the partition key, defining
    more than one partition key column creates a compound partition key
    """

    index = False
    """
    bool flag, indicates an index should be created for this column
    """

    custom_index = False
    """
    bool flag, indicates an index is managed outside of cqlengine. This is
    useful if you want to do filter queries on fields that have custom
    indexes.
    """

    db_field = None
    """
    the fieldname this field will map to in the database
    """

    default = None
    """
    the default value, can be a value or a callable (no args)
    """

    required = False
    """
    boolean, is the field required? Model validation will raise and
    exception if required is set to True and there is a None value assigned
    """

    clustering_order = None
    """
    only applicable on clustering keys (primary keys that are not partition keys)
    determines the order that the clustering keys are sorted on disk
    """

    discriminator_column = False
    """
    boolean, if set to True, this column will be used for discriminating records
    of inherited models.

    Should only be set on a column of an abstract model being used for inheritance.

    There may only be one discriminator column per model. See :attr:`~.__discriminator_value__`
    for how to specify the value of this column on specialized models.
    """

    static = False
    """
    boolean, if set to True, this is a static column, with a single value per partition
    """

    def __init__(self,
                 primary_key=False,
                 partition_key=False,
                 index=False,
                 db_field=None,
                 default=None,
                 required=False,
                 clustering_order=None,
                 discriminator_column=False,
                 static=False,
                 custom_index=False):
        self.partition_key = partition_key
        self.primary_key = partition_key or primary_key
        self.index = index
        self.custom_index = custom_index
        self.db_field = db_field
        self.default = default
        self.required = required
        self.clustering_order = clustering_order
        self.discriminator_column = discriminator_column

        # the column name in the model definition
        self.column_name = None
        self._partition_key_index = None
        self.static = static

        self.value = None

        # keep track of instantiation order
        self.position = Column.instance_counter
        Column.instance_counter += 1

    def __ne__(self, other):
        if isinstance(other, Column):
            return self.position != other.position
        return NotImplemented

    def __eq__(self, other):
        if isinstance(other, Column):
            return self.position == other.position
        return NotImplemented

    def __lt__(self, other):
        if isinstance(other, Column):
            return self.position < other.position
        return NotImplemented

    def __le__(self, other):
        if isinstance(other, Column):
            return self.position <= other.position
        return NotImplemented

    def __gt__(self, other):
        if isinstance(other, Column):
            return self.position > other.position
        return NotImplemented

    def __ge__(self, other):
        if isinstance(other, Column):
            return self.position >= other.position
        return NotImplemented

    def __hash__(self):
        return id(self)

    def validate(self, value):
        """
        Returns a cleaned and validated value. Raises a ValidationError
        if there's a problem
        """
        if value is None:
            if self.required:
                raise ValidationError('{0} - None values are not allowed'.format(self.column_name or self.db_field))
        return value

    def to_python(self, value):
        """
        Converts data from the database into python values
        raises a ValidationError if the value can't be converted
        """
        return value

    def to_database(self, value):
        """
        Converts python value into database value
        """
        return value

    @property
    def has_default(self):
        return self.default is not None

    @property
    def is_primary_key(self):
        return self.primary_key

    @property
    def can_delete(self):
        return not self.primary_key

    def get_default(self):
        if self.has_default:
            if callable(self.default):
                return self.default()
            else:
                return self.default

    def get_column_def(self):
        """
        Returns a column definition for CQL table definition
        """
        static = "static" if self.static else ""
        return '{0} {1} {2}'.format(self.cql, self.db_type, static)

    # TODO: make columns use cqltypes under the hood
    # until then, this bridges the gap in using types along with cassandra.metadata for CQL generation
    def cql_parameterized_type(self):
        return self.db_type

    def set_column_name(self, name):
        """
        Sets the column name during document class construction
        This value will be ignored if db_field is set in __init__
        """
        self.column_name = name

    @property
    def db_field_name(self):
        """ Returns the name of the cql name of this column """
        return self.db_field if self.db_field is not None else self.column_name

    @property
    def db_index_name(self):
        """ Returns the name of the cql index """
        return 'index_{0}'.format(self.db_field_name)

    @property
    def has_index(self):
        return self.index or self.custom_index

    @property
    def cql(self):
        return self.get_cql()

    def get_cql(self):
        return '"{0}"'.format(self.db_field_name)

    def _val_is_null(self, val):
        """ determines if the given value equates to a null value for the given column type """
        return val is None

    @property
    def sub_types(self):
        return []

    @property
    def cql_type(self):
        return _cqltypes[self.db_type]


class Blob(Column):
    """
    Stores a raw binary value
    """
    db_type = 'blob'

    def to_database(self, value):

        if not isinstance(value, (bytes, bytearray)):
            raise Exception("expecting a binary, got a %s" % type(value))

        val = super(Bytes, self).to_database(value)
        return bytearray(val)


Bytes = Blob


class Inet(Column):
    """
    Stores an IP address in IPv4 or IPv6 format
    """
    db_type = 'inet'


class Text(Column):
    """
    Stores a UTF-8 encoded string
    """
    db_type = 'text'

    def __init__(self, min_length=None, max_length=None, **kwargs):
        """
        :param int min_length: Sets the minimum length of this string, for validation purposes.
            Defaults to 1 if this is a ``required`` column. Otherwise, None.
        :param int max_length: Sets the maximum length of this string, for validation purposes.
        """
        self.min_length = (
            1 if min_length is None and kwargs.get('required', False)
            else min_length)
        self.max_length = max_length

        if self.min_length is not None:
            if self.min_length < 0:
                raise ValueError(
                    'Minimum length is not allowed to be negative.')

        if self.max_length is not None:
            if self.max_length < 0:
                raise ValueError(
                    'Maximum length is not allowed to be negative.')

        if self.min_length is not None and self.max_length is not None:
            if self.max_length < self.min_length:
                raise ValueError(
                    'Maximum length must be greater or equal '
                    'to minimum length.')

        super(Text, self).__init__(**kwargs)

    def validate(self, value):
        value = super(Text, self).validate(value)
        if not isinstance(value, (str, bytearray)) and value is not None:
            raise ValidationError('{0} {1} is not a string'.format(self.column_name, type(value)))
        if self.max_length is not None:
            if value and len(value) > self.max_length:
                raise ValidationError('{0} is longer than {1} characters'.format(self.column_name, self.max_length))
        if self.min_length:
            if (self.min_length and not value) or len(value) < self.min_length:
                raise ValidationError('{0} is shorter than {1} characters'.format(self.column_name, self.min_length))
        return value


class Ascii(Text):
    """
    Stores a US-ASCII character string
    """
    db_type = 'ascii'

    def validate(self, value):
        """ Only allow ASCII and None values.

        Check against US-ASCII, a.k.a. 7-bit ASCII, a.k.a. ISO646-US, a.k.a.
        the Basic Latin block of the Unicode character set.

        Source: https://github.com/apache/cassandra/blob
        /3dcbe90e02440e6ee534f643c7603d50ca08482b/src/java/org/apache/cassandra
        /serializers/AsciiSerializer.java#L29
        """
        value = super(Ascii, self).validate(value)
        if value:
            charset = value if isinstance(
                value, (bytearray, )) else map(ord, value)
            if not set(range(128)).issuperset(charset):
                raise ValidationError(
                    '{!r} is not an ASCII string.'.format(value))
        return value


class Integer(Column):
    """
    Stores a 32-bit signed integer value
    """

    db_type = 'int'

    def validate(self, value):
        val = super(Integer, self).validate(value)
        if val is None:
            return
        try:
            return int(val)
        except (TypeError, ValueError):
            raise ValidationError("{0} {1} can't be converted to integral value".format(self.column_name, value))

    def to_python(self, value):
        return self.validate(value)

    def to_database(self, value):
        return self.validate(value)


class TinyInt(Integer):
    """
    Stores an 8-bit signed integer value

    .. versionadded:: 2.6.0

    requires C* 2.2+ and protocol v4+
    """
    db_type = 'tinyint'


class SmallInt(Integer):
    """
    Stores a 16-bit signed integer value

    .. versionadded:: 2.6.0

    requires C* 2.2+ and protocol v4+
    """
    db_type = 'smallint'


class BigInt(Integer):
    """
    Stores a 64-bit signed integer value
    """
    db_type = 'bigint'


class VarInt(Column):
    """
    Stores an arbitrary-precision integer
    """
    db_type = 'varint'

    def validate(self, value):
        val = super(VarInt, self).validate(value)
        if val is None:
            return
        try:
            return int(val)
        except (TypeError, ValueError):
            raise ValidationError(
                "{0} {1} can't be converted to integral value".format(self.column_name, value))

    def to_python(self, value):
        return self.validate(value)

    def to_database(self, value):
        return self.validate(value)


class CounterValueManager(BaseValueManager):
    def __init__(self, instance, column, value):
        super(CounterValueManager, self).__init__(instance, column, value)
        self.value = self.value or 0
        self.previous_value = self.previous_value or 0


class Counter(Integer):
    """
    Stores a counter that can be incremented and decremented
    """
    db_type = 'counter'

    value_manager = CounterValueManager

    def __init__(self,
                 index=False,
                 db_field=None,
                 required=False):
        super(Counter, self).__init__(
            primary_key=False,
            partition_key=False,
            index=index,
            db_field=db_field,
            default=0,
            required=required,
        )


class DateTime(Column):
    """
    Stores a datetime value
    """
    db_type = 'timestamp'

    truncate_microseconds = False
    """
    Set this ``True`` to have model instances truncate the date, quantizing it in the same way it will be in the database.
    This allows equality comparison between assigned values and values read back from the database::

        DateTime.truncate_microseconds = True
        assert Model.create(id=0, d=datetime.utcnow()) == Model.objects(id=0).first()

    Defaults to ``False`` to preserve legacy behavior. May change in the future.
    """

    def to_python(self, value):
        if value is None:
            return
        if isinstance(value, datetime):
            if DateTime.truncate_microseconds:
                us = value.microsecond
                truncated_us = us // 1000 * 1000
                return value - timedelta(microseconds=us - truncated_us)
            else:
                return value
        elif isinstance(value, date):
            return datetime(*(value.timetuple()[:6]))

        return datetime.utcfromtimestamp(value)

    def to_database(self, value):
        value = super(DateTime, self).to_database(value)
        if value is None:
            return
        if not isinstance(value, datetime):
            if isinstance(value, date):
                value = datetime(value.year, value.month, value.day)
            else:
                raise ValidationError("{0} '{1}' is not a datetime object".format(self.column_name, value))
        epoch = datetime(1970, 1, 1, tzinfo=value.tzinfo)
        offset = get_total_seconds(epoch.tzinfo.utcoffset(epoch)) if epoch.tzinfo else 0

        return int((get_total_seconds(value - epoch) - offset) * 1000)


class Date(Column):
    """
    Stores a simple date, with no time-of-day

    .. versionchanged:: 2.6.0

        removed overload of Date and DateTime. DateTime is a drop-in replacement for legacy models

    requires C* 2.2+ and protocol v4+
    """
    db_type = 'date'

    def to_database(self, value):
        if value is None:
            return

        # need to translate to int version because some dates are not representable in
        # string form (datetime limitation)
        d = value if isinstance(value, util.Date) else util.Date(value)
        return d.days_from_epoch + SimpleDateType.EPOCH_OFFSET_DAYS

    def to_python(self, value):
        if value is None:
            return
        if isinstance(value, util.Date):
            return value
        if isinstance(value, datetime):
            value = value.date()
        return util.Date(value)

class Time(Column):
    """
    Stores a timezone-naive time-of-day, with nanosecond precision

    .. versionadded:: 2.6.0

    requires C* 2.2+ and protocol v4+
    """
    db_type = 'time'

    def to_database(self, value):
        value = super(Time, self).to_database(value)
        if value is None:
            return
        # str(util.Time) yields desired CQL encoding
        return value if isinstance(value, util.Time) else util.Time(value)

    def to_python(self, value):
        value = super(Time, self).to_database(value)
        if value is None:
            return
        if isinstance(value, util.Time):
            return value
        return util.Time(value)

class Duration(Column):
    """
    Stores a duration (months, days, nanoseconds)

    .. versionadded:: 3.10.0

    requires C* 3.10+ and protocol v4+
    """
    db_type = 'duration'

    def validate(self, value):
        val = super(Duration, self).validate(value)
        if val is None:
            return
        if not isinstance(val, _Duration):
            raise TypeError('{0} {1} is not a valid Duration.'.format(self.column_name, value))
        return val


class UUID(Column):
    """
    Stores a type 1 or 4 UUID
    """
    db_type = 'uuid'

    def validate(self, value):
        val = super(UUID, self).validate(value)
        if val is None:
            return
        if isinstance(val, _UUID):
            return val
        if isinstance(val, str):
            try:
                return _UUID(val)
            except ValueError:
                # fall-through to error
                pass
        raise ValidationError("{0} {1} is not a valid uuid".format(
            self.column_name, value))

    def to_python(self, value):
        return self.validate(value)

    def to_database(self, value):
        return self.validate(value)


class TimeUUID(UUID):
    """
    UUID containing timestamp
    """

    db_type = 'timeuuid'


class Boolean(Column):
    """
    Stores a boolean True or False value
    """
    db_type = 'boolean'

    def validate(self, value):
        """ Always returns a Python boolean. """
        value = super(Boolean, self).validate(value)

        if value is not None:
            value = bool(value)

        return value

    def to_python(self, value):
        return self.validate(value)


class BaseFloat(Column):
    def validate(self, value):
        value = super(BaseFloat, self).validate(value)
        if value is None:
            return
        try:
            return float(value)
        except (TypeError, ValueError):
            raise ValidationError("{0} {1} is not a valid float".format(self.column_name, value))

    def to_python(self, value):
        return self.validate(value)

    def to_database(self, value):
        return self.validate(value)


class Float(BaseFloat):
    """
    Stores a single-precision floating-point value
    """
    db_type = 'float'


class Double(BaseFloat):
    """
    Stores a double-precision floating-point value
    """
    db_type = 'double'


class Decimal(Column):
    """
    Stores a variable precision decimal value
    """
    db_type = 'decimal'

    def validate(self, value):
        from decimal import Decimal as _Decimal
        from decimal import InvalidOperation
        val = super(Decimal, self).validate(value)
        if val is None:
            return
        try:
            return _Decimal(repr(val)) if isinstance(val, float) else _Decimal(val)
        except InvalidOperation:
            raise ValidationError("{0} '{1}' can't be coerced to decimal".format(self.column_name, val))

    def to_python(self, value):
        return self.validate(value)

    def to_database(self, value):
        return self.validate(value)


class BaseCollectionColumn(Column):
    """
    Base Container type for collection-like columns.

    http://cassandra.apache.org/doc/cql3/CQL-3.0.html#collections
    """
    def __init__(self, types, **kwargs):
        """
        :param types: a sequence of sub types in this collection
        """
        instances = []
        for t in types:
            inheritance_comparator = issubclass if isinstance(t, type) else isinstance
            if not inheritance_comparator(t, Column):
                raise ValidationError("%s is not a column class" % (t,))
            if t.db_type is None:
                raise ValidationError("%s is an abstract type" % (t,))
            inst = t() if isinstance(t, type) else t
            if isinstance(t, BaseCollectionColumn):
                inst._freeze_db_type()
            instances.append(inst)

        self.types = instances
        super(BaseCollectionColumn, self).__init__(**kwargs)

    def validate(self, value):
        value = super(BaseCollectionColumn, self).validate(value)
        # It is dangerous to let collections have more than 65535.
        # See: https://issues.apache.org/jira/browse/CASSANDRA-5428
        if value is not None and len(value) > 65535:
            raise ValidationError("{0} Collection can't have more than 65535 elements.".format(self.column_name))
        return value

    def _val_is_null(self, val):
        return not val

    def _freeze_db_type(self):
        if not self.db_type.startswith('frozen'):
            self.db_type = "frozen<%s>" % (self.db_type,)

    @property
    def sub_types(self):
        return self.types

    @property
    def cql_type(self):
        return _cqltypes[self.__class__.__name__.lower()].apply_parameters([c.cql_type for c in self.types])


class Tuple(BaseCollectionColumn):
    """
    Stores a fixed-length set of positional values

    http://docs.datastax.com/en/cql/3.1/cql/cql_reference/tupleType.html
    """
    def __init__(self, *args, **kwargs):
        """
        :param args: column types representing tuple composition
        """
        if not args:
            raise ValueError("Tuple must specify at least one inner type")
        super(Tuple, self).__init__(args, **kwargs)
        self.db_type = 'tuple<{0}>'.format(', '.join(typ.db_type for typ in self.types))

    def validate(self, value):
        val = super(Tuple, self).validate(value)
        if val is None:
            return
        if len(val) > len(self.types):
            raise ValidationError("Value %r has more fields than tuple definition (%s)" %
                                  (val, ', '.join(t for t in self.types)))
        return tuple(t.validate(v) for t, v in zip(self.types, val))

    def to_python(self, value):
        if value is None:
            return tuple()
        return tuple(t.to_python(v) for t, v in zip(self.types, value))

    def to_database(self, value):
        if value is None:
            return
        return tuple(t.to_database(v) for t, v in zip(self.types, value))


class BaseContainerColumn(BaseCollectionColumn):
    pass


class Set(BaseContainerColumn):
    """
    Stores a set of unordered, unique values

    http://www.datastax.com/documentation/cql/3.1/cql/cql_using/use_set_t.html
    """

    _python_type_hashable = False

    def __init__(self, value_type, strict=True, default=set, **kwargs):
        """
        :param value_type: a column class indicating the types of the value
        :param strict: sets whether non set values will be coerced to set
            type on validation, or raise a validation error, defaults to True
        """
        self.strict = strict
        super(Set, self).__init__((value_type,), default=default, **kwargs)
        self.value_col = self.types[0]
        if not self.value_col._python_type_hashable:
            raise ValidationError("Cannot create a Set with unhashable value type (see PYTHON-494)")
        self.db_type = 'set<{0}>'.format(self.value_col.db_type)

    def validate(self, value):
        val = super(Set, self).validate(value)
        if val is None:
            return
        types = (set, util.SortedSet) if self.strict else (set, util.SortedSet, list, tuple)
        if not isinstance(val, types):
            if self.strict:
                raise ValidationError('{0} {1} is not a set object'.format(self.column_name, val))
            else:
                raise ValidationError('{0} {1} cannot be coerced to a set object'.format(self.column_name, val))

        if None in val:
            raise ValidationError("{0} None not allowed in a set".format(self.column_name))
        # TODO: stop doing this conversion because it doesn't support non-hashable collections as keys (cassandra does)
        # will need to start using the cassandra.util types in the next major rev (PYTHON-494)
        return set(self.value_col.validate(v) for v in val)

    def to_python(self, value):
        if value is None:
            return set()
        return set(self.value_col.to_python(v) for v in value)

    def to_database(self, value):
        if value is None:
            return None
        return set(self.value_col.to_database(v) for v in value)


class List(BaseContainerColumn):
    """
    Stores a list of ordered values

    http://www.datastax.com/documentation/cql/3.1/cql/cql_using/use_list_t.html
    """

    _python_type_hashable = False

    def __init__(self, value_type, default=list, **kwargs):
        """
        :param value_type: a column class indicating the types of the value
        """
        super(List, self).__init__((value_type,), default=default, **kwargs)
        self.value_col = self.types[0]
        self.db_type = 'list<{0}>'.format(self.value_col.db_type)

    def validate(self, value):
        val = super(List, self).validate(value)
        if val is None:
            return
        if not isinstance(val, (set, list, tuple)):
            raise ValidationError('{0} {1} is not a list object'.format(self.column_name, val))
        if None in val:
            raise ValidationError("{0} None is not allowed in a list".format(self.column_name))
        return [self.value_col.validate(v) for v in val]

    def to_python(self, value):
        if value is None:
            return []
        return [self.value_col.to_python(v) for v in value]

    def to_database(self, value):
        if value is None:
            return None
        return [self.value_col.to_database(v) for v in value]


class Map(BaseContainerColumn):
    """
    Stores a key -> value map (dictionary)

    https://docs.datastax.com/en/dse/6.7/cql/cql/cql_using/useMap.html
    """

    _python_type_hashable = False

    def __init__(self, key_type, value_type, default=dict, **kwargs):
        """
        :param key_type: a column class indicating the types of the key
        :param value_type: a column class indicating the types of the value
        """
        super(Map, self).__init__((key_type, value_type), default=default, **kwargs)
        self.key_col = self.types[0]
        self.value_col = self.types[1]

        if not self.key_col._python_type_hashable:
            raise ValidationError("Cannot create a Map with unhashable key type (see PYTHON-494)")

        self.db_type = 'map<{0}, {1}>'.format(self.key_col.db_type, self.value_col.db_type)

    def validate(self, value):
        val = super(Map, self).validate(value)
        if val is None:
            return
        if not isinstance(val, (dict, util.OrderedMap)):
            raise ValidationError('{0} {1} is not a dict object'.format(self.column_name, val))
        if None in val:
            raise ValidationError("{0} None is not allowed in a map".format(self.column_name))
        # TODO: stop doing this conversion because it doesn't support non-hashable collections as keys (cassandra does)
        # will need to start using the cassandra.util types in the next major rev (PYTHON-494)
        return dict((self.key_col.validate(k), self.value_col.validate(v)) for k, v in val.items())

    def to_python(self, value):
        if value is None:
            return {}
        if value is not None:
            return dict((self.key_col.to_python(k), self.value_col.to_python(v)) for k, v in value.items())

    def to_database(self, value):
        if value is None:
            return None
        return dict((self.key_col.to_database(k), self.value_col.to_database(v)) for k, v in value.items())


class UDTValueManager(BaseValueManager):
    @property
    def changed(self):
        if self.explicit:
            return self.value != self.previous_value

        default_value = self.column.get_default()
        if not self.column._val_is_null(default_value):
            return self.value != default_value
        elif self.previous_value is None:
            return not self.column._val_is_null(self.value) and self.value.has_changed_fields()

        return False

    def reset_previous_value(self):
        if self.value is not None:
            self.value.reset_changed_fields()
        self.previous_value = copy(self.value)


class UserDefinedType(Column):
    """
    User Defined Type column

    http://www.datastax.com/documentation/cql/3.1/cql/cql_using/cqlUseUDT.html

    These columns are represented by a specialization of :class:`cassandra.cqlengine.usertype.UserType`.

    Please see :ref:`user_types` for examples and discussion.
    """

    value_manager = UDTValueManager

    def __init__(self, user_type, **kwargs):
        """
        :param type user_type: specifies the :class:`~.cqlengine.usertype.UserType` model of the column
        """
        self.user_type = user_type
        self.db_type = "frozen<%s>" % user_type.type_name()
        super(UserDefinedType, self).__init__(**kwargs)

    @property
    def sub_types(self):
        return list(self.user_type._fields.values())

    @property
    def cql_type(self):
        return UserType.make_udt_class(keyspace='', udt_name=self.user_type.type_name(),
                                       field_names=[c.db_field_name for c in self.user_type._fields.values()],
                                       field_types=[c.cql_type for c in self.user_type._fields.values()])

    def validate(self, value):
        val = super(UserDefinedType, self).validate(value)
        if val is None:
            return
        val.validate()
        return val

    def to_python(self, value):
        if value is None:
            return

        for name, field in self.user_type._fields.items():
            if value[name] is not None or isinstance(field, BaseContainerColumn):
                value[name] = field.to_python(value[name])

        return value

    def to_database(self, value):
        if value is None:
            return

        copied_value = deepcopy(value)
        for name, field in self.user_type._fields.items():
            if copied_value[name] is not None or isinstance(field, BaseContainerColumn):
                copied_value[name] = field.to_database(copied_value[name])

        return copied_value


def resolve_udts(col_def, out_list):
    for col in col_def.sub_types:
        resolve_udts(col, out_list)
    if isinstance(col_def, UserDefinedType):
        out_list.append(col_def.user_type)


class _PartitionKeysToken(Column):
    """
    virtual column representing token of partition columns.
    Used by filter(pk__token=Token(...)) filters
    """

    def __init__(self, model):
        self.partition_columns = list(model._partition_keys.values())
        super(_PartitionKeysToken, self).__init__(partition_key=True)

    @property
    def db_field_name(self):
        return 'token({0})'.format(', '.join(['"{0}"'.format(c.db_field_name) for c in self.partition_columns]))