# Copyright 2009-2015 MongoDB, 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.

"""Cursor class to iterate over Mongo query results."""

import copy
import datetime

from collections import deque

from bson import RE_TYPE
from bson.code import Code
from bson.py3compat import (iteritems,
                            integer_types,
                            string_type)
from bson.son import SON
from pymongo import helpers
from pymongo.common import validate_boolean, validate_is_mapping
from pymongo.errors import (AutoReconnect,
                            ConnectionFailure,
                            InvalidOperation,
                            NotMasterError,
                            OperationFailure)
from pymongo.message import _CursorAddress, _GetMore, _Query, _convert_exception
from pymongo.read_preferences import ReadPreference

_QUERY_OPTIONS = {
    "tailable_cursor": 2,
    "slave_okay": 4,
    "oplog_replay": 8,
    "no_timeout": 16,
    "await_data": 32,
    "exhaust": 64,
    "partial": 128}


class CursorType(object):
    NON_TAILABLE = 0
    """The standard cursor type."""

    TAILABLE = _QUERY_OPTIONS["tailable_cursor"]
    """The tailable cursor type.

    Tailable cursors are only for use with capped collections. They are not
    closed when the last data is retrieved but are kept open and the cursor
    location marks the final document position. If more data is received
    iteration of the cursor will continue from the last document received.
    """

    TAILABLE_AWAIT = TAILABLE | _QUERY_OPTIONS["await_data"]
    """A tailable cursor with the await option set.

    Creates a tailable cursor that will wait for a few seconds after returning
    the full result set so that it can capture and return additional data added
    during the query.
    """

    EXHAUST = _QUERY_OPTIONS["exhaust"]
    """An exhaust cursor.

    MongoDB will stream batched results to the client without waiting for the
    client to request each batch, reducing latency.
    """


# This has to be an old style class due to
# http://bugs.jython.org/issue1057
class _SocketManager:
    """Used with exhaust cursors to ensure the socket is returned.
    """
    def __init__(self, sock, pool):
        self.sock = sock
        self.pool = pool
        self.__closed = False

    def __del__(self):
        self.close()

    def close(self):
        """Return this instance's socket to the connection pool.
        """
        if not self.__closed:
            self.__closed = True
            self.pool.return_socket(self.sock)
            self.sock, self.pool = None, None


class Cursor(object):
    """A cursor / iterator over Mongo query results.
    """

    def __init__(self, collection, filter=None, projection=None, skip=0,
                 limit=0, no_cursor_timeout=False,
                 cursor_type=CursorType.NON_TAILABLE,
                 sort=None, allow_partial_results=False, oplog_replay=False,
                 modifiers=None, batch_size=0, manipulate=True):
        """Create a new cursor.

        Should not be called directly by application developers - see
        :meth:`~pymongo.collection.Collection.find` instead.

        .. mongodoc:: cursors
        """
        self.__id = None

        spec = filter
        if spec is None:
            spec = {}

        validate_is_mapping("filter", spec)
        if not isinstance(skip, int):
            raise TypeError("skip must be an instance of int")
        if not isinstance(limit, int):
            raise TypeError("limit must be an instance of int")
        validate_boolean("no_cursor_timeout", no_cursor_timeout)
        if cursor_type not in (CursorType.NON_TAILABLE, CursorType.TAILABLE,
                               CursorType.TAILABLE_AWAIT, CursorType.EXHAUST):
            raise ValueError("not a valid value for cursor_type")
        validate_boolean("allow_partial_results", allow_partial_results)
        validate_boolean("oplog_replay", oplog_replay)
        if modifiers is not None:
            validate_is_mapping("modifiers", modifiers)
        if not isinstance(batch_size, integer_types):
            raise TypeError("batch_size must be an integer")
        if batch_size < 0:
            raise ValueError("batch_size must be >= 0")

        if projection is not None:
            if not projection:
                projection = {"_id": 1}
            projection = helpers._fields_list_to_dict(projection, "projection")

        self.__collection = collection
        self.__spec = spec
        self.__projection = projection
        self.__skip = skip
        self.__limit = limit
        self.__batch_size = batch_size
        self.__modifiers = modifiers and modifiers.copy() or {}
        self.__ordering = sort and helpers._index_document(sort) or None
        self.__max_scan = None
        self.__explain = False
        self.__hint = None
        self.__comment = None
        self.__max_time_ms = None
        self.__max_await_time_ms = None
        self.__max = None
        self.__min = None
        self.__manipulate = manipulate

        # Exhaust cursor support
        self.__exhaust = False
        self.__exhaust_mgr = None
        if cursor_type == CursorType.EXHAUST:
            if self.__collection.database.client.is_mongos:
                raise InvalidOperation('Exhaust cursors are '
                                       'not supported by mongos')
            if limit:
                raise InvalidOperation("Can't use limit and exhaust together.")
            self.__exhaust = True

        # This is ugly. People want to be able to do cursor[5:5] and
        # get an empty result set (old behavior was an
        # exception). It's hard to do that right, though, because the
        # server uses limit(0) to mean 'no limit'. So we set __empty
        # in that case and check for it when iterating. We also unset
        # it anytime we change __limit.
        self.__empty = False

        self.__data = deque()
        self.__address = None
        self.__retrieved = 0
        self.__killed = False

        self.__codec_options = collection.codec_options
        self.__read_preference = collection.read_preference
        self.__read_concern = collection.read_concern

        self.__query_flags = cursor_type
        if self.__read_preference != ReadPreference.PRIMARY:
            self.__query_flags |= _QUERY_OPTIONS["slave_okay"]
        if no_cursor_timeout:
            self.__query_flags |= _QUERY_OPTIONS["no_timeout"]
        if allow_partial_results:
            self.__query_flags |= _QUERY_OPTIONS["partial"]
        if oplog_replay:
            self.__query_flags |= _QUERY_OPTIONS["oplog_replay"]

    @property
    def collection(self):
        """The :class:`~pymongo.collection.Collection` that this
        :class:`Cursor` is iterating.
        """
        return self.__collection

    @property
    def retrieved(self):
        """The number of documents retrieved so far.
        """
        return self.__retrieved

    def __del__(self):
        if self.__id and not self.__killed:
            self.__die()

    def rewind(self):
        """Rewind this cursor to its unevaluated state.

        Reset this cursor if it has been partially or completely evaluated.
        Any options that are present on the cursor will remain in effect.
        Future iterating performed on this cursor will cause new queries to
        be sent to the server, even if the resultant data has already been
        retrieved by this cursor.
        """
        self.__data = deque()
        self.__id = None
        self.__address = None
        self.__retrieved = 0
        self.__killed = False

        return self

    def clone(self):
        """Get a clone of this cursor.

        Returns a new Cursor instance with options matching those that have
        been set on the current instance. The clone will be completely
        unevaluated, even if the current instance has been partially or
        completely evaluated.
        """
        return self._clone(True)

    def _clone(self, deepcopy=True):
        """Internal clone helper."""
        clone = self._clone_base()
        values_to_clone = ("spec", "projection", "skip", "limit",
                           "max_time_ms", "max_await_time_ms", "comment",
                           "max", "min", "ordering", "explain", "hint",
                           "batch_size", "max_scan", "manipulate",
                           "query_flags", "modifiers")
        data = dict((k, v) for k, v in iteritems(self.__dict__)
                    if k.startswith('_Cursor__') and k[9:] in values_to_clone)
        if deepcopy:
            data = self._deepcopy(data)
        clone.__dict__.update(data)
        return clone

    def _clone_base(self):
        """Creates an empty Cursor object for information to be copied into.
        """
        return Cursor(self.__collection)

    def __die(self):
        """Closes this cursor.
        """
        if self.__id and not self.__killed:
            if self.__exhaust and self.__exhaust_mgr:
                # If this is an exhaust cursor and we haven't completely
                # exhausted the result set we *must* close the socket
                # to stop the server from sending more data.
                self.__exhaust_mgr.sock.close()
            else:
                self.__collection.database.client.close_cursor(
                    self.__id,
                    _CursorAddress(
                        self.__address, self.__collection.full_name))
        if self.__exhaust and self.__exhaust_mgr:
            self.__exhaust_mgr.close()
        self.__killed = True

    def close(self):
        """Explicitly close / kill this cursor. Required for PyPy, Jython and
        other Python implementations that don't use reference counting
        garbage collection.
        """
        self.__die()

    def __query_spec(self):
        """Get the spec to use for a query.
        """
        operators = self.__modifiers.copy()
        if self.__ordering:
            operators["$orderby"] = self.__ordering
        if self.__explain:
            operators["$explain"] = True
        if self.__hint:
            operators["$hint"] = self.__hint
        if self.__comment:
            operators["$comment"] = self.__comment
        if self.__max_scan:
            operators["$maxScan"] = self.__max_scan
        if self.__max_time_ms is not None:
            operators["$maxTimeMS"] = self.__max_time_ms
        if self.__max:
            operators["$max"] = self.__max
        if self.__min:
            operators["$min"] = self.__min

        if operators:
            # Make a shallow copy so we can cleanly rewind or clone.
            spec = self.__spec.copy()

            # White-listed commands must be wrapped in $query.
            if "$query" not in spec:
                # $query has to come first
                spec = SON([("$query", spec)])

            if not isinstance(spec, SON):
                # Ensure the spec is SON. As order is important this will
                # ensure its set before merging in any extra operators.
                spec = SON(spec)

            spec.update(operators)
            return spec
        # Have to wrap with $query if "query" is the first key.
        # We can't just use $query anytime "query" is a key as
        # that breaks commands like count and find_and_modify.
        # Checking spec.keys()[0] covers the case that the spec
        # was passed as an instance of SON or OrderedDict.
        elif ("query" in self.__spec and
              (len(self.__spec) == 1 or
               next(iter(self.__spec)) == "query")):
            return SON({"$query": self.__spec})

        return self.__spec

    def __check_okay_to_chain(self):
        """Check if it is okay to chain more options onto this cursor.
        """
        if self.__retrieved or self.__id is not None:
            raise InvalidOperation("cannot set options after executing query")

    def add_option(self, mask):
        """Set arbitrary query flags using a bitmask.

        To set the tailable flag:
        cursor.add_option(2)
        """
        if not isinstance(mask, int):
            raise TypeError("mask must be an int")
        self.__check_okay_to_chain()

        if mask & _QUERY_OPTIONS["exhaust"]:
            if self.__limit:
                raise InvalidOperation("Can't use limit and exhaust together.")
            if self.__collection.database.client.is_mongos:
                raise InvalidOperation('Exhaust cursors are '
                                       'not supported by mongos')
            self.__exhaust = True

        self.__query_flags |= mask
        return self

    def remove_option(self, mask):
        """Unset arbitrary query flags using a bitmask.

        To unset the tailable flag:
        cursor.remove_option(2)
        """
        if not isinstance(mask, int):
            raise TypeError("mask must be an int")
        self.__check_okay_to_chain()

        if mask & _QUERY_OPTIONS["exhaust"]:
            self.__exhaust = False

        self.__query_flags &= ~mask
        return self

    def limit(self, limit):
        """Limits the number of results to be returned by this cursor.

        Raises :exc:`TypeError` if `limit` is not an integer. Raises
        :exc:`~pymongo.errors.InvalidOperation` if this :class:`Cursor`
        has already been used. The last `limit` applied to this cursor
        takes precedence. A limit of ``0`` is equivalent to no limit.

        :Parameters:
          - `limit`: the number of results to return

        .. mongodoc:: limit
        """
        if not isinstance(limit, integer_types):
            raise TypeError("limit must be an integer")
        if self.__exhaust:
            raise InvalidOperation("Can't use limit and exhaust together.")
        self.__check_okay_to_chain()

        self.__empty = False
        self.__limit = limit
        return self

    def batch_size(self, batch_size):
        """Limits the number of documents returned in one batch. Each batch
        requires a round trip to the server. It can be adjusted to optimize
        performance and limit data transfer.

        .. note:: batch_size can not override MongoDB's internal limits on the
           amount of data it will return to the client in a single batch (i.e
           if you set batch size to 1,000,000,000, MongoDB will currently only
           return 4-16MB of results per batch).

        Raises :exc:`TypeError` if `batch_size` is not an integer.
        Raises :exc:`ValueError` if `batch_size` is less than ``0``.
        Raises :exc:`~pymongo.errors.InvalidOperation` if this
        :class:`Cursor` has already been used. The last `batch_size`
        applied to this cursor takes precedence.

        :Parameters:
          - `batch_size`: The size of each batch of results requested.
        """
        if not isinstance(batch_size, integer_types):
            raise TypeError("batch_size must be an integer")
        if batch_size < 0:
            raise ValueError("batch_size must be >= 0")
        self.__check_okay_to_chain()

        self.__batch_size = batch_size == 1 and 2 or batch_size
        return self

    def skip(self, skip):
        """Skips the first `skip` results of this cursor.

        Raises :exc:`TypeError` if `skip` is not an integer. Raises
        :exc:`ValueError` if `skip` is less than ``0``. Raises
        :exc:`~pymongo.errors.InvalidOperation` if this :class:`Cursor` has
        already been used. The last `skip` applied to this cursor takes
        precedence.

        :Parameters:
          - `skip`: the number of results to skip
        """
        if not isinstance(skip, integer_types):
            raise TypeError("skip must be an integer")
        if skip < 0:
            raise ValueError("skip must be >= 0")
        self.__check_okay_to_chain()

        self.__skip = skip
        return self

    def max_time_ms(self, max_time_ms):
        """Specifies a time limit for a query operation. If the specified
        time is exceeded, the operation will be aborted and
        :exc:`~pymongo.errors.ExecutionTimeout` is raised. If `max_time_ms`
        is ``None`` no limit is applied.

        Raises :exc:`TypeError` if `max_time_ms` is not an integer or ``None``.
        Raises :exc:`~pymongo.errors.InvalidOperation` if this :class:`Cursor`
        has already been used.

        :Parameters:
          - `max_time_ms`: the time limit after which the operation is aborted
        """
        if (not isinstance(max_time_ms, integer_types)
                and max_time_ms is not None):
            raise TypeError("max_time_ms must be an integer or None")
        self.__check_okay_to_chain()

        self.__max_time_ms = max_time_ms
        return self

    def max_await_time_ms(self, max_await_time_ms):
        """Specifies a time limit for a getMore operation on a
        :attr:`~pymongo.cursor.CursorType.TAILABLE_AWAIT` cursor. For all other
        types of cursor max_await_time_ms is ignored.

        Raises :exc:`TypeError` if `max_await_time_ms` is not an integer or
        ``None``. Raises :exc:`~pymongo.errors.InvalidOperation` if this
        :class:`Cursor` has already been used.

        .. note:: `max_await_time_ms` requires server version **>= 3.2**

        :Parameters:
          - `max_await_time_ms`: the time limit after which the operation is
            aborted

        .. versionadded:: 3.2
        """
        if (not isinstance(max_await_time_ms, integer_types)
                and max_await_time_ms is not None):
            raise TypeError("max_await_time_ms must be an integer or None")
        self.__check_okay_to_chain()

        # Ignore max_await_time_ms if not tailable or await_data is False.
        if self.__query_flags & CursorType.TAILABLE_AWAIT:
            self.__max_await_time_ms = max_await_time_ms

        return self

    def __getitem__(self, index):
        """Get a single document or a slice of documents from this cursor.

        Raises :class:`~pymongo.errors.InvalidOperation` if this
        cursor has already been used.

        To get a single document use an integral index, e.g.::

          >>> db.test.find()[50]

        An :class:`IndexError` will be raised if the index is negative
        or greater than the amount of documents in this cursor. Any
        limit previously applied to this cursor will be ignored.

        To get a slice of documents use a slice index, e.g.::

          >>> db.test.find()[20:25]

        This will return this cursor with a limit of ``5`` and skip of
        ``20`` applied.  Using a slice index will override any prior
        limits or skips applied to this cursor (including those
        applied through previous calls to this method). Raises
        :class:`IndexError` when the slice has a step, a negative
        start value, or a stop value less than or equal to the start
        value.

        :Parameters:
          - `index`: An integer or slice index to be applied to this cursor
        """
        self.__check_okay_to_chain()
        self.__empty = False
        if isinstance(index, slice):
            if index.step is not None:
                raise IndexError("Cursor instances do not support slice steps")

            skip = 0
            if index.start is not None:
                if index.start < 0:
                    raise IndexError("Cursor instances do not support"
                                     "negative indices")
                skip = index.start

            if index.stop is not None:
                limit = index.stop - skip
                if limit < 0:
                    raise IndexError("stop index must be greater than start"
                                     "index for slice %r" % index)
                if limit == 0:
                    self.__empty = True
            else:
                limit = 0

            self.__skip = skip
            self.__limit = limit
            return self

        if isinstance(index, integer_types):
            if index < 0:
                raise IndexError("Cursor instances do not support negative"
                                 "indices")
            clone = self.clone()
            clone.skip(index + self.__skip)
            clone.limit(-1)  # use a hard limit
            for doc in clone:
                return doc
            raise IndexError("no such item for Cursor instance")
        raise TypeError("index %r cannot be applied to Cursor "
                        "instances" % index)

    def max_scan(self, max_scan):
        """Limit the number of documents to scan when performing the query.

        Raises :class:`~pymongo.errors.InvalidOperation` if this
        cursor has already been used. Only the last :meth:`max_scan`
        applied to this cursor has any effect.

        :Parameters:
          - `max_scan`: the maximum number of documents to scan
        """
        self.__check_okay_to_chain()
        self.__max_scan = max_scan
        return self

    def max(self, spec):
        """Adds `max` operator that specifies upper bound for specific index.

        :Parameters:
          - `spec`: a list of field, limit pairs specifying the exclusive
            upper bound for all keys of a specific index in order.

        .. versionadded:: 2.7
        """
        if not isinstance(spec, (list, tuple)):
            raise TypeError("spec must be an instance of list or tuple")

        self.__check_okay_to_chain()
        self.__max = SON(spec)
        return self

    def min(self, spec):
        """Adds `min` operator that specifies lower bound for specific index.

        :Parameters:
          - `spec`: a list of field, limit pairs specifying the inclusive
            lower bound for all keys of a specific index in order.

        .. versionadded:: 2.7
        """
        if not isinstance(spec, (list, tuple)):
            raise TypeError("spec must be an instance of list or tuple")

        self.__check_okay_to_chain()
        self.__min = SON(spec)
        return self

    def sort(self, key_or_list, direction=None):
        """Sorts this cursor's results.

        Pass a field name and a direction, either
        :data:`~pymongo.ASCENDING` or :data:`~pymongo.DESCENDING`::

            for doc in collection.find().sort('field', pymongo.ASCENDING):
                print(doc)

        To sort by multiple fields, pass a list of (key, direction) pairs::

            for doc in collection.find().sort([
                    ('field1', pymongo.ASCENDING),
                    ('field2', pymongo.DESCENDING)]):
                print(doc)

        Beginning with MongoDB version 2.6, text search results can be
        sorted by relevance::

            cursor = db.test.find(
                {'$text': {'$search': 'some words'}},
                {'score': {'$meta': 'textScore'}})

            # Sort by 'score' field.
            cursor.sort([('score', {'$meta': 'textScore'})])

            for doc in cursor:
                print(doc)

        Raises :class:`~pymongo.errors.InvalidOperation` if this cursor has
        already been used. Only the last :meth:`sort` applied to this
        cursor has any effect.

        :Parameters:
          - `key_or_list`: a single key or a list of (key, direction)
            pairs specifying the keys to sort on
          - `direction` (optional): only used if `key_or_list` is a single
            key, if not given :data:`~pymongo.ASCENDING` is assumed
        """
        self.__check_okay_to_chain()
        keys = helpers._index_list(key_or_list, direction)
        self.__ordering = helpers._index_document(keys)
        return self

    def count(self, with_limit_and_skip=False):
        """Get the size of the results set for this query.

        Returns the number of documents in the results set for this query. Does
        not take :meth:`limit` and :meth:`skip` into account by default - set
        `with_limit_and_skip` to ``True`` if that is the desired behavior.
        Raises :class:`~pymongo.errors.OperationFailure` on a database error.

        When used with MongoDB >= 2.6, :meth:`~count` uses any :meth:`~hint`
        applied to the query. In the following example the hint is passed to
        the count command:

          collection.find({'field': 'value'}).hint('field_1').count()

        The :meth:`count` method obeys the
        :attr:`~pymongo.collection.Collection.read_preference` of the
        :class:`~pymongo.collection.Collection` instance on which
        :meth:`~pymongo.collection.Collection.find` was called.

        :Parameters:
          - `with_limit_and_skip` (optional): take any :meth:`limit` or
            :meth:`skip` that has been applied to this cursor into account when
            getting the count

        .. note:: The `with_limit_and_skip` parameter requires server
           version **>= 1.1.4-**

        .. versionchanged:: 2.8
           The :meth:`~count` method now supports :meth:`~hint`.
        """
        validate_boolean("with_limit_and_skip", with_limit_and_skip)
        cmd = SON([("count", self.__collection.name),
                   ("query", self.__spec)])
        if self.__max_time_ms is not None:
            cmd["maxTimeMS"] = self.__max_time_ms
        if self.__comment:
            cmd["$comment"] = self.__comment

        if self.__hint is not None:
            cmd["hint"] = self.__hint

        if with_limit_and_skip:
            if self.__limit:
                cmd["limit"] = self.__limit
            if self.__skip:
                cmd["skip"] = self.__skip

        return self.__collection._count(cmd)

    def distinct(self, key):
        """Get a list of distinct values for `key` among all documents
        in the result set of this query.

        Raises :class:`TypeError` if `key` is not an instance of
        :class:`basestring` (:class:`str` in python 3).

        The :meth:`distinct` method obeys the
        :attr:`~pymongo.collection.Collection.read_preference` of the
        :class:`~pymongo.collection.Collection` instance on which
        :meth:`~pymongo.collection.Collection.find` was called.

        :Parameters:
          - `key`: name of key for which we want to get the distinct values

        .. seealso:: :meth:`pymongo.collection.Collection.distinct`
        """
        options = {}
        if self.__spec:
            options["query"] = self.__spec
        if self.__max_time_ms is not None:
            options['maxTimeMS'] = self.__max_time_ms
        if self.__comment:
            options['$comment'] = self.__comment

        return self.__collection.distinct(key, **options)

    def explain(self):
        """Returns an explain plan record for this cursor.

        .. mongodoc:: explain
        """
        c = self.clone()
        c.__explain = True

        # always use a hard limit for explains
        if c.__limit:
            c.__limit = -abs(c.__limit)
        return next(c)

    def hint(self, index):
        """Adds a 'hint', telling Mongo the proper index to use for the query.

        Judicious use of hints can greatly improve query
        performance. When doing a query on multiple fields (at least
        one of which is indexed) pass the indexed field as a hint to
        the query. Hinting will not do anything if the corresponding
        index does not exist. Raises
        :class:`~pymongo.errors.InvalidOperation` if this cursor has
        already been used.

        `index` should be an index as passed to
        :meth:`~pymongo.collection.Collection.create_index`
        (e.g. ``[('field', ASCENDING)]``) or the name of the index.
        If `index` is ``None`` any existing hint for this query is
        cleared. The last hint applied to this cursor takes precedence
        over all others.

        :Parameters:
          - `index`: index to hint on (as an index specifier)

        .. versionchanged:: 2.8
           The :meth:`~hint` method accepts the name of the index.
        """
        self.__check_okay_to_chain()
        if index is None:
            self.__hint = None
            return self

        if isinstance(index, string_type):
            self.__hint = index
        else:
            self.__hint = helpers._index_document(index)
        return self

    def comment(self, comment):
        """Adds a 'comment' to the cursor.

        http://docs.mongodb.org/manual/reference/operator/comment/

        :Parameters:
          - `comment`: A string or document

        .. versionadded:: 2.7
        """
        self.__check_okay_to_chain()
        self.__comment = comment
        return self

    def where(self, code):
        """Adds a $where clause to this query.

        The `code` argument must be an instance of :class:`basestring`
        (:class:`str` in python 3) or :class:`~bson.code.Code`
        containing a JavaScript expression. This expression will be
        evaluated for each document scanned. Only those documents
        for which the expression evaluates to *true* will be returned
        as results. The keyword *this* refers to the object currently
        being scanned.

        Raises :class:`TypeError` if `code` is not an instance of
        :class:`basestring` (:class:`str` in python 3). Raises
        :class:`~pymongo.errors.InvalidOperation` if this
        :class:`Cursor` has already been used. Only the last call to
        :meth:`where` applied to a :class:`Cursor` has any effect.

        :Parameters:
          - `code`: JavaScript expression to use as a filter
        """
        self.__check_okay_to_chain()
        if not isinstance(code, Code):
            code = Code(code)

        self.__spec["$where"] = code
        return self

    def __send_message(self, operation):
        """Send a query or getmore operation and handles the response.

        If operation is ``None`` this is an exhaust cursor, which reads
        the next result batch off the exhaust socket instead of
        sending getMore messages to the server.

        Can raise ConnectionFailure.
        """
        client = self.__collection.database.client
        listeners = client._event_listeners
        publish = listeners.enabled_for_commands
        from_command = False

        if operation:
            kwargs = {
                "read_preference": self.__read_preference,
                "exhaust": self.__exhaust,
            }
            if self.__address is not None:
                kwargs["address"] = self.__address

            try:
                response = client._send_message_with_response(operation,
                                                              **kwargs)
                self.__address = response.address
                if self.__exhaust:
                    # 'response' is an ExhaustResponse.
                    self.__exhaust_mgr = _SocketManager(response.socket_info,
                                                        response.pool)

                cmd_name = operation.name
                data = response.data
                cmd_duration = response.duration
                rqst_id = response.request_id
                from_command = response.from_command
            except AutoReconnect:
                # Don't try to send kill cursors on another socket
                # or to another server. It can cause a _pinValue
                # assertion on some server releases if we get here
                # due to a socket timeout.
                self.__killed = True
                raise
        else:
            # Exhaust cursor - no getMore message.
            rqst_id = 0
            cmd_name = 'getMore'
            if publish:
                # Fake a getMore command.
                cmd = SON([('getMore', self.__id),
                           ('collection', self.__collection.name)])
                if self.__batch_size:
                    cmd['batchSize'] = self.__batch_size
                if self.__max_time_ms:
                    cmd['maxTimeMS'] = self.__max_time_ms
                listeners.publish_command_start(
                    cmd, self.__collection.database.name, 0, self.__address)
                start = datetime.datetime.now()
            try:
                data = self.__exhaust_mgr.sock.receive_message(1, None)
            except Exception as exc:
                if publish:
                    duration = datetime.datetime.now() - start
                    listeners.publish_command_failure(
                        duration, _convert_exception(exc), cmd_name, rqst_id,
                        self.__address)
                if isinstance(exc, ConnectionFailure):
                    self.__die()
                raise
            if publish:
                cmd_duration = datetime.datetime.now() - start

        if publish:
            start = datetime.datetime.now()
        try:
            doc = helpers._unpack_response(response=data,
                                           cursor_id=self.__id,
                                           codec_options=self.__codec_options)
            if from_command:
                helpers._check_command_response(doc['data'][0])
        except OperationFailure as exc:
            self.__killed = True

            # Make sure exhaust socket is returned immediately, if necessary.
            self.__die()

            if publish:
                duration = (datetime.datetime.now() - start) + cmd_duration
                listeners.publish_command_failure(
                    duration, exc.details, cmd_name, rqst_id, self.__address)

            # If this is a tailable cursor the error is likely
            # due to capped collection roll over. Setting
            # self.__killed to True ensures Cursor.alive will be
            # False. No need to re-raise.
            if self.__query_flags & _QUERY_OPTIONS["tailable_cursor"]:
                return
            raise
        except NotMasterError as exc:
            # Don't send kill cursors to another server after a "not master"
            # error. It's completely pointless.
            self.__killed = True

            # Make sure exhaust socket is returned immediately, if necessary.
            self.__die()

            if publish:
                duration = (datetime.datetime.now() - start) + cmd_duration
                listeners.publish_command_failure(
                    duration, exc.details, cmd_name, rqst_id, self.__address)

            client._reset_server_and_request_check(self.__address)
            raise
        except Exception as exc:
            if publish:
                duration = (datetime.datetime.now() - start) + cmd_duration
                listeners.publish_command_failure(
                    duration, _convert_exception(exc), cmd_name, rqst_id,
                    self.__address)
            raise

        if publish:
            duration = (datetime.datetime.now() - start) + cmd_duration
            # Must publish in find / getMore / explain command response format.
            if from_command:
                res = doc['data'][0]
            elif cmd_name == "explain":
                res = doc["data"][0] if doc["number_returned"] else {}
            else:
                res = {"cursor": {"id": doc["cursor_id"],
                                  "ns": self.__collection.full_name},
                       "ok": 1}
                if cmd_name == "find":
                    res["cursor"]["firstBatch"] = doc["data"]
                else:
                    res["cursor"]["nextBatch"] = doc["data"]
            listeners.publish_command_success(
                duration, res, cmd_name, rqst_id, self.__address)

        if from_command and cmd_name != "explain":
            cursor = doc['data'][0]['cursor']
            self.__id = cursor['id']
            if cmd_name == 'find':
                documents = cursor['firstBatch']
            else:
                documents = cursor['nextBatch']
            self.__data = deque(documents)
            self.__retrieved += len(documents)
        else:
            self.__id = doc["cursor_id"]
            self.__data = deque(doc["data"])
            self.__retrieved += doc["number_returned"]

        if self.__id == 0:
            self.__killed = True


        if self.__limit and self.__id and self.__limit <= self.__retrieved:
            self.__die()

        # Don't wait for garbage collection to call __del__, return the
        # socket to the pool now.
        if self.__exhaust and self.__id == 0:
            self.__exhaust_mgr.close()

    def _refresh(self):
        """Refreshes the cursor with more data from Mongo.

        Returns the length of self.__data after refresh. Will exit early if
        self.__data is already non-empty. Raises OperationFailure when the
        cursor cannot be refreshed due to an error on the query.
        """
        if len(self.__data) or self.__killed:
            return len(self.__data)

        if self.__id is None:  # Query
            ntoreturn = self.__batch_size
            if self.__limit:
                if self.__batch_size:
                    ntoreturn = min(self.__limit, self.__batch_size)
                else:
                    ntoreturn = self.__limit
            self.__send_message(_Query(self.__query_flags,
                                       self.__collection.database.name,
                                       self.__collection.name,
                                       self.__skip,
                                       ntoreturn,
                                       self.__query_spec(),
                                       self.__projection,
                                       self.__codec_options,
                                       self.__read_preference,
                                       self.__limit,
                                       self.__batch_size,
                                       self.__read_concern))
            if not self.__id:
                self.__killed = True
        elif self.__id:  # Get More
            if self.__limit:
                limit = self.__limit - self.__retrieved
                if self.__batch_size:
                    limit = min(limit, self.__batch_size)
            else:
                limit = self.__batch_size

            # Exhaust cursors don't send getMore messages.
            if self.__exhaust:
                self.__send_message(None)
            else:
                self.__send_message(_GetMore(self.__collection.database.name,
                                             self.__collection.name,
                                             limit,
                                             self.__id,
                                             self.__codec_options,
                                             self.__max_await_time_ms))

        else:  # Cursor id is zero nothing else to return
            self.__killed = True

        return len(self.__data)

    @property
    def alive(self):
        """Does this cursor have the potential to return more data?

        This is mostly useful with `tailable cursors
        <http://www.mongodb.org/display/DOCS/Tailable+Cursors>`_
        since they will stop iterating even though they *may* return more
        results in the future.

        With regular cursors, simply use a for loop instead of :attr:`alive`::

            for doc in collection.find():
                print(doc)

        .. note:: Even if :attr:`alive` is True, :meth:`next` can raise
          :exc:`StopIteration`. :attr:`alive` can also be True while iterating
          a cursor from a failed server. In this case :attr:`alive` will
          return False after :meth:`next` fails to retrieve the next batch
          of results from the server.
        """
        return bool(len(self.__data) or (not self.__killed))

    @property
    def cursor_id(self):
        """Returns the id of the cursor

        Useful if you need to manage cursor ids and want to handle killing
        cursors manually using
        :meth:`~pymongo.mongo_client.MongoClient.kill_cursors`

        .. versionadded:: 2.2
        """
        return self.__id

    @property
    def address(self):
        """The (host, port) of the server used, or None.

        .. versionchanged:: 3.0
           Renamed from "conn_id".
        """
        return self.__address

    def __iter__(self):
        return self

    def next(self):
        """Advance the cursor."""
        if self.__empty:
            raise StopIteration
        _db = self.__collection.database
        if len(self.__data) or self._refresh():
            if self.__manipulate:
                return _db._fix_outgoing(self.__data.popleft(),
                                         self.__collection)
            else:
                return self.__data.popleft()
        else:
            raise StopIteration

    __next__ = next

    def __enter__(self):
        return self

    def __exit__(self, exc_type, exc_val, exc_tb):
        self.__die()

    def __copy__(self):
        """Support function for `copy.copy()`.

        .. versionadded:: 2.4
        """
        return self._clone(deepcopy=False)

    def __deepcopy__(self, memo):
        """Support function for `copy.deepcopy()`.

        .. versionadded:: 2.4
        """
        return self._clone(deepcopy=True)

    def _deepcopy(self, x, memo=None):
        """Deepcopy helper for the data dictionary or list.

        Regular expressions cannot be deep copied but as they are immutable we
        don't have to copy them when cloning.
        """
        if not hasattr(x, 'items'):
            y, is_list, iterator = [], True, enumerate(x)
        else:
            y, is_list, iterator = {}, False, iteritems(x)

        if memo is None:
            memo = {}
        val_id = id(x)
        if val_id in memo:
            return memo.get(val_id)
        memo[val_id] = y

        for key, value in iterator:
            if isinstance(value, (dict, list)) and not isinstance(value, SON):
                value = self._deepcopy(value, memo)
            elif not isinstance(value, RE_TYPE):
                value = copy.deepcopy(value, memo)

            if is_list:
                y.append(value)
            else:
                if not isinstance(key, RE_TYPE):
                    key = copy.deepcopy(key, memo)
                y[key] = value
        return y
