pymongo的一些操作

 參考:http://www.yiibai.com/mongodb/mongodb_drop_collection.html

http://www.cnblogs.com/zhouxuchen/p/5544227.html

pymongo的一些操作:

啟動

sudo service mongod start

遠程連接的時候, 配置mongodb.conf  (通過 $ whereis mongod  找到位置)    #bind_ip = 127.0.0.1    改為0.0.0.0  ,和django是一樣的

數據庫 聚集(1層)

#!/usr/bin/python
#coding=utf-8

import datetime,time
from pymongo import MongoClient

#連接到數據庫
# client = MongoClient('localhost', 27017)
client = MongoClient("127.0.0.1", 27017)

#list all databases
print client.database_names()   #database list

#delete specific database
# client.drop_database('tieba')   #delete
# db = client['tieba']            #沒有就新建

#list all collection names
print db.collection_names(include_system_collections=False)

#delete specific collection
#db["mycollection"].drop()

聚集下的item:增刪改查(2層)

insert_one,增加一個item，如果你傳遞給insert_one()方法的參數不包含_id字段，MongoClient將自動添加這個字段並且生成一個ObjectId設置為這個字段的值。

 def insert_one(self, document, bypass_document_validation=False):
        """Insert a single document.

          >>> db.test.count({'x': 1})
          0
          >>> result = db.test.insert_one({'x': 1})
          >>> result.inserted_id
          ObjectId('54f112defba522406c9cc208')
          >>> db.test.find_one({'x': 1})
          {u'x': 1, u'_id': ObjectId('54f112defba522406c9cc208')}

        :Parameters:
          - `document`: The document to insert. Must be a mutable mapping
            type. If the document does not have an _id field one will be
            added automatically.
          - `bypass_document_validation`: (optional) If ``True``, allows the
            write to opt-out of document level validation. Default is
            ``False``.

insert_many,增加一個可循環的item

    def insert_many(self, documents, ordered=True,
                    bypass_document_validation=False):
        """Insert an iterable of documents.

          >>> db.test.count()
          0
          >>> result = db.test.insert_many([{'x': i} for i in range(2)])
          >>> result.inserted_ids
          [ObjectId('54f113fffba522406c9cc20e'), ObjectId('54f113fffba522406c9cc20f')]
          >>> db.test.count()
          2

        :Parameters:
          - `documents`: A iterable of documents to insert.
          - `ordered` (optional): If ``True`` (the default) documents will be
            inserted on the server serially, in the order provided. If an error
            occurs all remaining inserts are aborted. If ``False``, documents
            will be inserted on the server in arbitrary order, possibly in
            parallel, and all document inserts will be attempted.
          - `bypass_document_validation`: (optional) If ``True``, allows the
            write to opt-out of document level validation. Default is
            ``False``.

update_one,注意一些操作符,如$set,$unset,$inc ,設置,刪除,加法,對字段的操作,upsert=True,沒有就增加

更新頂級字段

如下操作將更新第一個符合name等於Juni這個條件的文檔。使用$set操作符更新cuisine字段且將lastModified修改為當前日期。

result = db.restaurants.update_one(
    {"name": "Juni"},
    {
        "$set": {
            "cuisine": "American (New)"
        },
        "$currentDate": {"lastModified": True}
    }
)

更新嵌入式文檔中的字段

要更新一個嵌入式文檔中的字段，需要使用點操作符。當使用點操作符時，使用點操作符需要使用雙引號將字段名包裹。下面的操作將更新address字段中的street值。

result = db.restaurants.update_one(
    {"restaurant_id": "41156888"},
    {"$set": {"address.street": "East 31st Street"}}
)

    def update_one(self, filter, update, upsert=False,
                   bypass_document_validation=False,
                   collation=None):
        """Update a single document matching the filter.

          >>> for doc in db.test.find():
          ...     print(doc)
          ...
          {u'x': 1, u'_id': 0}
          {u'x': 1, u'_id': 1}
          {u'x': 1, u'_id': 2}
          >>> result = db.test.update_one({'x': 1}, {'$inc': {'x': 3}})
          >>> result.matched_count
          1
          >>> result.modified_count
          1
          >>> for doc in db.test.find():
          ...     print(doc)
          ...
          {u'x': 4, u'_id': 0}
          {u'x': 1, u'_id': 1}
          {u'x': 1, u'_id': 2}

        :Parameters:
          - `filter`: A query that matches the document to update.
          - `update`: The modifications to apply.
          - `upsert` (optional): If ``True``, perform an insert if no documents
            match the filter.
          - `bypass_document_validation`: (optional) If ``True``, allows the
            write to opt-out of document level validation. Default is
            ``False``.
          - `collation` (optional): An instance of
            :class:`~pymongo.collation.Collation`. This option is only supported
            on MongoDB 3.4 and above.

update_many,更改了所有匹配的item

    def update_many(self, filter, update, upsert=False,
                    bypass_document_validation=False, collation=None):
        """Update one or more documents that match the filter.

          >>> for doc in db.test.find():
          ...     print(doc)
          ...
          {u'x': 1, u'_id': 0}
          {u'x': 1, u'_id': 1}
          {u'x': 1, u'_id': 2}
          >>> result = db.test.update_many({'x': 1}, {'$inc': {'x': 3}})
          >>> result.matched_count
          3
          >>> result.modified_count
          3
          >>> for doc in db.test.find():
          ...     print(doc)
          ...
          {u'x': 4, u'_id': 0}
          {u'x': 4, u'_id': 1}
          {u'x': 4, u'_id': 2}

        :Parameters:
          - `filter`: A query that matches the documents to update.
          - `update`: The modifications to apply.
          - `upsert` (optional): If ``True``, perform an insert if no documents
            match the filter.
          - `bypass_document_validation` (optional): If ``True``, allows the
            write to opt-out of document level validation. Default is
            ``False``.
          - `collation` (optional): An instance of
            :class:`~pymongo.collation.Collation`. This option is only supported
            on MongoDB 3.4 and above.

replace_one,替換,這個是沒有操作符的

替換一個文檔

要替換整個文檔（除了_id字段），將一個完整的文檔作為第二個參數傳給update()方法。替代文檔對應原來的文檔可以有不同的字段。在替代文檔中，你可以忽略_id字段因為它是不變的。如果你包含了_id字段，那它必須和原文檔的值相同。

重要：
在更新之后，該文檔將只包含替代文檔的字段。

在如下的更新操作后，被修改的文檔將只剩下_id、name和address字段。該文檔將不再包含restaurant_id、cuisine、grades以及borough字段。

result = db.restaurants.replace_one(
    {"restaurant_id": "41704620"},
    {
        "name": "Vella 2",
        "address": {
            "coord": [-73.9557413, 40.7720266],
            "building": "1480",
            "street": "2 Avenue",
            "zipcode": "10075"
        }
    }
)

 

    def replace_one(self, filter, replacement, upsert=False,
                    bypass_document_validation=False, collation=None):
        """Replace a single document matching the filter.

          >>> for doc in db.test.find({}):
          ...     print(doc)
          ...
          {u'x': 1, u'_id': ObjectId('54f4c5befba5220aa4d6dee7')}
          >>> result = db.test.replace_one({'x': 1}, {'y': 1})
          >>> result.matched_count
          1
          >>> result.modified_count
          1
          >>> for doc in db.test.find({}):
          ...     print(doc)
          ...
          {u'y': 1, u'_id': ObjectId('54f4c5befba5220aa4d6dee7')}

        The *upsert* option can be used to insert a new document if a matching
        document does not exist.

          >>> result = db.test.replace_one({'x': 1}, {'x': 1}, True)
          >>> result.matched_count
          0
          >>> result.modified_count
          0
          >>> result.upserted_id
          ObjectId('54f11e5c8891e756a6e1abd4')
          >>> db.test.find_one({'x': 1})
          {u'x': 1, u'_id': ObjectId('54f11e5c8891e756a6e1abd4')}

        :Parameters:
          - `filter`: A query that matches the document to replace.
          - `replacement`: The new document.
          - `upsert` (optional): If ``True``, perform an insert if no documents
            match the filter.
          - `bypass_document_validation`: (optional) If ``True``, allows the
            write to opt-out of document level validation. Default is
            ``False``.
          - `collation` (optional): An instance of
            :class:`~pymongo.collation.Collation`. This option is only supported
            on MongoDB 3.4 and above.

delete_one,刪除一個item

"""Delete a single document matching the filter.

          >>> db.test.count({'x': 1})
          3
          >>> result = db.test.delete_one({'x': 1})
          >>> result.deleted_count
          1
          >>> db.test.count({'x': 1})
          2

        :Parameters:
          - `filter`: A query that matches the document to delete.
          - `collation` (optional): An instance of
            :class:`~pymongo.collation.Collation`. This option is only supported
            on MongoDB 3.4 and above.

delete_many,只要符合匹配規則的,都刪除

要刪除一個集合中的所有文檔，給delete_many()方法傳遞一個空的條件參數即可。

result = db.restaurants.delete_many({})

銷毀一個集合

刪除所有文檔的操作只會清空集合中的文檔。該集合以及集合的索引將依舊存在。要清空一個集合，銷毀該集合以及它的索引並且重建集合和索引可能是相比於清空一個集合更加高效的操作方式。使用drop()方法可以銷毀一個集合，包括它所有的索引。

db.restaurants.drop()

 

    def delete_many(self, filter, collation=None):
        """Delete one or more documents matching the filter.

          >>> db.test.count({'x': 1})
          3
          >>> result = db.test.delete_many({'x': 1})
          >>> result.deleted_count
          3
          >>> db.test.count({'x': 1})
          0

        :Parameters:
          - `filter`: A query that matches the documents to delete.
          - `collation` (optional): An instance of
            :class:`~pymongo.collation.Collation`. This option is only supported
            on MongoDB 3.4 and above.

find_one

    def find_one(self, filter=None, *args, **kwargs):
        """Get a single document from the database.

        All arguments to :meth:`find` are also valid arguments for
        :meth:`find_one`, although any `limit` argument will be
        ignored. Returns a single document, or ``None`` if no matching
        document is found.

        The :meth:`find_one` method obeys the :attr:`read_preference` of
        this :class:`Collection`.

        :Parameters:

          - `filter` (optional): a dictionary specifying
            the query to be performed OR any other type to be used as
            the value for a query for ``"_id"``.

          - `*args` (optional): any additional positional arguments
            are the same as the arguments to :meth:`find`.

          - `**kwargs` (optional): any additional keyword arguments
            are the same as the arguments to :meth:`find`.

              >>> collection.find_one(max_time_ms=100)
        """

find,調用find()方式不傳值即可得到集合中所有的文檔,

cursor = db.restaurants.find()   

返回restaurants集合中所有文檔。

如下所示的操作將查詢borough字段等於Manhattan的文檔。

cursor = db.restaurants.find({"borough": "Manhattan"})

邏輯與

你可以使用一個列表指定一個邏輯與條件查詢操作，使用逗號分隔條件。

cursor = db.restaurants.find({"cuisine": "Italian", "address.zipcode": "10075"})

邏輯或

你可以使用$or操作符進行邏輯或條件的指定。

cursor = db.restaurants.find(
    {"$or": [{"cuisine": "Italian"}, {"address.zipcode": "10075"}]})

大於（$gt）操作符,操作符：$eq $gt $gte $in $lt $lte $ne $nin，等等，還有很多

查詢字段grades包含一個嵌入式文檔，其中score大於30。

cursor = db.restaurants.find({"grades.score": {"$gt": 30}})

對結果進行排序

要指定結果集的順序，可以通過追加sort()方法進行查詢。給sort()方法傳遞需要排序的字段和配需類型等。

• pymongo.ASCENDING表示升序排序。
• pymongo.DESCENDING表示降序排序。

如果要通過多個鍵星星排序，可以傳遞鍵的列表和以及對應的排序類型的列表。舉例來說，如下操作將返回restaurants集合中所有的文檔，並且先通過borough字段進行升序排序，然后在每個borough內，通過"address.zipcode"字段進行升序排序。

import pymongo
cursor = db.restaurants.find().sort([
    ("borough", pymongo.ASCENDING),
    ("address.zipcode", pymongo.ASCENDING)
])

 

"""Query the database.

        The `filter` argument is a prototype document that all results
        must match. For example:

        >>> db.test.find({"hello": "world"})

        only matches documents that have a key "hello" with value
        "world".  Matches can have other keys *in addition* to
        "hello". The `projection` argument is used to specify a subset
        of fields that should be included in the result documents. By
        limiting results to a certain subset of fields you can cut
        down on network traffic and decoding time.

        Raises :class:`TypeError` if any of the arguments are of
        improper type. Returns an instance of
        :class:`~pymongo.cursor.Cursor` corresponding to this query.

        The :meth:`find` method obeys the :attr:`read_preference` of
        this :class:`Collection`.

        :Parameters:
          - `filter` (optional): a SON object specifying elements which
            must be present for a document to be included in the
            result set
          - `projection` (optional): a list of field names that should be
            returned in the result set or a dict specifying the fields
            to include or exclude. If `projection` is a list "_id" will
            always be returned. Use a dict to exclude fields from
            the result (e.g. projection={'_id': False}).
          - `skip` (optional): the number of documents to omit (from
            the start of the result set) when returning the results
          - `limit` (optional): the maximum number of results to
            return
          - `no_cursor_timeout` (optional): if False (the default), any
            returned cursor is closed by the server after 10 minutes of
            inactivity. If set to True, the returned cursor will never
            time out on the server. Care should be taken to ensure that
            cursors with no_cursor_timeout turned on are properly closed.
          - `cursor_type` (optional): the type of cursor to return. The valid
            options are defined by :class:`~pymongo.cursor.CursorType`:

            - :attr:`~pymongo.cursor.CursorType.NON_TAILABLE` - the result of
              this find call will return a standard cursor over the result set.
            - :attr:`~pymongo.cursor.CursorType.TAILABLE` - the result of this
              find call will be a tailable cursor - 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. For details, see the `tailable cursor documentation
              <http://www.mongodb.org/display/DOCS/Tailable+Cursors>`_.
            - :attr:`~pymongo.cursor.CursorType.TAILABLE_AWAIT` - the result
              of this find call will be a tailable cursor with the await flag
              set. The server 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.
            - :attr:`~pymongo.cursor.CursorType.EXHAUST` - the result of this
              find call will be an exhaust cursor. MongoDB will stream batched
              results to the client without waiting for the client to request
              each batch, reducing latency. See notes on compatibility below.

          - `sort` (optional): a list of (key, direction) pairs
            specifying the sort order for this query. See
            :meth:`~pymongo.cursor.Cursor.sort` for details.
          - `allow_partial_results` (optional): if True, mongos will return
            partial results if some shards are down instead of returning an
            error.
          - `oplog_replay` (optional): If True, set the oplogReplay query
            flag.
          - `batch_size` (optional): Limits the number of documents returned in
            a single batch.
          - `manipulate` (optional): **DEPRECATED** - If True (the default),
            apply any outgoing SON manipulators before returning.
          - `collation` (optional): An instance of
            :class:`~pymongo.collation.Collation`. This option is only supported
            on MongoDB 3.4 and above.
          - `return_key` (optional): If True, return only the index keys in
            each document.
          - `show_record_id` (optional): If True, adds a field ``$recordId`` in
            each document with the storage engine's internal record identifier.
          - `snapshot` (optional): If True, prevents the cursor from returning
            a document more than once because of an intervening write
            operation.
          - `hint` (optional): An index, in the same format as passed to
            :meth:`~pymongo.collection.Collection.create_index` (e.g.
            ``[('field', ASCENDING)]``). Pass this as an alternative to calling
            :meth:`~pymongo.cursor.Cursor.hint` on the cursor to tell Mongo the
            proper index to use for the query.
          - `max_time_ms` (optional): 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. Pass
            this as an alternative to calling
            :meth:`~pymongo.cursor.Cursor.max_time_ms` on the cursor.
          - `max_scan` (optional): The maximum number of documents to scan.
            Pass this as an alternative to calling
            :meth:`~pymongo.cursor.Cursor.max_scan` on the cursor.
          - `min` (optional): A list of field, limit pairs specifying the
            inclusive lower bound for all keys of a specific index in order.
            Pass this as an alternative to calling
            :meth:`~pymongo.cursor.Cursor.min` on the cursor.
          - `max` (optional): A list of field, limit pairs specifying the
            exclusive upper bound for all keys of a specific index in order.
            Pass this as an alternative to calling
            :meth:`~pymongo.cursor.Cursor.max` on the cursor.
          - `comment` (optional): A string or document. Pass this as an
            alternative to calling :meth:`~pymongo.cursor.Cursor.comment` on the
            cursor.
          - `modifiers` (optional): **DEPRECATED** - A dict specifying
            additional MongoDB query modifiers. Use the keyword arguments listed
            above instead.

        .. note:: There are a number of caveats to using
          :attr:`~pymongo.cursor.CursorType.EXHAUST` as cursor_type:

          - The `limit` option can not be used with an exhaust cursor.

          - Exhaust cursors are not supported by mongos and can not be
            used with a sharded cluster.

          - A :class:`~pymongo.cursor.Cursor` instance created with the
            :attr:`~pymongo.cursor.CursorType.EXHAUST` cursor_type requires an
            exclusive :class:`~socket.socket` connection to MongoDB. If the
            :class:`~pymongo.cursor.Cursor` is discarded without being
            completely iterated the underlying :class:`~socket.socket`
            connection will be closed and discarded without being returned to
            the connection pool.

find_one_and_delete

    def find_one_and_delete(self, filter,
                            projection=None, sort=None, **kwargs):
        """Finds a single document and deletes it, returning the document.

          >>> db.test.count({'x': 1})
          2
          >>> db.test.find_one_and_delete({'x': 1})
          {u'x': 1, u'_id': ObjectId('54f4e12bfba5220aa4d6dee8')}
          >>> db.test.count({'x': 1})
          1

        If multiple documents match *filter*, a *sort* can be applied.

          >>> for doc in db.test.find({'x': 1}):
          ...     print(doc)
          ...
          {u'x': 1, u'_id': 0}
          {u'x': 1, u'_id': 1}
          {u'x': 1, u'_id': 2}
          >>> db.test.find_one_and_delete(
          ...     {'x': 1}, sort=[('_id', pymongo.DESCENDING)])
          {u'x': 1, u'_id': 2}

        The *projection* option can be used to limit the fields returned.

          >>> db.test.find_one_and_delete({'x': 1}, projection={'_id': False})
          {u'x': 1}

        :Parameters:
          - `filter`: A query that matches the document to delete.
          - `projection` (optional): a list of field names that should be
            returned in the result document or a mapping specifying the fields
            to include or exclude. If `projection` is a list "_id" will
            always be returned. Use a mapping to exclude fields from
            the result (e.g. projection={'_id': False}).
          - `sort` (optional): a list of (key, direction) pairs
            specifying the sort order for the query. If multiple documents
            match the query, they are sorted and the first is deleted.
          - `**kwargs` (optional): additional command arguments can be passed
            as keyword arguments (for example maxTimeMS can be used with
            recent server versions).

find_one_and_replace

    def find_one_and_replace(self, filter, replacement,
                             projection=None, sort=None, upsert=False,
                             return_document=ReturnDocument.BEFORE, **kwargs):
        """Finds a single document and replaces it, returning either the
        original or the replaced document.

        The :meth:`find_one_and_replace` method differs from
        :meth:`find_one_and_update` by replacing the document matched by
        *filter*, rather than modifying the existing document.

          >>> for doc in db.test.find({}):
          ...     print(doc)
          ...
          {u'x': 1, u'_id': 0}
          {u'x': 1, u'_id': 1}
          {u'x': 1, u'_id': 2}
          >>> db.test.find_one_and_replace({'x': 1}, {'y': 1})
          {u'x': 1, u'_id': 0}
          >>> for doc in db.test.find({}):
          ...     print(doc)
          ...
          {u'y': 1, u'_id': 0}
          {u'x': 1, u'_id': 1}
          {u'x': 1, u'_id': 2}

        :Parameters:
          - `filter`: A query that matches the document to replace.
          - `replacement`: The replacement document.
          - `projection` (optional): A list of field names that should be
            returned in the result document or a mapping specifying the fields
            to include or exclude. If `projection` is a list "_id" will
            always be returned. Use a mapping to exclude fields from
            the result (e.g. projection={'_id': False}).
          - `sort` (optional): a list of (key, direction) pairs
            specifying the sort order for the query. If multiple documents
            match the query, they are sorted and the first is replaced.
          - `upsert` (optional): When ``True``, inserts a new document if no
            document matches the query. Defaults to ``False``.
          - `return_document`: If
            :attr:`ReturnDocument.BEFORE` (the default),
            returns the original document before it was replaced, or ``None``
            if no document matches. If
            :attr:`ReturnDocument.AFTER`, returns the replaced
            or inserted document.
          - `**kwargs` (optional): additional command arguments can be passed
            as keyword arguments (for example maxTimeMS can be used with
            recent server versions).

find_one_and_update

    def find_one_and_update(self, filter, update,
                            projection=None, sort=None, upsert=False,
                            return_document=ReturnDocument.BEFORE, **kwargs):
        """Finds a single document and updates it, returning either the
        original or the updated document.

          >>> db.test.find_one_and_update(
          ...    {'_id': 665}, {'$inc': {'count': 1}, '$set': {'done': True}})
          {u'_id': 665, u'done': False, u'count': 25}}

        By default :meth:`find_one_and_update` returns the original version of
        the document before the update was applied. To return the updated
        version of the document instead, use the *return_document* option.

          >>> from pymongo import ReturnDocument
          >>> db.example.find_one_and_update(
          ...     {'_id': 'userid'},
          ...     {'$inc': {'seq': 1}},
          ...     return_document=ReturnDocument.AFTER)
          {u'_id': u'userid', u'seq': 1}

        You can limit the fields returned with the *projection* option.

          >>> db.example.find_one_and_update(
          ...     {'_id': 'userid'},
          ...     {'$inc': {'seq': 1}},
          ...     projection={'seq': True, '_id': False},
          ...     return_document=ReturnDocument.AFTER)
          {u'seq': 2}

        The *upsert* option can be used to create the document if it doesn't
        already exist.

          >>> db.example.delete_many({}).deleted_count
          1
          >>> db.example.find_one_and_update(
          ...     {'_id': 'userid'},
          ...     {'$inc': {'seq': 1}},
          ...     projection={'seq': True, '_id': False},
          ...     upsert=True,
          ...     return_document=ReturnDocument.AFTER)
          {u'seq': 1}

        If multiple documents match *filter*, a *sort* can be applied.

          >>> for doc in db.test.find({'done': True}):
          ...     print(doc)
          ...
          {u'_id': 665, u'done': True, u'result': {u'count': 26}}
          {u'_id': 701, u'done': True, u'result': {u'count': 17}}
          >>> db.test.find_one_and_update(
          ...     {'done': True},
          ...     {'$set': {'final': True}},
          ...     sort=[('_id', pymongo.DESCENDING)])
          {u'_id': 701, u'done': True, u'result': {u'count': 17}}

        :Parameters:
          - `filter`: A query that matches the document to update.
          - `update`: The update operations to apply.
          - `projection` (optional): A list of field names that should be
            returned in the result document or a mapping specifying the fields
            to include or exclude. If `projection` is a list "_id" will
            always be returned. Use a dict to exclude fields from
            the result (e.g. projection={'_id': False}).
          - `sort` (optional): a list of (key, direction) pairs
            specifying the sort order for the query. If multiple documents
            match the query, they are sorted and the first is updated.
          - `upsert` (optional): When ``True``, inserts a new document if no
            document matches the query. Defaults to ``False``.
          - `return_document`: If
            :attr:`ReturnDocument.BEFORE` (the default),
            returns the original document before it was updated, or ``None``
            if no document matches. If
            :attr:`ReturnDocument.AFTER`, returns the updated
            or inserted document.
          - `**kwargs` (optional): additional command arguments can be passed
            as keyword arguments (for example maxTimeMS can be used with
            recent server versions).

 

3字段的一些操作(3層)

1.增加字段：collection.update({"_id":1},{"$set":{"new_field":0}}) #紅色為查找條件，綠色為新增字段（當document中沒有new_field這個字段時，則新增這個字段）

2.刪除字段：collection.update({"_id":1},{"$unset":{"new_field":1}}) #紅色為查找條件，綠色為刪除字段

3.按條件查找：collection.find_one({"_id":1}) #紅色為查找條件

                           collection.find({"_id":1})
4.統計不含有某一字段的記錄數：dbName.collectionName.find({fieldName:null}).count()

5.求某字段最大值：collection.find().sort({"_id":-1}).limit(1)#得到的記錄為集合中"_id"值最大的那一條