> help                                               关于mongodb主要方法的帮助
    db.help()                    关于数据库方法的帮助
    db.mycoll.help()             关于数据库表方法的帮助
    sh.help()                    分片方法的帮助
    rs.help()                    副本集方法的帮助
    help admin                   行政帮助
    help connect                 连接到数据库的帮助
    help keys                    键的快捷方式
    help misc                    misc things to know
    help mr                      mapreduce（一种分布式并行编程模型）

    show dbs                     展示所有数据库的名称
    show collections             展示当前数据库的所有集合（表）名称
    show users                   展示当前数据库的所有用户
    show profile                 显示时间为>= 1ms的最近的system.profile条目
    show logs                    显示可访问的log名称
    show log [name]              打印出内存中日志的最后一段，global是默认值
    use <db_name>                设置当前数据库
    db.mycoll.find()             查询集合mycoll中所有的记录
    db.mycoll.find( { a : 1 } )  查询集合mycoll中的记录，只查询满足条件a=1的记录
    it                           最后一行求值的结果;用于进一步迭代（目前不知道在哪使用）
    DBQuery.shellBatchSize = x   设置shell上显示的项目的默认数量（目前不知道在哪使用）
    exit                         退出mongo shell

> db.help()
DB methods:
    db.adminCommand(nameOrDocument) - 切换到'admin' db，并运行命令 [只调用 db.runCommand(...)]（目前不知道在哪使用）
    db.aggregate([pipeline], {options}) - 对该数据库执行无集合聚合;返回一个指针（目前不知道在哪使用）
    db.auth(username, password)（目前不知道在哪使用）
    db.cloneDatabase(fromhost) - will only function with MongoDB 4.0 and below
    db.commandHelp(name) returns the help for the command
    db.copyDatabase(fromdb, todb, fromhost) - will only function with MongoDB 4.0 and below
    db.createCollection(name, {size: ..., capped: ..., max: ...})
    db.createUser(userDocument)
    db.createView(name, viewOn, [{$operator: {...}}, ...], {viewOptions})
    db.currentOp() displays currently executing operations in the db
    db.dropDatabase(writeConcern)
    db.dropUser(username)
    db.eval() - deprecated
    db.fsyncLock() flush data to disk and lock server for backups
    db.fsyncUnlock() unlocks server following a db.fsyncLock()
    db.getCollection(cname) same as db['cname'] or db.cname
    db.getCollectionInfos([filter]) - returns a list that contains the names and options of the db's collections
    db.getCollectionNames()
    db.getLastError() - just returns the err msg string
    db.getLastErrorObj() - return full status object
    db.getLogComponents()
    db.getMongo() get the server connection object
    db.getMongo().setSlaveOk() allow queries on a replication slave server
    db.getName()
    db.getProfilingLevel() - deprecated
    db.getProfilingStatus() - returns if profiling is on and slow threshold
    db.getReplicationInfo()
    db.getSiblingDB(name) get the db at the same server as this one
    db.getWriteConcern() - returns the write concern used for any operations on this db, inherited from server object if set
    db.hostInfo() get details about the server's host
    db.isMaster() check replica primary status
    db.killOp(opid) kills the current operation in the db
    db.listCommands() lists all the db commands
    db.loadServerScripts() loads all the scripts in db.system.js
    db.logout()
    db.printCollectionStats()
    db.printReplicationInfo()
    db.printShardingStatus()
    db.printSlaveReplicationInfo()
    db.resetError()
    db.runCommand(cmdObj) run a database command.  if cmdObj is a string, turns it into {cmdObj: 1}
    db.serverStatus()
    db.setLogLevel(level,<component>)
    db.setProfilingLevel(level,slowms) 0=off 1=slow 2=all
    db.setVerboseShell(flag) display extra information in shell output
    db.setWriteConcern(<write concern doc>) - sets the write concern for writes to the db
    db.shutdownServer()
    db.stats()
    db.unsetWriteConcern(<write concern doc>) - unsets the write concern for writes to the db
    db.version() current version of the server
    db.watch() - opens a change stream cursor for a database to report on all  changes to its non-system collections.

> db.demo.help()
DBCollection help
    db.demo.find().help() - 展示 DBCursor 方法帮助
    db.demo.bulkWrite( operations, <optional params> ) - bulk execute write operations, optional parameters are: w, wtimeout, j
    db.demo.count( query = {}, <optional params> ) - 计算匹配查询的文档数量，可选参数有:limit、skip、hint、maxTimeMS
    db.demo.countDocuments( query = {}, <optional params> ) - count the number of documents that matches the query, optional parameters are: limit, skip, hint, maxTimeMS
    db.demo.estimatedDocumentCount( <optional params> ) - estimate the document count using collection metadata, optional parameters are: maxTimeMS
    db.demo.convertToCapped(maxBytes) - calls {convertToCapped:'demo', size:maxBytes}} command
    db.demo.createIndex(keypattern[,options])
    db.demo.createIndexes([keypatterns], <options>)
    db.demo.dataSize()
    db.demo.deleteOne( filter, <optional params> ) - delete first matching document, optional parameters are: w, wtimeout, j
    db.demo.deleteMany( filter, <optional params> ) - delete all matching documents, optional parameters are: w, wtimeout, j
    db.demo.distinct( key, query, <optional params> ) - e.g. db.demo.distinct( 'x' ), optional parameters are: maxTimeMS
    db.demo.drop() drop the collection
    db.demo.dropIndex(index) - e.g. db.demo.dropIndex( "indexName" ) or db.demo.dropIndex( { "indexKey" : 1 } )
    db.demo.hideIndex(index) - e.g. db.demo.hideIndex( "indexName" ) or db.demo.hideIndex( { "indexKey" : 1 } )
    db.demo.unhideIndex(index) - e.g. db.demo.unhideIndex( "indexName" ) or db.demo.unhideIndex( { "indexKey" : 1 } )
    db.demo.dropIndexes()
    db.demo.ensureIndex(keypattern[,options]) - DEPRECATED, use createIndex() instead
    db.demo.explain().help() - show explain help
    db.demo.reIndex()
    db.demo.find([query],[fields]) - query is an optional query filter. fields is optional set of fields to return.
                                                  e.g. db.demo.find( {x:77} , {name:1, x:1} )
    db.demo.find(...).count()
    db.demo.find(...).limit(n)
    db.demo.find(...).skip(n)
    db.demo.find(...).sort(...)
    db.demo.findOne([query], [fields], [options], [readConcern])
    db.demo.findOneAndDelete( filter, <optional params> ) - delete first matching document, optional parameters are: projection, sort, maxTimeMS
    db.demo.findOneAndReplace( filter, replacement, <optional params> ) - replace first matching document, optional parameters are: projection, sort, maxTimeMS, upsert, returnNewDocument
    db.demo.findOneAndUpdate( filter, <update object or pipeline>, <optional params> ) - update first matching document, optional parameters are: projection, sort, maxTimeMS, upsert, returnNewDocument
    db.demo.getDB() get DB object associated with collection
    db.demo.getPlanCache() get query plan cache associated with collection
    db.demo.getIndexes()
    db.demo.insert(obj)
    db.demo.insertOne( obj, <optional params> ) - insert a document, optional parameters are: w, wtimeout, j
    db.demo.insertMany( [objects], <optional params> ) - insert multiple documents, optional parameters are: w, wtimeout, j
    db.demo.mapReduce( mapFunction , reduceFunction , <optional params> )
    db.demo.aggregate( [pipeline], <optional params> ) - performs an aggregation on a collection; returns a cursor
    db.demo.remove(query)
    db.demo.replaceOne( filter, replacement, <optional params> ) - replace the first matching document, optional parameters are: upsert, w, wtimeout, j
    db.demo.renameCollection( newName , <dropTarget> ) renames the collection.
    db.demo.runCommand( name , <options> ) runs a db command with the given name where the first param is the collection name
    db.demo.save(obj)
    db.demo.stats({scale: N, indexDetails: true/false, indexDetailsKey: <index key>, indexDetailsName: <index name>})
    db.demo.storageSize() - includes free space allocated to this collection
    db.demo.totalIndexSize() - size in bytes of all the indexes
    db.demo.totalSize() - storage allocated for all data and indexes
    db.demo.update( query, <update object or pipeline>[, upsert_bool, multi_bool] ) - instead of two flags, you can pass an object with fields: upsert, multi, hint
    db.demo.updateOne( filter, <update object or pipeline>, <optional params> ) - update the first matching document, optional parameters are: upsert, w, wtimeout, j, hint
    db.demo.updateMany( filter, <update object or pipeline>, <optional params> ) - update all matching documents, optional parameters are: upsert, w, wtimeout, j, hint
    db.demo.validate( <full> ) - SLOW
    db.demo.getShardVersion() - only for use with sharding
    db.demo.getShardDistribution() - prints statistics about data distribution in the cluster
    db.demo.getSplitKeysForChunks( <maxChunkSize> ) - calculates split points over all chunks and returns splitter function
    db.demo.getWriteConcern() - returns the write concern used for any operations on this collection, inherited from server/db if set
    db.demo.setWriteConcern( <write concern doc> ) - sets the write concern for writes to the collection
    db.demo.unsetWriteConcern( <write concern doc> ) - unsets the write concern for writes to the collection
    db.demo.latencyStats() - display operation latency histograms for this collection

> help keys
Tab completion and command history is available at the command prompt.
Some emacs keystrokes are available too:
  Ctrl-A start of line
  Ctrl-E end of line
  Ctrl-K del to end of line

Multi-line commands
You can enter a multi line javascript expression.  If parens, braces, etc. are not closed, you will see a new line
beginning with '...' characters.  Type the rest of your expression.  Press Ctrl-C to abort the data entry if you
get stuck.

> db.HouseHold.find().help()
find(<predicate>, <projection>) modifiers
    .sort({...})
    .limit(<n>)
    .skip(<n>)
    .batchSize(<n>) - sets the number of docs to return per getMore
    .collation({...})
    .hint({...})
    .readConcern(<level>)
    .readPref(<mode>, <tagset>)
    .count(<applySkipLimit>) - total # of objects matching query. by default ignores skip,limit
    .size() - total # of objects cursor would return, honors skip,limit
    .explain(<verbosity>) - accepted verbosities are {'queryPlanner', 'executionStats', 'allPlansExecution'}
    .min({...})
    .max({...})
    .maxTimeMS(<n>)
    .comment(<comment>)
    .tailable(<isAwaitData>)
    .noCursorTimeout()
    .allowPartialResults()
    .returnKey()
    .showRecordId() - adds a $recordId field to each returned object
    .allowDiskUse() - allow using disk in completing the query

Cursor methods
    .toArray() - iterates through docs and returns an array of the results
    .forEach(<func>)
    .map(<func>)
    .hasNext()
    .next()
    .close()
    .objsLeftInBatch() - returns count of docs left in current batch (when exhausted, a new getMore will be issued)
    .itcount() - iterates through documents and counts them
    .pretty() - pretty print each document, possibly over multiple lines