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sudo rm /var/lib/mongodb/mongod.lock 
sudo -u mongodb mongod -f /etc/mongodb.conf --repair
sudo start mongodb

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> db.help()
DB methods:
	db.addUser(username, password[, readOnly=false])
	db.auth(username, password)
	db.cloneDatabase(fromhost)
	db.commandHelp(name) returns the help for the command
	db.copyDatabase(fromdb, todb, fromhost)
	db.createCollection(name, { size : ..., capped : ..., max : ... } )
	db.currentOp() displays the current operation in the db
	db.dropDatabase()

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> db.users.help()
DBCollection help
	db.users.find().help() - show DBCursor help
	db.users.count()
	db.users.dataSize()
	db.users.distinct( key ) - eg. db.users.distinct( 'x' )
	db.users.drop() drop the collection
	db.users.dropIndex(name)
	db.users.dropIndexes()
	db.users.ensureIndex(keypattern[,options]) - options is an object with these possible fields: name, unique, dropDups

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> db.system.js.save({_id:"last_n_users", value: function(n){return db.users.find().sort({created_at:-1}).limit(n).toArray()}})
> db.eval("last_n_users(3)")
[
	{
		"_id" : ObjectId("4e8c2a00de570598c220878b"),
		"username" : "Zoloo",
		"created_at" : ISODate("2011-10-04T16:00:00Z"),
		"password" : "secret",
		"last_visited_at" : ISODate("2011-10-04T16:00:00Z")
	},

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# Өгөгдөл 1
овог: Болд
нэр: Отгонхүү
хүйс: эрэгтэй
хаяг: БГД 2-5-4 тоот


# Өгөгдөл 2
нэр: Номин
хүйс: эмэгтэй

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private native void nativeMe()
{
   // Энд доод түвшний кодоо бичээрэй.
}

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# Hooks
unless Array::unique
  Array::unique = ->
    output = {}
    output[@[key]] = @[key] for key in [0...@length]
    value for key, value of output

unless Array::contains_array
  Array::contains_array = (arr)->
    @.every (val)->

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var Ability, ability, as_array, is_array, is_array_contains_array;

if (!Array.prototype.unique) {
  Array.prototype.unique = function() {
    var key, output, value, _i, _ref, _results;
    output = {};
    for (key = _i = 0, _ref = this.length; 0 <= _ref ? _i < _ref : _i > _ref; key = 0 <= _ref ? ++_i : --_i) {
      output[this[key]] = this[key];
    }
    _results = [];

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За тэгвэл чадлынхаа хэрээр тайлбарлах гээд үзье: Ангилалуудаа categories гэсэн хүснэгтэнд хадгальяа, 
тэр хүснэгт нь id, name, ancestry гэсэн талбаруудтай. 
Хамгийн эхний өгөгдөл буюу A-г хийхэд дараах байдалтай болно. 

id|name|ancestry 
1|A|null

Өөрөөр хэлбэл A нь root болно. Одоо B-г нэмвэл:

id|name|ancestry 

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# encoding: UTF-8
# This file is auto-generated from the current state of the database. Instead
# of editing this file, please use the migrations feature of Active Record to
# incrementally modify your database, and then regenerate this schema definition.
#
# Note that this schema.rb definition is the authoritative source for your
# database schema. If you need to create the application database on another
# system, you should be using db:schema:load, not running all the migrations
# from scratch. The latter is a flawed and unsustainable approach (the more migrations
# you'll amass, the slower it'll run and the greater likelihood for issues).