Symfony中Doctrine对应的Mongodb数据类型 data type
1. hash 就是 json对象2. collection 就是 数组
3. 若要知道如何使用referenceOne, referenceMany, embbedDocument等
主要查看：
http://doctrine-mongodb-odm.readthedocs.org/en/latest/reference/reference-mapping.html
http://docs.doctrine-project.org/projects/doctrine-mongodb-odm/en/latest/reference/embedded-mapping.html#embed-many
http://docs.doctrine-project.org/projects/doctrine-mongodb-odm/en/latest/reference/basic-mapping.html#doctrine-mapping-types
以上部分主要看yaml部分，就会知道如何编写 yml 了。
http://doctrine-mongodb-odm.readthedocs.org/en/latest/reference/basic-mapping.html
HasLifecycleCallbacks事件：
http://doctrine-mongodb-odm.readthedocs.org/en/latest/reference/annotations-reference.html
了解更多：
http://us.php.net/manual/zh/class.mongodbref.php
比较典型的yml：
Documents\User:
fields:
id:
type: id
id: true
name:
type: string
email:
type: string
referenceMany:
posts:
targetDocument: Documents\BlogPost
cascade: all
Documents\BlogPost:
fields:
id:
type: id
id: true
title:
type: string
body:
type: string
createdAt:
type: date
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