Doctrine: ODM vs. ORM [closed]

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I consider using ODM/ORM/mixing ODM and ORM entities in my new Doctrine 2.0 project. When should 开发者_运维问答I use ORM (MySQL InnoDB) and when ODM (MongoDB)? Should I mix it in one project?

Pseudo structure of my app (Google Reader like):

• user { id, login, password, email }
• channel { id, url }
• entry { id, channelId, title, body, date }
• entryValue { entryId, userId, value }
• userFriend { user1Id, user2Id }

Most popular functions:

• getValuesOfNewsByUsers(newsId, [userIdArray])
• getEntriesByUserWithMinValue(userId, minValue)
• areFriends(user1, user2)
• getEntryValues(userId, [entryIdArray])

I should be prepared for billions of rows in entry and entryValue tables.