将MongoDB文档映射到具有类型但没有嵌入式文档的案例类 [英] Mapping MongoDB documents to case class with types but without embedded documents
问题描述
子集看起来像是一个有趣的瘦MongoDB包装器.
Subset looks like an interesting, thin MongoDB wrapper.
在给出的示例之一中，有Tweets和Users.但是，User是Tweet的子文档.在经典SQL中，这将被标准化为两个单独的表，并具有从Tweet到User的外键.在MongoDB中，这不需要DBRef，存储用户的ObjectId就足够了.
In one of the examples given, there are Tweets and Users. However, User is a subdocument of Tweet. In classical SQL, this would be normalized into two separate tables with a foreign key from Tweet to User. In MongoDB, this wouldn't necessitate a DBRef, storing the user's ObjectId would be sufficient.
在Subset和Salat中，这都将导致以下案例类:
Both in Subset and Salat this would result in these case classes:
case class Tweet(_id: ObjectId, content: String, userId: ObjectId)
case class User(_id: ObjectId, name: String)
因此，不能保证Tweet中的ObjectId实际上解析为User(使它的类型安全性降低).我还必须为每个引用User的类编写相同的查询(或将其移至某些特征).
So there's no guarantee that the ObjectId in Tweet actually resolves to a User (making it less typesafe). I also have to write the same query for each class that references User (or move it to some trait).
所以我想实现的是在代码中包含case class Tweet(_id: ObjectId, content: String, userId: User)，在数据库中包含ObjectId.这可能吗?如果可以，怎么办?有什么好的选择?
So what I'd like to achieve is to have case class Tweet(_id: ObjectId, content: String, userId: User), in code, and the ObjectId in the database. Is this possible, and if so, how? What are good alternatives?
推荐答案
是的，有可能.实际上，它甚至比在"tweet"中包含"user"子文档还要简单.当用户"是一个引用时，它只是一个标量值，MongoDB和子集"没有查询子文档字段的机制.
Yes, it's possible. Actually it's even simpler than having a "user" sub-document in a "tweet". When "user" is a reference, it is just a scalar value, MongoDB and "Subset" has no mechanisms to query subdocument fields.
我为您准备了一个简单的可替换代码段(假定您有两个集合-"tweets"和"users").
I've prepared a simple REPLable snippet of code for you (it assumes you have two collections -- "tweets" and "users").
准备...
import org.bson.types.ObjectId
import com.mongodb._
import com.osinka.subset._
import Document.DocumentId
val db = new Mongo("localhost") getDB "test"
val tweets = db getCollection "tweets"
val users = db getCollection "users"
我们的User案例类
case class User(_id: ObjectId, name: String)
tweets和用户的许多字段
A number of fields for tweets and user
val content = "content".fieldOf[String]
val user = "user".fieldOf[User]
val name = "name".fieldOf[String]
更复杂的事情开始发生.我们需要的是能够根据字段名称获取ObjectId的ValueReader，然后转到另一个集合并从那里读取对象.
Here more complicated things start to happen. What we need is a ValueReader that's capable of getting ObjectId based on field name, but then goes to another collection and reads an object from there.
这可以写成一个单一的代码，一次完成所有事情(您可能会在回答历史中看到这样的变体)，但是将其表达为读者的组合会更加习惯.假设我们有一个ValueReader[User]从DBObject读取:
This can be written as a single piece of code, that does all things at once (you may see such a variant in the answer history), but it would be more idiomatic to express it as a combination of readers. Suppose we have a ValueReader[User] that reads from DBObject:
val userFromDBObject = ValueReader({
case DocumentId(id) ~ name(name) => User(id, name)
})
剩下的是通用ValueReader[T]，它期望ObjectId并使用提供的底层阅读器从特定集合中检索对象:
What's left is a generic ValueReader[T] that expects ObjectId and retrieves an object from a specific collection using supplied underlying reader:
class RefReader[T](val collection: DBCollection, val underlying: ValueReader[T]) extends ValueReader[T] {
override def unpack(o: Any):Option[T] =
o match {
case id: ObjectId =>
Option(collection findOne id) flatMap {underlying.unpack _}
case _ =>
None
}
}
然后，我们可以说我们从引用中读取User的类型类仅仅是
Then, we may say our type class for reading Users from references is merely
implicit val userReader = new RefReader[User](users, userFromDBObject)
(对于这个问题，我深表感谢，因为这个用例非常 很少见，我没有开发通用解决方案的真正动力.我认为 最后，我需要将这种帮助程序包括在子集"中. 非常感谢您对这种方法的反馈)
(I am grateful to you for this question, since this use case is quite rare and I had no real motivation to develop a generic solution. I think I need to include this kind of helper into "Subset" finally.. I shall appreciate your feedback on this approach)
这是您将如何使用它:
And this is how you would use it:
import collection.JavaConverters._
tweets.find.iterator.asScala foreach {
case Document.DocumentId(id) ~ content(content) ~ user(u) =>
println("%s - %s by %s".format(id, content, u))
}
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