Scala 用函数式编程的方法调用db4o

db4o 是一个开源的纯面向对象数据库，同时支持Java和dotNet。使用db4o可以用更加面向对象的方式存储数据，比JPA等通过ORM来协调数据库和对象之间的方案来的更为自然。

那么用函数式编程的眼光来看，应该如何使用db4o这样的库呢？首先我们来看一个db4o的包装类

xml 代码

package plan.util;

import com.db4o.query.Query

abstract class LazyQuery[QInterface, T]  (val criteria: List[Query=>Query]) extends Iterable[T] {

  def elements = {
    val init: Query = null
    // 等价于 criteria leftFold init ( query=>query(query=>query) )
    val query = (criteria :\ init)((f, q)=>f(q))
    Db4oUtil.iterate[T](query)
  }

  def factory: List[Query=>Query]=>QInterface

  def constrain(c: Query=>Query) = factory(c :: criteria)
}

object Db4oUtil {
  def iterate[T](q: Query) = {
    val set = q.execute()
    new Iterator[T] {
      def hasNext = set.hasNext
      def next = set.next.asInstanceOf[T]
    }
  }
}

看一个实际的例子，People和AutoInfo是POJO，People有一个指向AutoInfo的List。通过继承LazyQuery的Peoples可以很容易做一些查询、filter、以及复杂的操作。由于是functional的（所有的constrain都是query=>query的函数），所以各个查询条件可以互相组合，非常方便

xml 代码

package plan;

import plan.util.Db4oUtil
import plan.util.LazyQuery

import com.db4o.Db4o
import com.db4o.ObjectContainer
import com.db4o.query.Query

import scala.reflect.BeanProperty

class People {
  @BeanProperty
  var id: int =_

  @BeanProperty
  var name: String =_

  @BeanProperty
  var address: String =_

  @BeanProperty
  var autoInfoList: java.util.List = new java.util.ArrayList

  def addAutoInfo(autoInfo: AutoInfo) = {
    autoInfoList.add(autoInfo)
  }

  def autoCount = autoInfoList size
}

class AutoInfo {
  @BeanProperty
  var id: int = _

  @BeanProperty
  var name: String = _

  @BeanProperty
  var owner: People  = _
}

class Peoples(criteria: List[Query=>Query]) extends LazyQuery[Peoples, People](criteria) {

  override def factory = new Peoples(_)

  def withAuto() = {
    constrain { q=>
        q.descend("autoInfoList")
        q
    }
  }

  def totalAuto = (this.map(_.autoCount) :\ 0)( (f,g) => f + g )
}

object Db4oTest {
  var db: ObjectContainer = _
  def initData() = {
    val peo = new People
    peo.address = "北京"
    peo.name = "Chris"
    var auInfo = new AutoInfo
    auInfo.name = "雪铁龙"
    auInfo.owner = peo
    peo.addAutoInfo(auInfo)
    db.set(peo)

    val peo1 = new People
    peo1.address = "上海"
    peo1.name = "Louis"
    db.set(peo1)
  }
  private def query(klass: Class) = {
    val q = db.query()
    q.constrain(klass)
    q
  }
  def main(args: Array[String]) = {
    db = Db4o.openFile("auto.yap")
    try {
      //initData
      val peoples = new Peoples(List(q=>query(classOf[People])))

      // get total car all people have
      println( peoples.totalAuto )

      // lazy query all people
      for (peo <- peoples.elements) {
        println( peo.name )
      }
      // lazy query all people with autoinfo
      for (peo <- peoples.withAuto.elements) {
        println( peo.name )
      }
    }finally{
      db.close();
    }
  }
}