EDIT: Rewrote the question. Added bounty as its important for me. The final hint with which i can get findByAttributes to work (without reimplementing it in subclasses) will get my points.
In my app i'm doing typesafe database queries with the new JPA2 Criteria Query. Therefore I have a trait DAO which should be (re)usable for ALL entities in my application. So this is how the outline the current trait i'm using looks like (which works):
trait DAO[T, K](implicit m: Manifest[T]) {
@PersistenceContext
var em:EntityManager = _
lazy val cb:CriteriaBuilder = em.getCriteriaBuilder
def persist(entity: T)
def update(entity: T)
def remove(entity: T)
def findAll(): ArrayList[T]
// Pair of SingularAttribute and corresponding value
// (used for queries for multiple attributes)
type AttributeValuePair[A] = Pair[SingularAttribute[T, A], A]
// Query for entities where given attribute has given value
def findByAttribute[A](attribute:AttributeValuePair[A]):ArrayList[T]
// Query for entities with multiple attributes (like query by example)
def findByAttributes[A](attributes:AttributeValuePair[_]*):ArrayList[T]
}
In a concrete DAO, i'm extending this trait like this, setting the type and implementing the methods (removed all except the most important method):
class UserDAO extends DAO[User, Long] {
override type AttributeValuePair[T] = Pair[SingularAttribute[User, T], T]
override def findByAttributes[T](attributes:AttributeValuePair[_]*):ArrayList[User] = {
val cq = cb.createQuery(classOf[User])
val queryRoot = cq.from(classOf[User])
var criteria = cb.conjunction
for (pair <- attributes)
criteria = cb.and(cb.equal(queryRoot.get(pair._1), pair._2 ))
cq.where(Seq(criteria):_*)
val results = em.createQuery(cq).getResultList
results.asInstanceOf[ArrayList[User]]
}
}
BTW, findByAttributes is really nice to use. Example:
val userList = userEJB.findByAttributes(
User_.title -> Title.MR,
User_.email -> "[email protected]"
)
I realized, that findByAttributes is so generic, that its the same in ALL classes of my app that implement the DAO. The only thing that changes is the Type used in the method. So in another class wich inherits DAO, its
def findByAttributes[T](attributes:AttributeValuePair[_]*):ArrayList[Message] = {
val cq = cb.createQuery(classOf[Message])
val queryRoot = cq.from(classOf[Message])
var criteria = cb.conjunction
for (pair <- attributes)
criteria = cb.and(cb.equal(queryRoot.get(pair._1), pair._2 ))
cq.where(Seq(criteria):_*)
val results = em.createQuery(cq).getResultList
results.asInstanceOf[ArrayList[User]]
}
So i created a new abstract class named SuperDAO that should contain the implemented generic methods, so that i don't have to re-implement them in every subclass. After some help from Landei (thanks), the (most important part of my) current implementation of my SuperDAO looks like this
abstract class SuperDAO[T, K](implicit m: Manifest[T]) {
@PersistenceContext
var em:EntityManager = _
lazy val cb:CriteriaBuilder = em.getCriteriaBuilder
type AttributeValuePair[A] = Pair[SingularAttribute[T, A], A]
def findByAttributes(attributes:AttributeValuePair[_]*):ArrayList[T] = {
val cq = cb.createQuery(m.erasure)
val queryRoot = cq.from(m.erasure)
var criteria = cb.conjunction
for (pair <- attributes) {
criteria = cb.and(
cb.equal(
// gives compiler error
queryRoot.get[SingularAttribute[T,_]](pair._1)
)
,pair._2
)
}
cq.where(Seq(criteria):_*)
val results = em.createQuery(cq).getResultList
results.asInstanceOf[ArrayList[T]]
}
So the current problem is that the line with queryRoot.get produces the following error:
overloaded method value get with alternatives:
(java.lang.String)javax.persistence.criteria.Path
[javax.persistence.metamodel.SingularAttribute[T, _]] <and>
(javax.persistence.metamodel.SingularAttribute[_ >: Any,
javax.persistence.metamodel.SingularAttribute[T,_]])
javax.persistence.criteria.Path
[javax.persistence.metamodel.SingularAttribute[T, _]]
cannot be applied to
(javax.persistence.metamodel.SingularAttribute[T,_$1])
Whats meant with $1???
If needed: SingularAttribute Javadoc
EDIT @Landei:
Changing the method signature to
def findByAttributesOld[A](attributes:AttributeValuePair[A]*):ArrayList[T] = {
And the queryRoot.get to
queryRoot.get[A](pair._1.asInstanceOf[SingularAttribute[T,A]])
Results in the (much shorter!) error:
overloaded method value get with alternatives:
(java.lang.String)javax.persistence.criteria.Path[A] <and>
(javax.persistence.metamodel.SingularAttribute[_ >: Any, A])
javax.persistence.criteria.Path[A] cannot be applied to
(javax.persistence.metamodel.SingularAttribute[T,A])
The solution of @Sandor Murakozi seems to work. Have to test it a bit. But i would also appreciate a shorter solution, if its possible at all!
def fBA(...): SomeType = { ... }you're defining a method, not a a function. There are many ways to get functions in Scala. E.g., partial application, which can be used to lift a method to a corresponding function:def mFBA(...): ... = { ... }; val fFBA = mFBA _. – Ric