Two Seq Comparison in Scala

Question

Here is a scenario of determining updated data in a data update. A Seq of case class objects have their IDs and the case class has an override equals method which makes a data comparison of its attributes excluding the ID field. To find out any updated data entries, I need to retrieve the data from DB. And a comparison of two sequences is needed. What is a Scala approach to find out any objects which are updated?

Just having a thought: creating a map with the object ID as the key and the object as its value. That might work.

(Update) Here is a solution I come out

val existingDataList: Foo = ...
val existingDataMap: Map[Long, Foo] = existingDataList.map(d => d.id -> d)(collection.breakOut)

// To find out updated data    
val updatedData = inputData.filter(d => existingDataMap.get(d.id) != d)

Answer 1

If I understand you right, you've already done the most of the hard work by overriding equals - and of course you MUST also override hashCode correspondingly e.g.:

case class Thing(id:Long, foo:String, bar:Int) {
  override def equals(that:Any):Boolean = {
    that match {
      case Thing(_, f, b) => (f == this.foo) && (b == this.bar)
      case _ => false
    }
  }

  override def hashCode:Int = {
    // A quick hack. You probably shouldn't do this for real; 
    // set id to 0 and use the default hashing algorithm:
    ScalaRunTime._hashCode(this.copy(id = 0))
  }
}

Now we define a few Thing instances:

val t1 = Thing(111, "t1", 1)
val t1c = Thing(112, "t1", 1) // Same as t1 but with a new ID
val t2 = Thing(222, "t2", 2)
val t3 = Thing(333, "t3", 3)
val t4 = Thing(444, "t4", 4)
val t4m = Thing(444, "t4m", 4)  // Same as t4 but with a modified "foo"

Let's make a couple of sequences:

val orig = Seq(t1, t2, t3, t4)
val mod = Seq(t1c, t2, t3, t4m)

And now a diff tells us everything we need to know:

mod.diff(orig)
// => returns Seq(t4m) - just what we wanted