merge 2 lists A over B in scala - list

Question

I have 2 immutable case classes A(source, key, value) and B(source, key, value)
I want to add A over B in such a way when 'source' and 'key' doesn't exist, to be added from A to the B and when 'source' and 'key' exist to replace the value from B with the one from A. The same way 'merge_array' function from php works on a multidimensional array.
I tried with 'A.union(B).groupBy(.key)' and then 'groupBy(.source)' and get the 1st value. But then I realized that I can never be sure that first value will always be the value of A.
I'm quite new to scala and I really ran out of ideas how I could do this from a functional immutable point of view.
Anyone has any idea how I could do this?
Thank you
Edit:
case class TranslationValue(source: String, key: String, value: String)
def main(args:Array[String]):Unit = {
println(merge(data1.toSet, data2.toSet))
}
def merge(a: Set[TranslationValue], b: Set[TranslationValue]) = {
a.union(b).groupBy(_.key).flatMap{ case (s, v) =>
v.groupBy(_.source).flatMap{case (s1, v1) => {
for (res <- 0 to 0) yield v1.head
}
}
}
}
Example
data1 has this data
Set(
TranslationValue(messages,No,No),
TranslationValue(messages,OrdRef,Order Reference),
TranslationValue(messages,OrdId,Order Id)
)
data2 has this data
Set(
TranslationValue(messages,No,No),
TranslationValue(messages,OrdRef,OrderRef)
TranslationValue(messages,Name,Name)
)
putting data1 over data2 I want to obtain
List(
TranslationValue(messages,No,No),
TranslationValue(messages,OrdRef,Order Reference),
TranslationValue(messages,OrdId,Order Id)
TranslationValue(messages,Name,Name)
)
I know that what I do can be done better, but like I said, I'm learning :)

Answer 1

you can group in one go:
def merge(a: Seq[TranslationValue], b: Seq[TranslationValue]) = {
a.union(b).groupBy(t=>(t.key,t.source)).map(c=>c._2.head)
}
i think you could also override the equals method for TranslationValue so that two translation values are equal when source and key are the same(the hashcode method has also to be overridden). Then a.union(b) would be enough.
edit:
It seems Set doesnt guarantee order of items(Scala: Can I rely on the order of items in a Set?), but a seq should.