Using distinct in Spring data over multiple columns - spring

Question

My domain model is like this:
CollectedData {
String name;
String description;
int count;
int xAxis,
int yAxis
}
Using Spring data repository query, I would like to retrieve all the unique rows (unique with name, xAxis, yAxis)
I am trying something like this
#Query("select distinct a.name, a.xAxis, a.yAxis from CollectedData a")
List<CollectedData> findAllDistinctData();
So, when I do
List<CollectedData> records= findAllDistinctData();
for (CollectedData record : records) { //Exception on this line
}
Exception
[Ljava.lang.Object; cannot be cast to CollectedData.
Is there any other way to write query for this ?

Answer 1

#Query return ArrayList of Object(s) instead of specific type of object. so you have to define some thing like
#Query("select distinct a.name, a.xAxis, a.yAxis from CollectedData a")
List<Object> findAllDistinctData();
then cast according to your requirement,
List<Object> cdataList=findAllDistinctData();
for (Object cdata:cdataList) {
Object[] obj= (Object[]) cdata;
String name = (String)obj[0];
String description = (String)obj[1];;
...
}

Answer 2

Instead of returning an object you can use JPA's constructor expression feature to return a more specific object holding only the columns you're interested in. See also following answer:
JPQL Constructor Expression - org.hibernate.hql.ast.QuerySyntaxException:Table is not mapped
According to your example you could create a new Object with only the columns you are interested in:
SELECT DISTINCT new com.mypackage.MyInterestingCollectedData(a.name, a.xAxis, a.yAxis) from CollectedData a

Answer 3

If you want to select complete object based on distinct values of multiple columns,
In that case the native query would be the option.
e.g.
#Query(
value = "select distinct on (column1, column2, column3) * From my_table where someId=: order by column1 asc,column2 desc,column3 desc,column4 desc",
nativeQuery = true
)
fun finalAllDistinctBy(containerId: String): List<MyTable>