efficient way of transforming a dataframe to map in scala

Question

I have a use case where I want to convert the dataframe to a map. For that i am using groupbykey and mapGroups operations. But they are running into memory issues.

Eg:

EmployeeDataFrame

EmployeeId, JobLevel, JobCode
1, 4, 50
1, 5, 60
2, 4, 70
2, 5, 80
3, 7, 90

case class EmployeeModel(EmployeeId: String ,JobLevel: String, JobCode: String)

val ds : Dataset[EmployeeModel] = EmployeeDataFrame.as[EmployeeModel]
    val groupedData = ds
      .groupByKey(_.EmployeeId)
      .mapGroups((key, rows) => (key, rows.toList))
      .collect()
      .toMap

Expected Map

1, [(),()]
2, [(), ()]
3, [()]

Is there a better way of doing this?