Does Appian 16.2 support @ElementCollection?

Something thats done in JAVA will look like -

@Entity
@Table(name="college")
public class College implements Serializable {

@ElementCollection
@CollectionTable(name="student", joinColumns=@JoinColumn(name="college_id"))
@Column(name="student_name")
private Set<String> students;
          
}

Perhaps I misunderstand what you are looking for but, can you not accomplish (almost) the same thing via nested CDTs?

Take a look at this Query Recipe: forum.appian.com/.../Query_Recipes.html
I'm not sure if you have a more complex use case, but for your example I believe you could simply query the "student" table and filter on "college_id", then use the dot properties as mentioned in the query recipe to return just the IDs.

This is an api we are trying to optimize. The list of id's are for nested child cdt's. We have a list of parents being returned by api and each parent has multiple children. Instead of sending the whole cdt I just want to send ids with each parent.

You can create a small helper expression that extracts the IDs of the children from a single parent and appends it to a rule input of type integer?list. Then use the reduce() function to build a list of IDs while iterating on the list of parents. Did this before, works like a charm.