Question

Let’s say I have a table called Person with the columns:

id, name, parent_id

and let’s say I have some data like this:

1, Bob, null
2, Mary, 1
3, Tim, 1
4, Sally, 3

So Bob has 2 kids: Mary and Tim.
And Tim has 1 kid: Sally (who grandparent is Bob)

What is the easiest way to write a JPA query such that I could find all descendants of Bob? (i.e. the result would return Mary, Tim and Sally)

Answer 1

I don’t think you can do this with plain JPA.

But if you can modify your table like this

id, name, parent_path(string)

and your data will look like this

1, Bob, null
2, Mary, 1
3, Tim, 1
4, Sally, 1/3
5, John, 1/3/4
6, Huge, 1/3/4/5

then you can query for all the descendants using like clause.
For example:

select p from Person p where p.parentPath like '1/%'

here 1 – is parentPath + entityId, so for Bob like clause looks like

like '1/%'

because Bob’s parentPath is null and Bob’s id is 1.

And for Sally the query will look like this

select p from Person p where p.parentPath like '1/3/4/%'

because Sally’s parentPath is ‘1/3’ and Sally’s id is 4.

if you have to add a new child you just need to set its parent_path to

parent.parentPath + '/' + parent.id