Rails 4 select Uniq

Question

I have the table Items:

id   enrollment_id   turma_id
11   2               2
12   2               3
13   2               2
14   2               2
15   2               3    

I want my result to be this:

id   enrollment_id   turma_id
11   2               2
12   2               3  

That is I want all the items, but that does not repeat the class, I am trying so:

@itens = Item.where(turma_id: @turma.id).uniq

But it’s not working. Keep returning the repeated.

Answer 1

If you don’t need any information beyond the separate fields you can do:

@itens = Item.select(:enrollment_id, :turma_id).uniq

In this case, objects of type Item with id equal to nil.

Or

@itens = Item.uniq.pluck(:enrollment_id, :turma_id)

With the pluck are not instantiated objects of type Item, but an array of arrays of type [enrollment_id, turma_id]. In this case Pluck has to be the last method called, so Uniq comes before.

If you need ids or other information other than that are not part of distinct, in Postgresql you can use distinct on:

@itens = Item.select('distinct on (enrollment_id, turma_id) *')

In case you wanted to get the first records guaranteed, you can add order by id.