Adding and removing elements from jsonb field using embedded schema

KiKi

I’m working on a small project where I’m trying to learn how to use embedded schemas. So I have albums and tracks. Album can have many tracks and they are saved in jsonb field using embedded schema. I want to be able to add new tracks and also, delete them.

In create_track() function, first I get existing tracks, then there is a map with a new track and then they both get merged, put into changeset and album get updated.

def create_track(%Album{} = album, attrs \\ %{}) do
    existing_tracks = album.tracks
    
    new_track = 
      %TrackEmbed{
        # this is just to show how TrackEmbed looks, real data comes from attrs
        name: "track name"
      }
      
    tracks = [new_track | existing_tracks]
    changeset = Ecto.Changeset.change(album)

    changeset
    |> Ecto.Changeset.put_embed(:tracks, tracks)
    |> Repo.update()
  end

To delete a track, I get existing tracks, find a list index where track I want to delete is and then I remove it from the list and the album gets updated.


def delete_track(%Album{} = album, attrs \\ %{}) do
    existing_tracks = album.tracks
    
    # real data comes from attrs here too
    existing_track_index = Enum.find_index(existing_tracks, fn x -> x.name == "track name" end)
    
    tracks = List.delete_at(existing_tracks, existing_track_index)
    changeset = Ecto.Changeset.change(album)

    changeset
    |> Ecto.Changeset.put_embed(:tracks, tracks)
    |> Repo.update()
end

The code I have here works but here I’m getting an existing list and just adding or removing from it, so I was wondering if there is maybe a better way?

3 comments

/phoenix #ecto #embedded_schema

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2021-02-02 17:24:49 UTC

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dimitarvp

Nothing dramatically better exists, as far I am aware. You could use cast_embed but that doesn’t help you much because you would still need to load the old value of the list and then update it, meaning that put_embed is the better choice here since it assumes the exact same use case.

cast_embed would be a good choice if you were receiving an entirely new list with which overwrites the old one (and usually if that new list is coming from external request).

Post #1

KiKi

There are several tutorials about embedded schemas out there and no one wrote anything about this so I just wanted to check if I’m doing something wrong. Thanks!

Post #2

dimitarvp

Well, if you are storing things in PostgreSQL you could reach for its various JSON functions: PostgreSQL: Documentation: 13: 9.16. JSON Functions and Operators

This SO thread might help a bit in orienting you how are they used: https://stackoverflow.com/questions/23490965/postgresql-remove-attribute-from-json-column

This one seems to give you info on how to delete an object inside a JSONB array by index: https://dba.stackexchange.com/questions/164869/delete-an-object-from-within-an-array-using-jsonb-in-postgresql

(Not very useful if you don’t have the index though.)

But I’d strongly caution against poking in the guts of your DB with DB-specific functions unless you can gain a big performance win. Say if you notice that the requests to your app that are supposed to delete tracks have abnormally high latency (which in Elixir/Phoenix land basically means anything above 30ms ) then it’s worth the pursuit. Otherwise I wouldn’t.