Where to set associations with put_assoc?

Questions & Help Questions
1

Hello folks,

I’m trying to build an application similar to Goodreads, and as such I’ve run into many_to_many relationships, such as an author having multiple books, and a book having multiple offers. I’m beginning to wrap my head around using put_assoc, and I was just wondering if there is a preferred place to actually set the associations. So far I’ve considered putting it into my changeset directly like so

def changeset(book, attrs) do
    required_fields = [:title, :description, :isbn_13, :slug]
    optional_fields = []

    book
    |> cast(attrs, required_fields ++ optional_fields)
    |> validate_required(required_fields)
    |> put_assoc(:authors, attrs.authors)
end

or doing it within the context that I am using to work with all the various objects in the data model.

def create_book(attrs) do
    %Book{}
    |> Book.changeset(attrs)
    |> Ecto.changeset.put_assoc(:authors, attrs.authors)
    |> Repo.insert()
end

Is there any advantage to either approach, or an idiomatic way in Elixir?

Thanks in advance.

2

The way you’re retrieving the association data suggests that it’s provided by some form filled by an user. So this is untrusted data and you want then to use cast_assoc instead.

Now about the better place to call such an Ecto function I think it depends wether you’ll be always using exclusively your changeset along the said association or wether sometimes you want to use it without that association. In the first case I would just put it in the changeset.

Here some blog post about put_assoc vs cats_assoc.