How to update a many-to-many association with Ecto

I have a Phoenix app with two resources that have a many to many relationship. To simplify, let’s call them posts and tags. When I create a post from scratch, I can associate existing tags with it. I can also edit a post that has no tags to associate tags with it. However, whenever I want to edit which tags are associated with a post, I get this error:

you are attempting to change relation :tags of MyApp.Posts.Post but the `:on_replace` option of
this relation is set to `:raise`

Here is my changeset - I’m using put_assoc which has 4 arguments, but according to the docs, the opts one isn’t used so I can’t set the on_replace option:

def changeset(%__MODULE__{} = user, attrs) do
    tag_ids = if attrs["tags"], do: attrs["tags"], else: []
    tags = Tags.by_ids(tag_ids)

    user
    |> Repo.preload(:tags)
    |> cast(attrs, [:title, :body])
    |> put_assoc(:tags, tags)
    |> validate_required([:body])
  end

The idea is to only update the association between the two - I would never create or delete tags themselves from the posts form. Should I be updating them in a different way?

(cross-posted on stackoverflow)