Fill in `has_many` relationship while creating a new record

Questions / Help
,
1

As it says in the title, I am trying to fill in a has_many relationship while creating a new record, but everything that I’ve been able to find requires the record to already exist before filling in the has_many.

Here’s the setup that I have:

schema "course" do
  has_many(:tiers, CourseTiers)
end

schema "tier" do
  has_many(:courses, CourseTiers)
end

schema "course_tiers" do
  belongs_to(:course, Course)
  belongs_to(:tier, Tier)
  field(:order, :integer)
end

The problem comes whenever I try to create a course with the reference to the tiers in the same changeset as the new course. Since neither the course_tiers nor the course exist yet, I haven’t been able to use either build_assoc, cast_assoc, or put_assoc effectively.

Is there any workaround to this that does not involve using multi or performing the insert operation in multiple steps?

2

cast_assoc should 100% be able to construct this shape - but your associations are declared strangely.

For instance, has_many(:tiers, CourseTiers) says that you want an association named tiers on Course that has CourseTiers structs in it. That’s… not probably what you want.

A more-standard set of associations:

schema "course" do
  has_many(:course_tiers, CourseTiers)
  has_many(:tiers, Tier, through: :course_tiers)
end

schema "tier" do
  has_many(:course_tiers, CourseTiers)
  has_many(:courses, Course, through: :course_tiers)
end

# join table schema same as before

Then the changeset for a Course can cast_assoc :course_tiers, and CourseTiers.changeset can cast_assoc :tiers.

General aside: Ecto doesn’t care much about pluralization, but human readers might. Naming tables with singular forms (course, tier) and schemas with plural ones (CourseTiers) feels odd IMO.

3

The CourseTiers naming thing was just an issue because I did that example without copypasting but it is indeed CourseTier in my code.

Alright, I tried renaming the association to :course_tiers but it still gives me an error. This is part of the changeset error, the one which shows the invalid changeset (and just one of the elements):

course_tier_groups: [
       #Ecto.Changeset<
         action: :insert,
         changes: %{order: 0, tier_group_id: 14},
         errors: [course_id: {"can't be blank", [validation: :required]}],
         data: #LLServer.Schema.CourseTierGroup<>,
         valid?: false
       >,

I’ll re-post the structure as it is right now (instead of tiers I have tier groups, it was just easier to write it down but it’s the same thing):

Course schema:

schema "course" do
...
  has_many(:course_tier_groups, CourseTierGroup, on_replace: :delete)
...
end

Course changeset:

def changeset(%Course{} = struct, params \\ %{}) do
  struct
  |> Repo.preload([:scopes, :type, :course_tier_groups])
  |> cast(params, [:title, :description, :tag, :type_metadata])
  |> put_scopes
  |> put_type
  |> cast_assoc(:course_tier_groups, with: &CourseTierGroup.changeset/2)
  |> validate_required([:title, :description, :tag])
  |> unique_constraint([:tag])
end

CourseTierGroup schema:

schema "course_tier_group" do
  belongs_to(:course, Course)
  belongs_to(:tier_group, TierGroup)
  field(:order, :integer)
end

TierGroup schema:

schema "tier_group" do
...
  has_many(:tiers, TierInTierGroup, on_replace: :delete)
  # Won't post more, irrelevant, but this just keeps going a bit
...
end

What I pass to the changeset on create:
[%{"tier_group_id" => X, "order" => Y}, ...]

On update it works but only if I manually put the course_id before calling the API.

Then when that is passed the course_id property of course_tier_group doesn’t get filled and it throws an error since the changeset is invalid.

4

Don’t use validate_required to verify associations; see also the docs:

Do not use this function to validate associations that are required, instead pass the :required option to cast_assoc/3 or cast_embed/3.

1 Like
5

As I understand it, I have to use cast_assoc in CourseTierGroup in order for it to work, right? The thing is, if I do so, I would have to pass the whole tier_group object through the API since cast_assoc expects a whole object to do an update operation in case IDs match. Is that correct? I do not want to perform such update operation, I just want to set the IDs of the respective props in CourseTierGroup.

6

I’ve been trying to change up the format but I still can’t get this to work :confused:

7

Did you manage to make this work eventually?

8

Yes, I ended up splitting creation and updating in 2 different flows:

  • In create, I run a multi where I first create the course then I create the associations with the course’s new ID
  • In update, I just fill in the course_id and run it through the changeset normally

It’s so much more complicated than I expected it to be, but it does the job.