Handling nested associations with cast_assoc

Hi everyone,

I finally had a bit of time to followup on this post: Best way to handle associations in single html form

In my previous post I just wanted to create a handle a nested association in the same html form. Specifically to create the parent and child in one go.

In this post I want to go a little bit further and try to explain to new comers how I manage to implement a full CRUD for this scenarios.

Please feel free to comment in this post how did you handle this scenario.

You can find all the code for this implementation in bitbucket here.

Handle a nested (has_many) association in the same html form.
Schema: Invoice --> has_many --> details

Key points:

  1. You are using one single page form to handle both parent and child associations.
  2. When creating the association for the first time you don’t have the parent’s id yet so you have to save the parent’s part fist then use its id to associate each child.
  3. I will be using drab library to add fields to the html form dynamically. So you will have to add it to your project.
  4. I will use cast_assoc to handle the update and delete of the nested association.


To keep this post as short as possible, please refer to my previous post where I coverd the create action only. You can find it here.


Please refer to the cast_assoc official documentation for further reference but I will enumerate some important points:

  1. cast_assoc requires you to preload the nested associations records

     def get_invoice!(id) do
       Repo.get!(Invoice, id)
       |> Repo.preload(:details)
  2. Based in the id number cast_assoc will set the changeset action accordingly (insert, update, delete, replace) and the database will follow.

  3. cast_assoc will handle all the database insert, update and delete actions.

So how does cast_assoc works. Lets see the code:

def update_changeset(%Invoice{} = invoice, attrs) do
  |> cast(attrs, [:provider])
  |> validate_required([:provider])
  |> cast_assoc(:details, with: &Multipartform.Documents.Detail.update_changeset(&1, &2, invoice))

Note that cast_assoc/3 takes 3 arguments: the changeset, the nested association as atom and a function. cast_assoc will invoque the function for each child record. The function must return a changeset of Details for each child record that is received in the attrs[“details”].

Also note that this function has three arguments. The first is the %Details{} struct and the second is the corresponding attributes in attrs[“details”] based on the id. So cast_assoc matches the id’s and calls the function with the arguments. Lets see the code of this function:

  @doc false
  def update_changeset(%Detail{invoice_id: nil}, attrs, invoice) do
    Ecto.build_assoc(invoice, :details)
    |> cast(attrs, [:tax, :code, :desc, :up, :invoice_id])
    |> validate_required([:tax, :code, :desc, :up, :invoice_id])
  @doc false
  def update_changeset(%Detail{} = detail, attrs, _invoice) do
    |> cast(attrs, [:tax, :code, :desc, :up, :invoice_id, :delete])
    |> validate_required([:tax, :code, :desc, :up, :invoice_id])
    |> mark_for_delete()

  @doc false
  defp mark_for_delete(changeset) do
    if get_change(changeset, :delete) do
      %{ changeset | action: :delete }

Two important thing to note:

  1. We are pattern matching on the first argument of the function header (Detail Struct). This argument is the record we preloaded. If there is no matching id means that the record found is new and this argument is initialized with nil values.
  2. When the invoice_id is nil in the struct means that the record is new and we have to build the association between the invoice and the detail.


To delete a nested association you just have to delete the entire record in the html form so it won’t be part of the attrs[“details”]. cast_assoc will delete the record form the database. This is a straight forward operation but because it is a destructive operation the recommended way is to mark the record for delete and change the changeset action to delete. This lets you do something else with the data insted of just deleting it.

Note that for each record we check the changeset for the key “delete” in the changes and if so we change the changeset action to delete.

I hope this helps explanation helps.

Best regards


@joaquinalcerro can you explain a bit this part I’m not sure I fully understand it Multipartform.Documents.Detail.update_changeset(&1, &2, invoice)

So cast_assoc takes a function, in this case, Multiplatform.Documents.Detail.update_changeset/3. This functions must return a changeset and will be used to match the preloaded record from the database against the params posted in the html form.

I use the capture operator “&” to facilitate the reading. &1 refers to the first argument and &2 to the second. The first argument will be the data record loaded from the database and the second will be the data form the params posted. Both are passed to the function and returns a changeset with the proper action added. The action of a changeset might be:

action() :: nil | :insert | :update | :delete | :replace | :ignore

Thats why you have the following function header with three arguments:

def update_changeset(%Detail{} = detail, attrs, _invoice) do

Depending in the changeset’s action, cast_assoc will create, update, delete the record in the database or just ignore it.

Hope this clarifies it.

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