Two options. Either just do inserts directly to Files that include asset_id, or if you will always have the entire collection of files loaded, you can use nested changesets. So in your Asset changeset, you will have a function: cast_assoc(:files, required: false, with: &File.changeset/2)

When you update Asset though, you need to include at least the id for all existing files, as well as the fields you want to set for any new files. The former option is the only safe one if concurrent updates are possible; by default you will get an exception if you leave an ID out of your new payload.

Yes this was what I was referring to. That means there are existing associations you have not included in your payload. After the first execution there are two files that were assigned IDs. You need to include those in your next params:

params = %{
  files: [
    %{id: file1.id},
    %{id: file2.id},
    %{filename: "name3"},
    %{filename: "name4"} 
  ]
}