Update nested struct with Ecto

Questions & Help Questions
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1

Hello!
I have 2 schemas: Asset and File.

# asset schema
has_many :files, File

# file schema
belongs_to :asset, Asset

So it looks like:

%Asset{
   __meta__: #Ecto.Schema.Metadata<:loaded, "assets">,
   id: 1,
   name: "name",
   files: []
 }

Each time i need to update my asset.files (for example: add one file, then add yet 3 files in 10 minutes).
Actually I do not really understand how works put_assoc function, but it not saves old data, it removes data and set new data. Or maybe build_assoc is the only way to handle that?!
Maybe i’m explaining too bad, but it must work like in Rails that command asset.files << new_file