Well… you were indirectly right! 
Although not exactly a nested form’s problem but rather me adding an unwanted/unexpected collection member where it wasn’t meant to be. And masquerading this fact with some crashes 
I use Decimal functions in many places. It’s not about this. It was a side-thread caused by the temporarily hard to explain errors I encountered.
It is more about “how to handle such cases”. I could come up with:
- building a separate “read” model and transforming (normalising) data on reading from DB
- doing the normalising in the end on the inputs themselves
The first feels like an overkill for such relatively small problem to solve, while the latter feels a bit “hacky”, like hammering the transformed values into places against the framework’s will 
All that while doubting that my case is a very rare, isolated one and wondering if there is maybe already something built-in or a de facto standard Elixir/Ecto/Phoenix way of dealing with this kind of problems. Something I am simply not yet aware of.
I currently use input_value but having any default there doesn’t make much sense in the case at hand so I handle nil gracefully instead.
