Suppose you have two fields in a Phoenix LiveView form where both are user-editable, and each one can be used to calculate the other.

Example:

A → derive B
B → derive A

Both inputs are valid entry points for the user.

The problem is that phx-change submits the entire form on every input change.

Once the form is submitted, the changeset receives both values at the same time and no longer knows which field was the actual user input and which one should be treated as the derived value.

A few obvious approaches seem problematic:

• always prioritizing one field breaks UX
• relying on changed? can be unreliable because both fields may appear changed after recalculation
• moving logic outside the changeset makes validation and calculation harder to keep consistent

What is considered the idiomatic Phoenix/Ecto approach for this kind of bidirectional dependency?

How do you usually preserve which field was the real user input when the whole form is submitted on every change?