A call to inputs_for that uses :prepend seems to skip the prepended value when the containing form has errors (that is: was created from a changeset with errors). Is this a bug?

Here’s what a form looks like for an animal with no “service gaps”:

Screen Shot 2020-01-14 at 5.51.16 PM1374×738 39.6 KB

This form represents an animal that has_many service gaps. In this case, it has none.

The code that produced the form’s HTML looks like this:

form_for @changeset, "/not-used", fn f ->
   ... 
   inputs_for(f, :service_gaps, [prepend: [%ServiceGap{}]], fn gap_f -> ...

The form itself looks like this:

%Phoenix.HTML.Form{
  data: %Crit.Usables.Schemas.Animal{...
    service_gaps: [],...
  },
  errors: [],
}

All is OK in this case.

Now suppose I submit the form with an error in one of the animal fields. In this case, I try to rename it to an already-existing name. The resulting changeset gets fed to form_for and the form looks like this:

%Phoenix.HTML.Form{
  data: %Crit.Usables.Schemas.Animal{...
    service_gaps: [],...
  },
  errors: [
    name: {"has already been taken",
     [constraint: :unique, constraint_name: "unique_available_names"]}
  ],
}

In this case, the function argument to inputs_for is not called for the :prepend value. Since there are no actual service gaps, the function is never called. That means:

1. If a user chooses to edit an animal, she can add a service gap and change properties of the animal.
2. But, if she makes a mistake changing an animal’s properties, the ability to add a new service gap mysteriously disappears.

In the case where the animal already has service gaps, the non-prepend values for inputs_for are still processed. For example, here’s the initial form for an animal with a single service gap:

Screen Shot 2020-01-14 at 6.06.38 PM1326×540 24.7 KB

The same mistake (picking a new name the same as an existing name) produces this form:

Screen Shot 2020-01-14 at 6.06.54 PM1326×306 15.3 KB

(The “Add a gap” heading is wrong because the code assumes the first set of fields will be for the prepended empty Animal. But it’s not, because the empty Animal got skipped.)

So: it seems that, in the presence of an error in the top-level Form structure, inputs_for ignores specifically the prepended value used to create a new “assoc” value. But it processes other values.