Batch updating associations using fields_for and array names requires extra array

[sjmog]

Steps to reproduce

Application upgraded from 4.2 to 5.0.0 following the official upgrade guides.

Unusual behaviour in fields_for when used to batch create or update records using model[].

(Guidelines for creating a bug report are available
here)

Expected behavior

Something like the following should create an input to update an attribute of an association that belongs_to a record, customisable:

= form_for(controller: 'records', action: 'batch_create') do
  - @records.each do |record|
    = fields_for('associations[]', record.association) do |assoc|
      = assoc.number_field(:association_attribute)

Expected HTML:

<form foo="bar">
  <input type="number" name="associations[1][association_attribute]" />
  <input type="number" name="associations[2][association_attribute]" />
  <!-- repeat for each associated record -->
</form>

Actual behavior

HTML produced:

<form foo="bar">
  <input type="number" name="associations[association_attribute]" />
  <input type="number" name="associations[association_attribute]" />
  <!-- repeat for each associated record -->
</form>

This produces the same behaviour as if we had just used fields_for('associations')

System configuration

Rails version: 5.0.0

Ruby version: 2.2.3

[maclover7]

Can you create an executable test script using one of the templates here, that would demonstrate the problem you are seeing? Also, since you're saying this worked for you in Rails 4.2.x, can you do a git bisect to determine where a possible regression may have been introduced?

On a very quick look, looks like this could be related to #26931.