Fallbacks for an array of symbolized keys given as default to I18n.t are not retrieved from backend, only the initial key

# Gemfile
# gem 'rails' etc...
gem 'devise'

# app/models/user.rb
class User < ActiveRecord::Base
  devise database_authenticatable, :registerable, :recoverable, :validatable
end

# config/locales/devise.en.yml
en:
  errors:
    messages:
      not_found: "not found"

# config/initializers/i18n.rb
require 'i18n/backend/active_record'
require "i18n/backend/fallbacks"

I18n.default_locale = :'en-US'
I18n.available_locales = [I18n.default_locale]
I18n.fallbacks = true
I18n::Backend::ActiveRecord.send(:include, I18n::Backend::Fallbacks)
I18n.backend = I18n::Backend::ActiveRecord.new

# in rails console
User.new.errors.add :email, :not_found

# Generates the following SQL queries in the log file
   (0.3ms)  SELECT COUNT(*) FROM "translations"  WHERE "translations"."locale" = 'en-US' AND ("key" IN ('activerecord.errors.models.user.attributes.email.not_found') OR "key" LIKE 'activerecord.errors.models.user.attributes.email.not_found.%')
   (0.2ms)  SELECT COUNT(*) FROM "translations"  WHERE "translations"."locale" = 'en-US' AND ("key" IN ('activerecord.errors.models.user.not_found') OR "key" LIKE 'activerecord.errors.models.user.not_found.%')
   (0.2ms)  SELECT COUNT(*) FROM "translations"  WHERE "translations"."locale" = 'en-US' AND ("key" IN ('activerecord.errors.messages.not_found') OR "key" LIKE 'activerecord.errors.messages.not_found.%')
   (0.2ms)  SELECT COUNT(*) FROM "translations"  WHERE "translations"."locale" = 'en-US' AND ("key" IN ('errors.attributes.email.not_found') OR "key" LIKE 'errors.attributes.email.not_found.%')
   (0.2ms)  SELECT COUNT(*) FROM "translations"  WHERE "translations"."locale" = 'en-US' AND ("key" IN ('errors.messages.not_found') OR "key" LIKE 'errors.messages.not_found.%')
   (0.4ms)  SELECT COUNT(*) FROM "translations"  WHERE "translations"."locale" = 'en' AND ("key" IN ('activerecord.errors.models.user.attributes.email.not_found') OR "key" LIKE 'activerecord.errors.models.user.attributes.email.not_found.%')

# Console returns with output
[ "translation missing: en-US.activerecord.errors.models.user.attributes.email.not_found" ]

I think it should query all I18n keys given as defaults on the fallback locales too, not just the original, most significant key of the error message.

Whether the fallback queries should happen between each query for a default, or after all default keys have been tried for the original locale then the fallbacks, I think it should try the fallback for each fallback locale for the most significant key, then gradually going the least significant default key symbol and trying the fallback locales on the way.

So the queries should look something like this instead of the above:

# Generates the following SQL queries in the log file
   (0.3ms)  SELECT COUNT(*) FROM "translations"  WHERE "translations"."locale" = 'en-US' AND ("key" IN ('activerecord.errors.models.user.attributes.email.not_found') OR "key" LIKE 'activerecord.errors.models.user.attributes.email.not_found.%')
   (0.3ms)  SELECT COUNT(*) FROM "translations"  WHERE "translations"."locale" = 'en' AND ("key" IN ('activerecord.errors.models.user.attributes.email.not_found') OR "key" LIKE 'activerecord.errors.models.user.attributes.email.not_found.%')
   (0.2ms)  SELECT COUNT(*) FROM "translations"  WHERE "translations"."locale" = 'en-US' AND ("key" IN ('activerecord.errors.models.user.not_found') OR "key" LIKE 'activerecord.errors.models.user.not_found.%')
   (0.2ms)  SELECT COUNT(*) FROM "translations"  WHERE "translations"."locale" = 'en' AND ("key" IN ('activerecord.errors.models.user.not_found') OR "key" LIKE 'activerecord.errors.models.user.not_found.%')
   (0.2ms)  SELECT COUNT(*) FROM "translations"  WHERE "translations"."locale" = 'en-US' AND ("key" IN ('activerecord.errors.messages.not_found') OR "key" LIKE 'activerecord.errors.messages.not_found.%')
   (0.2ms)  SELECT COUNT(*) FROM "translations"  WHERE "translations"."locale" = 'en' AND ("key" IN ('activerecord.errors.messages.not_found') OR "key" LIKE 'activerecord.errors.messages.not_found.%')
   (0.2ms)  SELECT COUNT(*) FROM "translations"  WHERE "translations"."locale" = 'en-US' AND ("key" IN ('errors.attributes.email.not_found') OR "key" LIKE 'errors.attributes.email.not_found.%')
   (0.2ms)  SELECT COUNT(*) FROM "translations"  WHERE "translations"."locale" = 'en' AND ("key" IN ('errors.attributes.email.not_found') OR "key" LIKE 'errors.attributes.email.not_found.%')
   (0.2ms)  SELECT COUNT(*) FROM "translations"  WHERE "translations"."locale" = 'en-US' AND ("key" IN ('errors.messages.not_found') OR "key" LIKE 'errors.messages.not_found.%')
   (0.2ms)  SELECT COUNT(*) FROM "translations"  WHERE "translations"."locale" = 'en' AND ("key" IN ('errors.messages.not_found') OR "key" LIKE 'errors.messages.not_found.%')

Start at I18n::Backend::Fallbacks and I18n::Backend::ActiveRecord, using the initializer and Rails console example in the issue to reproduce the lookup. Verify that each default key is tried across the configured fallback locales, with the fallback queries visible in the generated SQL.