Validations create empty translations in DB

[mrbrdo]

I am using fallbacks. One of my locales is "en-GB", and I think the I18n fallbacks defaults to "en" as a fallback.
However this bug exists also without fallbacks, it's just easier to explain this kind of case.

During validations, translation_for will be called:
https://github.com/shioyama/mobility/blob/master/lib/mobility/backends/active_record/table.rb#L291

It will build a translation for this "en" locale, even though this is not necessary.
Additionally I have a presence validation on the Translation class (friendly_id slug). Therefore due to this, I cannot save my models, because fallbacks is generating empty translations on the model. Even if I did not have that validation, it would create unwanted empty translations in the DB upon saving.

Temporarily I fixed it by patching generate_fallbacks to exclude the 'en' locale, but that only partly fixes it - only for models which have translations for all locales I use.

Possible solution would be for the reader to not call build, and only writer to call build. I can whiff up a PR if you think that solution is the best we can do.

[shioyama]

Please write a failing test, that's much clearer than a long explanation.

[mrbrdo]

See this failing spec: https://github.com/mrbrdo/mobility/blob/mrbrdo/spec/mobility/backends/active_record/table_spec.rb#L112

Also, I'm not sure that a fallback for en-XX should always include en, even if it's explicitly not specified (such as fallbacks({ :'en-US' => 'de-DE', :pt => 'de-DE' }) will still include en and de fallbacks). This makes the above issue even worse, and I can imagine it causing problems in other situations too. I understand it from perspective of I18n with Yaml files, but it makes less sense for Mobility fallbacks, as if I am using en-US and not en, there is absolutely no reason I would have en translations in my DB.
I cannot add a failing spec for this part, because it is a discussion.

[mrbrdo]

@shioyama I think the fix could be as simple as #572
I did some testing with my app (Spree) and it seems its working well. My failing spec is now passing with this change. Some existing specs need to be updated due to the change.

[shioyama]

Unfortunately the fix in #572 doesn't work with caching (several tests are failing on that branch).