Handle partial GraphQL errors

[mrtnzlml]

Simple workaround for Relay bug (see: facebook/relay#1913).
I have no idea how to write tests for this part of application.
I would prefer to merge this and think about it later since we will
need to refactor it a little bit because of other tests...

Related issue: #98

Checklist:

• it works in landscape and portrait mode
• it works offline (if possible) otherwise cacheConfig.force=true is used

[mrtnzlml]

Please test these changes in Expo after build has finished:

exp://exp.host/@mrtnzlml/code-review-handle-partial-errors

[VaclavSir]

I also noticed that we create a new relay environment for every call of commitMutation and in the render method of QueryRenderer. Isn't the environment supposed to be a single instance for the whole application?

[VaclavSir]

Capital H, capital P.

[mrtnzlml]

Would you prefer first capitals everywhere? I did this change intentionally but I also prefer first capitals everywhere in English headings...

[VaclavSir]

Now that I tried to find some arguments, I'm not so sure anymore. There are two main standards - title case and sentence case. Until now I thought the title case is more "correct", but apparently the sentence case is also a valid option. Maybe we can skip this decision for now and continue using the sentence case.

https://en.wikipedia.org/wiki/Letter_case#Headings_and_publication_titles

[VaclavSir]

I'm trying to test this scenario by modifying the fetchQuery:

  const fetchQuery = async (operation, variables, cacheConfig) => {
    return {
      data: null,
      errors: [
        {
          message: 'Some message',
        },
      ],
    };

Then I get the following message, is this the expected behavior? Or am I testing it wrong (and how can I test it correctly)?

[mrtnzlml]

Yes, this is correct behavior. Data cannot be null because this indicates a fatal GraphQL error and not partial error. See documentation. This is the only situation when Relay actually works correctly and handles errors...

[mrtnzlml]

Ah, you are pointing to the docs. So yes, this is GeneralError component in action.

[mrtnzlml]

I also noticed that we create a new relay environment for every call of commitMutation and in the render method of QueryRenderer. Isn't the environment supposed to be a single instance for the whole application?

Sounds like a mistake but that's not relevant here. I should fix it though. Probably during refactoring of this section, not now. You can create a new issue...

[VaclavSir]

OK then.

[VaclavSir]

Issue created #101