Completed "Adding validation" (Section 11).

We now have errors in Post entry being reported correctly.

This is somewhat more complex than it should be, but you don't see that at first. Your Mutation command object returns an :errors object that is _almost_ compatible with Rails' ActiveModel::Errors class (and the difference accounts for the one line currently not covered by specs; line 39 of app/helpers/application_helper/explain_errors.rb).

Making validation Just Work with our current model object required adding (and setting) the :errors attribute on the Post model. Given the slight incompatibility between Mutations' error object (which uses #message_list to get a full list of messages) and Rails' ActiveModel::Errors (which uses #full_messages), some additional care will likely be required (as in the new `explain_errors` helper).

RSpec: 84 examples, 0 failures
Script: 0 tests: 0 failures, 0 errors, 0 pending, 0 omitted
RuboCop: 30 files inspected, no offenses detected
RuboCop global overrides in `.rubocop.yml`: 4
RuboCop local overrides in source and own specs: 1
Source lines not covered by specs: 1