Fix Rails 4.2 "can't modify frozen hash ActiveSupport::HashWithIndifferentAccess"

[stevenschobert]

Looks like in rails 4.2, the changed_attributes method returns a frozen hash when no attributes have changed, which raises an error when we try to delete keys from it.

Looks like other libraries have used the internal instance variable directly (mbleigh/acts-as-taggable-on#583) to access the mutable copy.

Our tests didn't catch it because the audit spec is based on a stubbed class. To add in a quick safe-guard, I added a spec that runs the audit against our built in User model. Since the Audit module is available to all of ActiveRecord, it gets tested indirectly.

Test Case:

describe "with an real model subclass" do
  it "should not raise an error if nothing has changed" do
    expect { User.new.send(:initialize_dup, nil) }.to_not raise_error
  end
end

1) Challah::Audit with an real model subclass should not raise an error if nothing has changed
   Failure/Error: expect { stub.send(:initialize_dup, nil) }.to_not raise_error

     expected no Exception, got #<RuntimeError: can't modify frozen ActiveSupport::HashWithIndifferentAccess> with backtrace:
       # ./lib/challah/audit.rb:99:in `block in clear_audit_attributes'
       # ./lib/challah/audit.rb:94:in `each'
       # ./lib/challah/audit.rb:94:in `clear_audit_attributes'
       # ./lib/challah/audit.rb:39:in `initialize_dup'
       # ./spec/lib/audit_spec.rb:109:in `block (4 levels) in <module:Challah>'
       # ./spec/lib/audit_spec.rb:109:in `block (3 levels) in <module:Challah>'
   # ./spec/lib/audit_spec.rb:109:in `block (3 levels) in <module:Challah>'

[stevenschobert]

@jdtornow looks like this change is failing in rails 5 beta, i'll dig into it. if you have thoughts in the mean time let me know!

[jdtornow]

Hey @stevenschobert thanks for finding this one!

See if this commit works in your local testing (works and passes tests on this end): 73ed0ca

Instead of modifying the hash (and therefore throwing those frozen errors) I switched it to just remove those keys in a reduce and return a new hash instead. I also changed the stub class to have a changed_attributes instance method just like in Rails itself. Seemed cleaner that way.

[stevenschobert]

@jdtornow yep that commit works locally for me! I figured you'd have a better refactor in mind 👍

[jdtornow]

Sweet thanks! I merged this into master from my other branch I changed. Will push a new version shortly :)