Allow ActiveRecord::Base.as_json to accept a frozen Hash

[betesh]

Summary

ActiveRecord::Base.as_json takes an options Hash, which is cloned. The clone is then modified.
Since Object#clone preserves frozen state, a frozen Hash will not work here.

This commit resolves that by changing clone to dup, which does not preserve frozen state.

Other Information

There are many situations where one would want to pass a frozen Hash to as_json. I came across this issue when trying to create a default JSON representation:

class User < ApplicationRecord
  DEFAULT_JSON = { only: [:name, :email] }.freeze

  def as_json(options = nil)
    super(options.presence || DEFAULT_JSON)
  end
end

Doing this in a seemingly innocuous and correct way, I got this stack trace:

 RuntimeError:
   can't modify frozen Hash
# ~/.rvm/gems/ruby-2.3.0/gems/activerecord-5.0.1/lib/active_record/serialization.rb:14:in `serializable_hash'
# ~/.rvm/gems/ruby-2.3.0/gems/activemodel-5.0.1/lib/active_model/serializers/json.rb:99:in `as_json'
# ./app/models/user.rb:5:in `as_json'

[rails-bot]

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[rafaelfranca]

Could you add a CHANGELOG entry?

[dennisbaskin]

👍

[betesh]

Changelog entry added

[rafaelfranca]

Could you squash your commits please?

[betesh]

There's a Rubocop violation. Should I squash the fix?

[rafaelfranca]

If it is in your changeset, yes.

[betesh]

Yes, it's in my changeset. Sorry that wasn't clear. Done.

[rafaelfranca]

Backported in 0f338df.