Is there an easy way to make a Rails ActiveRecord model read-only?

Question

I want to be able to create a record in the DB but then prevent Rails from making changes from that point on. I understand changes will still be possible at the DB level.

Answer 1

Looking at ActiveRecord::Persistence, everything ends up calling create_or_update behind the scenes.

def create_or_update
  raise ReadOnlyRecord if readonly?
  result = new_record? ? create : update
  result != false
end

So! Just:

def readonly?
  !new_record?
end

Answer 2

A custom validator can do this:

validate :nothing_changed, unless: :new_record? # make immutable

...

def nothing_changed
  errors.add(:base, "Record is read-only") if self.changed?
end

Answer 3

I've found a more concise solution, which uses the after_initialize callback:

class Post < ActiveRecord::Base
  after_initialize :readonly!
end

Answer 4

This seems to be fairly effective and is probably a bit overkill, but for my case, I really want to be sure my application will never create, save, update, or destroy any records in the model, ever.

module ReadOnlyModel
  def readonly?() true end
  def create_or_update() raise ActiveRecord::ReadOnlyRecord end
  before_create { raise ActiveRecord::ReadOnlyRecord }
  before_destroy { raise ActiveRecord::ReadOnlyRecord }
  before_save { raise ActiveRecord::ReadOnlyRecord }
  before_update { raise ActiveRecord::ReadOnlyRecord }
end

class MyModel < ActiveRecord::Base
  include ReadOnlyModel
  # ...
end

Since OP asked to be able to create and destroy but not save or update I believe this will work

module SaveAndDestroyOnlyModel
  before_save { raise ActiveRecord::ReadOnlyRecord }
  before_update { raise ActiveRecord::ReadOnlyRecord }
end

class MyModel < ActiveRecord::Base
  include SaveAndDestroyOnlyModel
  # ...
end

Not exactly the right exception, but close enough I think.

Answer 5

TL;DR for OP's

class YourModel < ActiveRecord::Base
  before_save { false } # prevent create & update, allows destroy

  # ... 
end

Generally

• To prevent creates only: before_create { false }
• To prevent updates only: before_update { false }
• To prevent destroys only: before_destroy { false } # does not prevent delete

See also: http://guides.rubyonrails.org/active_record_callbacks.html

Answer 6

Why not just create a user on the database that has read only access, and have rails use that account.

However if you want model level access, you can add the following to a specific model:

 def readonly?
    true
  end

  def before_destroy
    raise ActiveRecord::ReadOnlyRecord
  end