attr_accessible (documentation) says "the specified attributes are accessible and all others are protected" (think of it as whitelisting.)
whereas
attr_protected (documentation) says "the specified attributes are protected and all others are accessible" (think of it as blacklisting.)
A protected attribute is one that can only be modified explicitly (e.g. via attribute=) and can't be updated via mass assignment (e.g. using model.update_attributes or by passing attributes to new). The behaviour upon an attempt to update a protected attribute via mass assignment depends on the mass_assignment_sanitizer setting (see the update below).
The classic example would be if a User model had an is_admin attribute you could protect that attribute to prevent form submissions that would allow any user to be set as an administrator.
example:
class User < ActiveRecord::Base
# explicitly protect is_admin, any new attributes added to the model
# in future will be unprotected so we need to remember to come back
# and add any other sensitive attributes here in the future
attr_protected :is_admin
end
compared with:
class User < ActiveRecord::Base
# explicitly unprotect name and bio, any new attributes added to the model
# in the future will need to be listed here if we want them to be accessible
attr_accessible :name, :bio
end
Now, assuming is_admin attribute is protected:
> u = User.find_by_name('mikej')
> u.is_admin?
false
> u.update_attributes(:name => 'new name', :is_admin => true)
> u.is_admin?
false
> u.name
"new name"
> u.is_admin = true # setting it explicitly
> u.save
> u.is_admin?
true
Update: Later versions of Rails introduced the concept of a mass assignment sanitizer to control the behaviour upon attempts to update protected attributes via mass assignment. In Rails 3.2 and later this can be controlled by setting mass_assignment_sanitizer in config. The default is to just log the attempts and allow code execution to continue, but the standard environment config for development sets this to :strict which raises as exception on an attempt to update a protected attribute.
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