How to protect a Rails model attribute?

Question

My Invoice model has an address_id attribute, and I don't want this address_id to change FOREVER. So I don't want this to happen outside the class:

invoice.address_id = 1 invoice.address = some_address

Rails automatically adds this address_id attribute to the model from the invoice table, so how can I declare this attribute private/protected? Calling

attr_protected :address_id

is most likely not the solution since based on the documentation it only prevents mass assignments.

Thanks!

Ian Terrell · Accepted Answer · 2008-09-26 22:49:47Z

21

You want attr_readonly.

edited Sep 26, 2008 at 22:49

answered Sep 26, 2008 at 3:11

Ian Terrell

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4 Comments

Gishu Over a year ago

Wouldn't this deter Rails from updating and saving the Model back to the DB?

ryw Over a year ago

nice - didn't know the method existed ;) @Gishu no i don't think it would block saving model - it would just ignore changes to the fields listed in *attributes

gsmendoza Over a year ago

Is this method only available in Rails 2.*? I am running in Rails 1.2.6.

Ian Terrell Over a year ago

Not sure. Look it up. Add it in if it isn't. :)

ryw · Accepted Answer · 2008-09-26 03:08:11Z

Not as pretty as a one liner, but code below should work (and you could always do some metaprogramming to write an 'immutable' method)

def address_id=(id) if new_record? write_attribute(:address_id, id) else raise 'address is immutable!' end end

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