Odd behaviour of update_attributes in STI table

I just noticed this odd behaviour (in rails 2.2.2). It looks like
changing the type with update_attributes doesn’t work (but returns
true!), but changing the field and then saving it does work. Here i
have an STI situation where TeachingObject and LearningObject extend
Resource.

The bit where update_attributes returns true, yet seems to fail, is
rather disconcerting. Can anyone explain what’s going on here?

thanks -max

resource = Resource.first
=> #<TeachingObject id: 59, name: “Clarinet bite point close-up”,
description: “”, type: “TeachingObject”, >

resource.update_attributes(:type => “LearningObject”)
=> true

resource = Resource.first
=> #<TeachingObject id: 59, name: “Clarinet bite point close-up”,
description: “”, type: “TeachingObject”, >

resource.type = “LearningObject”
=> “LearningObject”

resource.save
=> true

resource = Resource.first
=> #<LearningObject id: 59, name: “Clarinet bite point close-up”,
description: “”, type: “LearningObject”, >

On Oct 19, 10:08 am, Max W. [email protected]
wrote:

I just noticed this odd behaviour (in rails 2.2.2). It looks like
changing the type with update_attributes doesn’t work (but returns
true!), but changing the field and then saving it does work. Here i
have an STI situation where TeachingObject and LearningObject extend
Resource.

The bit where update_attributes returns true, yet seems to fail, is
rather disconcerting. Can anyone explain what’s going on here?

I believe that the type column is marked as a protected attribute or
something along those lines.

Fred

Frederick C. wrote:

On Oct 19, 10:08�am, Max W. [email protected]
wrote:

I just noticed this odd behaviour (in rails 2.2.2). � It looks like
changing the type with update_attributes doesn’t work (but returns
true!), but changing the field and then saving it does work. �Here i
have an STI situation where TeachingObject and LearningObject extend
Resource.

The bit where update_attributes returns true, yet seems to fail, is
rather disconcerting. �Can anyone explain what’s going on here?

I believe that the type column is marked as a protected attribute or
something along those lines.

Fred

ahhh, well that would explain it. It would also explain why
update_attribute returns true - it just silently ignores the protected
fields. It’s a bit annoyin that the api doesn’t mention this: to quote

Single table inheritance

Active Record allows inheritance by storing the name of the class in a column that by default is named “type” (can be changed by overwriting Base.inheritance_column). This means that an inheritance looking like this:

class Company < ActiveRecord::Base; end
class Firm < Company; end
class Client < Company; end
class PriorityClient < Client; end

When you do Firm.create(:name => “37signals”), this record will be saved in the companies table with type = “Firm”. You can then fetch this row again using Company.find(:first, “name = ‘37signals’”) and it will return a Firm object.

If you don‘t have a type column defined in your table, single-table inheritance won‘t be triggered. In that case, it‘ll work just like normal subclasses with no special magic for differentiating between them or reloading the right type with find.

Note, all the attributes for all the cases are kept in the same table. Read more: www.martinfowler.com/eaaCatalog/singleTableInheritance.html

unquote.

anyway, thanks Fred.
max

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