SimpleDelegator and ActiveRecord::AssociationTypeMismatch

[Kris Leech]

When wrapping an object using SimpleDelegator, instead of extending it using a Module, the class changes which causes issues with ActiveRecord assosiations which are type checked, giving `ActiveRecord::AssociationTypeMismatch`.

customer   = Customer.new(user)

order.user = customer

order.save #=> ActiveRecord::AssociationTypeMismatch (Got Customer, Expected User)

I could do `order.user_id = customer.id` but it doesn't look so nice.

Does anyone know of a nice way of getting around this?

[Jim Gay]

You can explicitly get the object that you want from the Customer

order.user = customer.__getobj__

But it probably depends on what else you may be doing with the customer object and that's almost the same as your user_id = id example. 

You could also override the user= method to assign the id from whatever object you're handing to it

def user=(user)

  self.user_id = user.id

end

For situations similar to this I'm using a gem I wrote that applies methods from a module http://github.com/saturnflyer/casting

But for that you'd really need Ruby 2.x (though it would work in 1.9) and you'd need to accept that method_missing would be implemented on your user class. I tried to make the readme informative if you'd like to check it out. 

There's also http://github.com/steveklabnik/becoming that relies on the same behavior in Ruby 2.x

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Saturn Flyer LLC

571 403 0338

[Chris Flipse]

On Mon, Jun 24, 2013 at 10:24 AM, Jim Gay <j...@saturnflyer.com> wrote:

You can explicitly get the object that you want from the Customer

order.user = customer.__getobj__

But it probably depends on what else you may be doing with the customer object and that's almost the same as your user_id = id example. 

You could also override the user= method to assign the id from whatever object you're handing to it

def user=(user)

  self.user_id = user.id

end

Careful doing this, though, as that could likely break some of the save cascades that you may be expecting from active record; also if the user has already been loaded, I don't think that it will get _reloaded_ (ie, you may have different users, even after assigning the user.)  Fails the "least surprise" test.

 

For situations similar to this I'm using a gem I wrote that applies methods from a module http://github.com/saturnflyer/casting

But for that you'd really need Ruby 2.x (though it would work in 1.9) and you'd need to accept that method_missing would be implemented on your user class. I tried to make the readme informative if you'd like to check it out. 

There's also http://github.com/steveklabnik/becoming that relies on the same behavior in Ruby 2.x

On Jun 24, 2013, at 8:23, Kris Leech <kris....@gmail.com> wrote:

When wrapping an object using SimpleDelegator, instead of extending it using a Module, the class changes which causes issues with ActiveRecord assosiations which are type checked, giving `ActiveRecord::AssociationTypeMismatch`.

customer   = Customer.new(user)

order.user = customer

order.save #=> ActiveRecord::AssociationTypeMismatch (Got Customer, Expected User)

I could do `order.user_id = customer.id` but it doesn't look so nice.

Does anyone know of a nice way of getting around this?

--

Write intention revealing code #=> http://www.clean-ruby.com

Jim Gay

Saturn Flyer LLC

571 403 0338

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[Kris Leech]

Jim Gay wrote:

You can explicitly get the object that you want from the Customer

order.user = customer.__getobj__

That reminds me, I actually tried this:

class Customer < SimpleDelegator

  def class
    __getobj__.class
  end
end

but get the rarther odd error: User(#2154403100) expected, got User(#2154403100).

But it probably depends on what else you may be doing with the customer object and that's almost the same as your user_id = id example. 

You could also override the user= method to assign the id from whatever object you're handing to it

def user=(user)

  self.user_id = user.id

end

For situations similar to this I'm using a gem I wrote that applies methods from a module http://github.com/saturnflyer/casting

But for that you'd really need Ruby 2.x (though it would work in 1.9) and you'd need to accept that method_missing would be implemented on your user class. I tried to make the readme informative if you'd like to check it out. 

There's also http://github.com/steveklabnik/becoming that relies on the same behavior in Ruby 2.x

I will definitely check these gems out. Is rebinding methods a technique you feel you will stick with?

On Jun 24, 2013, at 8:23, Kris Leech <kris....@gmail.com> wrote:

When wrapping an object using SimpleDelegator, instead of extending it using a Module, the class changes which causes issues with ActiveRecord assosiations which are type checked, giving `ActiveRecord::AssociationTypeMismatch`.

customer   = Customer.new(user)

order.user = customer

order.save #=> ActiveRecord::AssociationTypeMismatch (Got Customer, Expected User)

I could do `order.user_id = customer.id` but it doesn't look so nice.

Does anyone know of a nice way of getting around this?

--

Write intention revealing code #=> http://www.clean-ruby.com

Jim Gay

Saturn Flyer LLC

571 403 0338

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[Steve Klabnik]

Yup, this is one of the reasons why I turned Avdi's code into
becoming; it's one of those features that's great in rails 90% of the
time but bad the other 10% :/

[Amos King]

Steve does that work with validations? Can I add validations on the fly and not have them add to the core User class?

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[Kris Leech]

I've been playing with casting, I'm still on 1.9 as I couldn't get Rails 4 generators not to segfault on Ruby 2.0, so have yet to try becoming.

So far its very pleasing. My only minor concern is that casting an object in a role does not change the inheritence chain, so there are no hints the object has been extended. Similar to what happens when a object/class is re-opened and behaviour added to the invisible singelton class. Out of interest is it the singelton class to which the methods are re-bound too?

[Greg H.]

I ran into the same issue but trying to do something else... 

class Customer < DelegateClass(User)

  def foo

    accounts.build(options)

  end

end

This will end up with a 

ActiveRecord::AssociationTypeMismatch: User(#123) expected, got User(#124)

The solution here is to start from the decorated object: 

class Customer < DelegateClass(User)

  def foo

    user.accounts.build(options)

  end

  private

  def user

    __getobj__

  end

end