Default `[]` on `type: :array` is returning the exact same 'default' object

[nathankot]

For example:

class FakeModel
  include MotionModel::Model
  include MotionModel::ArrayModelAdapter
  include MotionModel::Validatable

  columns test: { type: :array, default: [] }
end

@one = FakeModel.new
@two = FakeModel.new

@one.test.push(:one)

p @one.test.object_id == @two.test.object_id # => true
p @two.test.inspect # => "[:one]"

Not sure if desirable? Happy to look into this if not.

[sxross]

This is a Ruby thing. Once you assign an array, I believe it will be a different object. Otherwise, you'll have to copy the array (dup). You can test this:

@two.type = ['hello']

p @one.test.object_id == @two.test.object_id # => true
# ^^^ should be false

Steve

[nathankot]

Yep, although my use case is that I just want to append items directly to the
default array without having to think about whether it will affect other model
instances... wonder if its better to provide dups as the default?

Cheers,

Nathan Kot

On Mon, Apr 21, 2014 at 09:26:38AM -0700, s.ross wrote:

This is a Ruby thing. Once you assign an array, I believe it will be a different object. Otherwise, you'll have to copy the array (dup). You can test this:

@two.type = ['hello']

p @one.test.object_id == @two.test.object_id # => true
# ^^^ should be false

Steve

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#113 (comment)

[sxross]

I wonder if a more general solution might be to have the default take a value or a proc. So something like this would work:

columns test: { type: :array, default: lambda{[].dup}}

This would let you do things like generate UUIDs and stuff as defaults (!). Or is the syntax too ugly for words?

WDYT?

[nathankot]

Hmmmmmmm, I think its a bit ugly xD

I wonder how ActiveRecord does it? When I used it never ran into this problem before, so I'm assuming they do it
transparently but will have a look into that today after work!

Nathan Kot

On Mon, Apr 21, 2014 at 03:44:18PM -0700, s.ross wrote:

I wonder if a more general solution might be to have the default take a value or a proc. So something like this would work:

columns test: { type: :array, default: lambda{[].dup}}

This would let you do things like generate UUIDs and stuff as defaults (!). Or is the syntax too ugly for words?

WDYT?

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#113 (comment)

[DougPuchalski]

Well, you wouldn't need the dup:

columns test: { type: :array, default: -> { [] } }

Perhaps it would make sense to clone a non-primitive default value when creating an instance, but then you have to decide, shallow, deep, etc. Honestly I think rails can be a little to clever sometimes, I think the above is clear, it's saying that the default should be evaluated at the time it is created. Rails moved in this direction anyway, i.e. using procs for scopes and things like that.

[sxross]

I'm inclined to agree with @aceofspades on this. ActiveRecord doesn't really address mutable defaults (for lack of a better term). They defer it to after hooks. A stubby proc, lambda or method would be a clean general solution, IMO.

[nathankot]

I can live with the shorthand :) wonder if it's already supported by motion model?

[sxross]

I pushed a fix for this in the proc_defaults branch (https://github.com/sxross/MotionModel/tree/proc_defaults). Please have a look.

[nathankot]

perfect! looks good 👍