Allow nil input with default present

[antulik]

I resisted suggesting this for years, but the issue is keep coming back. I think it's time to fix it.

Problem

I want to provide a default when value is not given e.g. new html form, but allow user to clear value if needed.

integer :num, default: 100

• Currently it's impossible to allow nil, while providing default
• default syntax is confusing, as the behavior differs from Rails. I've seen many devs confused by this.

Proposed solution

Separate current default behavior into two options: default and allow_nil.

integer :num, default: 100, allow_nil: true

• allow_nil is chosen to match rails validations syntax and being self explanatory.
• no surprises, default is a default, allow_nil allows nil.

[antulik]

To keep this change backwards compatible my proposal is to treat default: nil as allow_nil: true

integer :num, default: nil # backwards compatible syntax

integer :num, allow_nil: true # same, new syntax

integer :num, default: nil, allow_nil: true # same, but `default: nil` is not necessary 

[AaronLasseigne]

This seems reasonable to me.