Motivation / Background

The :in option of numericality currently only accepts a static Range,
while other numericality options such as :greater_than and
:less_than_or_equal_to already support procs and symbols.

In practice, it is common for the valid range of a numeric attribute to depend
on other attributes of the same record. For example, a Product model may
define a minimum and maximum allowed price:

class Product < ApplicationRecord
  # columns: price, min_price, max_price
end

Validating that price falls between min_price and max_price currently
requires a custom validate method, even though the intent fits naturally
within numericality.

Allowing :in to accept a proc or symbol that resolves to a Range enables
this use case to be expressed declaratively:

validates :price, numericality: { in: ->(p) { p.min_price..p.max_price } }

This change brings the :in option in line with other numericality options and
with validators such as inclusion and exclusion, while remaining fully
backward compatible.