rails/rails — pull request #16304 by mcmire (2014-07-26 05:01:57+00:00)
Title: Don't encourage validating boolean columns

The validation guides mention declaring an inclusion validation against
a boolean column, like so:

validates_inclusion_of :boolean_column, in: [true, false]

Such a validation is unnecessary, as it is impossible for a boolean
column to *not* accept true or false as valid values. We should not be
encouraging people to do this.