Bug fix: uniqueness validation fails with scoped attributes of data type time

[mikeduynguyen]

PR to fix issue #1114

The bug

The validation fails but it should pass when the scope has data type time.

Example: model Schedule with the following attributes and their data types:
day_of_week: integer
start_time: time

class Schedule
    validates :day_of_week, uniqueness: { scope: :start_time }
end

Spec:

it { should validate_uniqueness_of(:day_of_week).scoped_to(:start_time) }

Failures:

  1) Schedule should require case sensitive unique value for day_of_week scoped to start_time
     Failure/Error: it { should validate_uniqueness_of(:day_of_week).scoped_to(:start_time) }
     
       Did not expect errors to include "has already been taken" when day_of_week is set to 0,
       got errors:
       * "has already been taken" (attribute: day_of_week, value: 0) (with different value of start_time)

This bug is caused by the method that calculates the next value to test for the uniqueness of the scope:
lib/shoulda/matchers/active_record/validate_uniqueness_of_matcher.rb

module Shoulda
  module Matchers
    module ActiveRecord
    ...
    def next_scalar_value_for(scope, previous_value)
        ...
        elsif previous_value.respond_to?(:to_datetime)
            previous_value.to_datetime.next
    ...

When previous_value is of type time:

previous_value -> 2018-07-02 11:20:14
previous_value.to_datetime.next -> 2018-07-03 11:20:14

The date is incremented by 1, however the time remains the same. As a result when the next value is set on the new record the uniqueness validation fails since the 2 records have the same time value.

The fix

This PR fixes the bug by adding in(60) to increment the timestamp by 60 second before incrementing the date with next:
previous_value.to_datetime.in(60).next

[mcmire]

Sorry for the long delay on merging this. I tweaked your branch slightly to get all CI to fully pass. This is in master now.