Behavior module allows to determine how to display deprecation messages. You can create a custom behavior or set any from the DEFAULT_BEHAVIORS constant. Available behaviors are:

raise

Raise ActiveSupport::DeprecationException.

stderr

Log all deprecation warnings to +$stderr+.

log

Log all deprecation warnings to Rails.logger.

notify

Use ActiveSupport::Notifications to notify deprecation.rails.

silence

Do nothing.

Setting behaviors only affects deprecations that happen after boot time. For more information you can read the documentation of the behavior= method.

Methods
B
Attributes
[RW] debug

Whether to print a backtrace along with the warning.
Instance Public methods
behavior()

Returns the current behavior or if one isn't set, defaults to :stderr.

Source: show | on GitHub 

# File activesupport/lib/active_support/deprecation/behaviors.rb, line 60
def behavior
  @behavior ||= [DEFAULT_BEHAVIORS[:stderr]]
end
behavior=(behavior)

Sets the behavior to the specified value. Can be a single value, array, or an object that responds to call.

Available behaviors:

raise

Raise ActiveSupport::DeprecationException.

stderr

Log all deprecation warnings to +$stderr+.

log

Log all deprecation warnings to Rails.logger.

notify

Use ActiveSupport::Notifications to notify deprecation.rails.

silence

Do nothing.

Setting behaviors only affects deprecations that happen after boot time. Deprecation warnings raised by gems are not affected by this setting because they happen before Rails boots up.

ActiveSupport::Deprecation.behavior = :stderr
ActiveSupport::Deprecation.behavior = [:stderr, :log]
ActiveSupport::Deprecation.behavior = MyCustomHandler
ActiveSupport::Deprecation.behavior = ->(message, callstack) {
  # custom stuff
}

Source: show | on GitHub 

# File activesupport/lib/active_support/deprecation/behaviors.rb, line 85
def behavior=(behavior)
  @behavior = Array(behavior).map { |b| DEFAULT_BEHAVIORS[b] || b }
end