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Update:

A bang can used in the below ways, in order of my personal preference.

1) An active record method raises an error if the method does not do what it says it will. (e.g. create!)
2) An active record method saves the record or a method saves an object (e.g. strip!)
3) A method does something “extra”, like posts to someplace, or does some action.
4) other things

The point is: only use a bang when you’ve really thought about whether it’s necessary, to save other developers the annoyance of having to check why you are using a bang.

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When writing methods for active record models, bangs should only come when a) something is being changed in the database or b) calling the method will raise an error if the record is not valid, or if no records are returned (e.g. find! / create!)

When to use a bang:

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# obviously we would write this better in prod, but the point is to show the point
class AModel < ActiveRecord::Base
  def hit!
    hits += 1
    save
  end
end

When not to use a bang:

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3

def modify_something
  self.hits += 1
end

In the first method, we are saving the object, in the second we will have to save it if we want the change to actually happen in the db.

The bang provides two cues to other developers.

1) that it’s not necessary to save the object after calling the method.

2) when you call the method, the db is going to be changed.

If bangs are used in other situations, the going gets rough, because it’s unclear what is being saved in what context.

Obviously, this is more a design decision than a hard rule, but I’ve found that it works well for me.