Ruby NoMethodError: undefined method `messages' for []:Array - ruby-on-rails

Question

Since google did not help me in my quest to figure this out, I figured I would share it for anyone else that runs into this problem:
When attempting to save a model, I get
NoMethodError: undefined method `messages' for []:Array
0: /.../active_record/autosave_association.rb:491:in `_ensure_no_duplicate_errors'
1: /.../active_support/callbacks.rb:413:in `block in make_lambda'
2: /.../active_support/callbacks.rb:246:in `block in halting'
3: /.../active_support/callbacks.rb:511:in `block in invoke_after'
4: /.../active_support/callbacks.rb:511:in `each'
5: /.../active_support/callbacks.rb:511:in `invoke_after'
6: /.../active_support/callbacks.rb:132:in `run_callbacks'
7: /.../active_support/callbacks.rb:825:in `_run_validation_callbacks'
8: /.../active_model/validations/callbacks.rb:110:in `run_validations!'
9: /.../active_model/validations.rb:335:in `valid?'
Example model:
class Foo < ActiveRecord::Base
def do_stuff
#errors = []
#errors << 'Bad stuff' if self.bar > 4
#errors
end
end
Example code:
foo = Foo.first
foo.do_stuff
foo.save # or foo.valid?, etc.

Answer 1

The Cause
The cause of this error message is that the Foo class creates an instance level #errors variable.
ActiveRecord keeps track of database errors in a model's errors variable. When you try to validate a model, it checks for error messages in this variable.
The Solution
Change #errors to a different name (e.g. #foo_errors).