update_attributes changes attributes even if validation fails

Question

For example, if I run test.update_attributes prop1: 'test', prop2: 'test2' when prop1 and prop2 have validations that prevent those values, test.prop1 will still be 'test' and test.prop2 will still be 'test2'. Why is this happening and how can I fix it?

Answer 1

According to the Rails docs for update_attributes, it's an alias of update. Its source is as follows:

# File activerecord/lib/active_record/persistence.rb, line 247
def update(attributes)
  # The following transaction covers any possible database side-effects of the
  # attributes assignment. For example, setting the IDs of a child collection.
  with_transaction_returning_status do
    assign_attributes(attributes)
    save
  end
end

So, it's wrapped in a DB transaction which is why the rollback happens. However, let's check out assign_attributes. According to its source:

# File activerecord/lib/active_record/attribute_assignment.rb, line 23
def assign_attributes(new_attributes)
  ...
  _assign_attribute(k, v)
  ...
end

That is defined as:

# File activerecord/lib/active_record/attribute_assignment.rb, line 53
def _assign_attribute(k, v)
  public_send("#{k}=", v)
rescue NoMethodError
  if respond_to?("#{k}=")
    raise
  else
    raise UnknownAttributeError.new(self, k)
  end
end

So, when you call test.update_attributes prop1: 'test', prop2: 'test', it basically boils down to:

test.prop1 = 'test'
test.prop2 = 'test'
test.save

If save fails the validations, our in-memory copy of test still has the modified prop1 and prop2 values. Hence, we need to use test.reload and the issue is resolved (i.e. our DB and in-memory versions are both unchanged).

tl;dr Use test.reload after the failed update_attributes call.