Rails Bug With Has Many Collections Empty? and Build

After my long rant about problems in Ruby on Rails has_many collections and empty? calls, we spent some more time and really condensed the problem. I can explain this problem much more clearly now.

Let's take the following code:


f = Foo.new
f.bars.build
f.bars.empty?
pp f.bars


What do you think will be printed out when we pp f.bars?

Well, if you're like me, you'd think it would return an array with one Bar in it. And, if you were like me, you'd be wrong.

Turns out that the call to f.bars.empty? doesn't realize you've ever added a new Bar to the collection of bars. Therefore, it will go into the database and do a select count(*) to determine the size of the collection. It will return zero, because at this point you haven't saved anything into the database. Not only is going to the database to determine the size of the collection wrong (in the line above, I've added a Bar though build so empty? should know there's at least one), but empty? in the process is caching the empty collection!

Because of this caching, our last line of code here is printing out an empty array. This effectively deletes the original Bar we added via build.

This has to be a bug. In fact, I'm filing a bug at Rails' Trac system.

How do you work around this? Just change your usage of empty? to length.zero?:


f = Foo.new
f.bars.build
f.bars.length.zero?
pp f.bars


The above code will work just fine!

Many thanks to Wenyi for researching this with me.

Popular posts from this blog

Lists and arrays in Dart

(This is Part 3 in a series about Dart. Check out Part 2, Function in Dart .) Warning : We expect the Dart libraries to undergo potentially sweeping changes before Dart goes to alpha. This document is relevant as of 2011-12-22. Intro Dart is a "batteries included" effort to help app developers build modern web apps. An important "battery" is the bundled core Dart libraries, providing common and rich functionality. Dart is building a solution for large, complex web apps, and providing well tested, integrated, and common libraries is key to helping a web app developer be more productive out of the box. The Collection libraries are a crucial set of APIs that Dart developers get for free. Much more than simple arrays and maps, the Collection library includes standard ways to filter, iterate, inspect, compose, and sort your data. This post specifically looks at List<E> , Dart's ordered, indexable collection of objects. Aside: The Dart project is lucky
Read more

The 29 Healthiest Foods on the Planet

The 29 Healthiest Foods on the Planet > The following is a "healthy food hot list" consisting of the 29 food that will give you the biggest nutritional bang for you caloric buck, as well as decrease your risk for deadly illnesses like cancer, diabetes and heart disease.
Read more

Converting Array to List in Scala

Now, this has to have a built-in somewhere in Scala , because it just seems too common. So, how to convert an Array to a List in Scala? Why do I need this? I needed to drop to Java for some functionality, which in this case returns an Array. I wanted to get that Array into a List to practice my functional programming skillz. **Update**: I figured out how to convert Arrays to Lists the Scala way. Turns out it's a piece of cake. val myList = List.fromArray(Array("one", "two", "three")) or val myList = Array("one","two","three").elements.toList The call to elements returns an Iterator , and from there you can convert to a List via toList . Nice. Because my first version wasn't actually tail recursive, what follows is a true tail recursive solution, if I were to implement this by hand. The above, built in mechanism is much better, though. object ArrayUtil { def toList[a](array: Array[a]): List[a] = { d
Read more