Fun coding tips – by Carsten Gehling

I use RVM all the time when I develop Ruby (and Rails) applications. It’s great for isolating Ruby environments and gem packages for a specific project. I also use git extensively – especially branching when developing new features for an app.

Sometimes a branch works with new gems, that I do not want to pollute my main project-specific gemset with. So I create a new RVM gemset for this particularly branch. This has had its own problems because a “git checkout” now also needed to be followed by a “rvm use” statement. Also having a .ruby-gemset file in the project root led to RVM resetting the current gemset as soon as I changed directory.

One solution could be to check-in the .ruby-version/.ruby-gemset files in each branch, but I don’t want to annoy fellow developers with my RVM files. So I came up with the solution below instead.

First for this example, I assume that you have a git repository containing a master branch and another branch. If not, here’s a quickstart to make this happen:

$ mkdir myproject && cd myproject

$ git init .

$ touch 1.txt

$ git add -A

$ git commit -m 'First master commit'
[master (root-commit) b578056] First master commit
 2 files changed, 2 insertions(+)
 create mode 100644 .gitignore
 create mode 100644 1.txt

$ git checkout -b somebranch
Switched to a new branch 'somebranch'

$ touch 2.txt

$ git add -A

$ git commit -m 'First somebranch commit'
[somebranch d10e2c1] First somebranch commit
 1 file changed, 0 insertions(+), 0 deletions(-)
 create mode 100644 2.txt

$ git checkout master
Switched to branch 'master'

$ git branch
* master

Okay with that squared away, let’s get on with the actual steps. You need to create a RVM gemset for each branch (master and somebranch) with the appropiate .ruby-version/.ruby-gemset files. The principle is, that you rename them, so they have a postfix named after the branch.

First create for the master branch:

$ rvm --create --ruby-version 2.0.0@myproject-master
ruby-2.0.0-p643 - #gemset created /home/carsten/.rvm/gems/ruby-2.0.0-p643@myproject-master
ruby-2.0.0-p643 - #generating myproject-master wrappers..........

$ ls -a
.  ..  .git  .ruby-gemset  .ruby-version

Then rename the files:

$ mv .ruby-gemset .ruby-gemset-master
$ mv .ruby-version .ruby-version-master

$ ls -a
.  ..  .git  .ruby-gemset-master  .ruby-version-master

Now do the same for the branch “somebranch”:

$ rvm --create --ruby-version 2.0.0@myproject-somebranch
ruby-2.0.0-p643 - #gemset created /home/carsten/.rvm/gems/ruby-2.0.0-p643@myproject-somebranch
ruby-2.0.0-p643 - #generating myproject-somebranch wrappers..........

$ ls -a
.  ..  .git  .ruby-gemset  .ruby-gemset-master  .ruby-version  .ruby-version-master

$ mv .ruby-version .ruby-version-somebranch
$ mv .ruby-gemset .ruby-gemset-somebranch

Since the files are renamed as shown, they will not by themselves trigger an RVM gemset switch, if you enter the directory.

If you look at your git repo now, you will see the files as untracked.

$ git status
On branch master
Untracked files:
  (use "git add ..." to include in what will be committed)


nothing added to commit but untracked files present (use "git add" to track)

As stated in the beginning, we don’t want these files in the repository, so create a .gitignore file and add this:



Now comes the fun part. We are going to make a hook in our git config. Create this file



# Find current branch name
BRANCH=`git rev-parse --abbrev-ref HEAD`

# Copy branch-specific RVM files, if available - default to master-branch
if [ -f .ruby-gemset-$BRANCH ];
  cp -f .ruby-gemset-$BRANCH .ruby-gemset
  cp -f .ruby-gemset-master .ruby-gemset

if [ -f .ruby-version-$BRANCH ];
  cp -f .ruby-version-$BRANCH .ruby-version
  cp -f .ruby-version-master .ruby-version

And make sure, that the file is executable, since it is just a script:

chmod +x .git/hooks/post-checkout

After you have done this, everytime you checkout a branch, this script will be called. If you checkout somebranch, it will look for a .ruby-version-somebranch and .ruby-gemset-somebranch and copy them to .ruby-version and .ruby-gemset respectively. If you create a new branch but do not make a specific gemset for this branch, the script will instead copy .ruby-version-master/.ruby-gemset-master. So always as a minimum create these.

You might think that now you a all done. However if you try to checkout a “somebranch” now, things will seem strange. The .ruby-gemset file will be an exact copy of .ruby-gemset-somebranch, but if you call “rvm current”, you will still be on the master gemset. Why is this so?

The thing is: .ruby-gemset is now placed correctly, but will not be read by RVM until you change into the actual directory. Try this:

$ git checkout somebranch 
Switched to branch 'somebranch'

$ rvm current
ruby-2.0.0-p643@myproject-master      < == Not what we expected

$ cd .

$ rvm current
ruby-2.0.0-p643@myproject-somebranch  <== Much better

But this is still an extra manual step, that complicates things. You WILL forget this at some point, so let's get rid of it.

You need to edit your $HOME/.bashrc file and add this line:


git () { /usr/bin/git "$@"; cd .; }

This changes the git command to a function, which calls the executable git in /usr/bin with all arguments, and afterwards does the otherwise harmless “cd .” which will then make RVM reload the .ruby-version/.ruby-gemset files.

Exit the shell and open a new to reload .bashrc, and you’re good to go.

Please let me know, if you have any trouble with the above. Everything has been tested, but evil typos may have creeped into the text.

Have fun!

Please read this post for my reasons behind this article series.

There are few things more aggravating – at least as a programmer working with multiple languages – than when a certain construction is implemented in two extremely different ways. I tend to mix them up and often try using the idioms from one language in the other.

One of these is the yield keyword, which is a very strong construction in functional programming. It exists in both Ruby and C# but works quite differently.

Let’s take Ruby first. yield basically breaks execution, and calls an anonymous code block that have been supplied to the function containing the yield statement. yield can take arguments as well.

yield is quite powerful to create enumerators. Let’s try to make the classic Fibonacci sequence as an “endless” enumerator. And then do the same in C#. Both with yield and look at the differences.


fib = do |obj|
  obj.yield i = 0
  obj.yield j = 1
  while true
    k = i + j
    obj.yield k
    i = j
    j = k

20.times { puts }


namespace fib
    class Program
        static public IEnumerable<int> Fibs(int size)
            int i = 0;
            yield return i;
            int j = 1;
            yield return j;

            for (int n = 0; n < size; n++)
                int k = i + j;
                yield return k;
                i = j;
                j = k;

        static void Main(string[] args)
            var fibs = Fibs(20);

            foreach (var f in fibs)

When you look at the two examples, there are some major differencies. In Ruby, the yield statement immediatly breaks execution of the code back to the caller's code-block (in this example the Enumerator itself). A state-machine stores all information about what was going on until the yield. So when the enumerator's next() method is called, the execution continues where it left off. This is why the loop in our Fibonacci generator can be infinite.

In C#, however, IEnumerable will run the Fibs function to the end. Each yield return statement will store its argument inside the Enumerable object, which can then later be iterated over. But that also means that you need to make sure that the generating loop exists at some point - hence the size argument.

Inspired by this SO question:

I’ve had a lot of trouble recently with Rails and PostGIS. One of the main problem arised with testing. When you run Rails tests (or RSpec for that matter) the test database is always dropped and a new is created. The problem with this is, that the PostGIS extension is not created in this new database. Therefore your schema-load will fail, if you have any postgis-specific fields/indices.

The solution to this is to use a PostgreSQL database template – and of course the activerecord-postgis-adapter found here:

First the template:

You need a PostgreSQL template with PostGIS functions support.

Create a template database:

$ psql -U postgres
> CREATE DATABASE template_postgis WITH TEMPLATE=template1 ENCODING='UTF8';
> \c template_postgis;

Load necessary PostGIS functions into template (your SQL files may be located in a different path, but they come with the PostGIS installation):

$ psql -U postgres -f  /usr/share/postgresql/9.1/contrib/postgis-2.1/postgis.sql template_postgis
$ psql -U postgres -f /usr/share/postgresql/9.1/contrib/postgis-2.1/spatial_ref_sys.sql template_postgis
$ psql -U postgres -f  /usr/share/postgresql/9.1/contrib/postgis-2.1/topology.sql template_postgis

Set database as template and grant permissions:

$ psql -U postgres template_postgis
> UPDATE pg_database SET datistemplate = TRUE WHERE datname = 'template_postgis';
> GRANT ALL ON geometry_columns TO PUBLIC;
> GRANT ALL ON spatial_ref_sys TO PUBLIC;

Now the adapter:

Add this gem to your Gemfile and run bundle:

gem 'activerecord-postgis-adapter'

After that add the adapter and the template to your config/database.yml like this:

  adapter: postgis
  template: template_postgis
  database: mydb_development

  adapter: postgis
  template: template_postgis
  database: mydb_test

And that should do the trick! Test it by running rake db:schema:dump and rake db:test:prepare.

Let me know, if you get any additional problems regarding this.

Please read this post for my reasons behind this article series.

Some people have asked me, why I try to do things in C# that are clearly Ruby-ideoms. The short answer is: Because I can. The longer answer involves thoughts about that you should never stop learning. For a programmer, this involves pushing your tools to their boundaries and beyond. Seeking new insights other places and apply them to your environment. Stuff like that.

Catch-all property (like Ruby’s method_missing)

One of the powerful features of Ruby is method_missing. With this seemingly innocent construct you are able to make a class respond to things that are not statically defined beforehand.

Typical uses are for ORMs like ActiveRecord, that enables you to map class properties to database table-fields without explicitly defining the fieldnames in your class.

The way ActiveRecord does this is, that the model base-class contains a Hash (Dictionary) called attr. When a table-record is loaded, all fields are loaded into this hash. And method_missing are then used to map property-names directly to keys in the hash.

user = User.find(13)

This can also be done in C# using DynamicObject.

DynamicObject is a rather bold introduction into a strong-typed language like C#, since it expose members such as properties and methods at run time, instead of at compile time.

using System;
using System.Collections.Generic;
using System.Dynamic;

public class MyFakeORM : DynamicObject {
    // For a clearer example, I don't go into the stuff about loading data from the database into this model

    // Dictionary to hold all fields from the loaded record
    Dictionary _attr = new Dictionary();

    // Catch-all methods for getting and setting a "missing" property
    public override bool TrySetMember(SetMemberBinder binder, object value) {
        _attr[binder.Name] = value;
        return true;

    public override bool TryGetMember(GetMemberBinder binder, out object result) {
        return _attr.TryGetValue(binder.Name, out result);

Note: For this to work, you also need to define your instance-variable as a dynamic instead of MyFakeORM:

dynamic user = new MyFakeORM(); = "";


If you want to enable access with both properties as shown above AND as a common Dictionary, you need to add setters and getters for the class:

using System;
using System.Collections.Generic;
using System.Dynamic;

public class MyFakeORM : DynamicObject {

    public object this[string key] {
        get {
            return _attr[key];
        set {
            _attr[key] = value;

    // The rest is as before

Now all of the following is valid:

dynamic user = new MyFakeORM(); = "";
user['email'] = "";


Please read this post for my reasons behind this article series.

Using flash messages between web requests

Ok I admit, this is not entirely a C# vs. Ruby issue. It is more Ruby On Rails done with MVC3. But it uses some of the dynamic stuff from C#.

In Rails you can push messages between requests by using a special flash component:

controller UserController < ApplicationController

def Login
  user = (try-to-login-user)
  if not user.LoggedIn?
    flash[:error] = "Invalid credentials"
    redirect_to :controller => "LoginForm"

This will use sessions to carry the error-message into the next request. You can now test for the flash-message in LoginForm’s view:


<% if flash[:error] %>
  <div class="error"><%=flash[:error]%></div>
<% end %>

After this request, the flash message is erased automatically.

The same thing can be made in .NET by using a combination of the dictionary TempData” and C#’s extension methods. And we can formalize it a bit more by using an enum to control how many different message types, that we will accept.

Create a file called “FlashHelper.cs” with the following content:

using System;
using System.Collections.Generic;
using System.Text;

namespace System.Web.Mvc
    public enum FlashType

    public class FlashData
        public FlashType Key { get; set; }
        public string Message { get; set; }

    public static class FlashHelpers
        public static void Flash(this Controller controller, FlashType key, string message)
            if (controller.TempData["flash"] == null)
                controller.TempData["flash"] = new List<FlashData>();

            var flashList = controller.TempData["flash"] as List<FlashData>;
            flashList.Add(new FlashData {Key = key, Message = message});

        public static MvcHtmlString Flash(this HtmlHelper helper)
            var sb = new StringBuilder();

            if (helper.ViewContext.TempData["flash"] != null)
                foreach (FlashData flash in helper.ViewContext.TempData["flash"] as List<FlashData>)
                    sb.AppendLine(string.Format("<div class=\"flash {0}\">", flash.Key.ToString().ToLower()));
            return MvcHtmlString.Create(sb.ToString());

What I am doing here is, that I extend the base MVC controller with a new method called “Flash()”. You call this in your controller to set a new message, supplying the type of message and the message-string itself.

The same goes for extending the Html helper. I define a method, that will print out all added messages, each of them wrapped into their own DIV tag with classes set appropiate of the message type.

So now we can write a C# MVC 3 example like the Rails one above.

The controller:

public class UserController : Controller
  public RedirectToRouteResult DoLogin(string username, string password)
    var user = (try-to-login-user);

    if (!user.LoggedIn()) {
      this.Flash(FlashType.Error, "Invalid credentials");
      return RedirectToAction("LoginForm");

And in your _layout.cshtml view:

@Html.Flash() @RenderBody()

If this example is run, and the user tries to login with invalid credentials, the @Html.Flash() call will render:

<section id="main">
    <div class="flash error">Invalid credentials</div>

which you can then style with CSS.

Skriv et svar

Din e-mailadresse vil ikke blive offentliggjort. Krævede felter er markeret med *