Ugly Duck Model Binding in Nancy

It's been a while since I last wrote about my pet web development project. I have made a couple large changes - most importantly, I am now using RavenDB. The second big change is that I have begun dual booting Windows 8 as my development system. While running Windows in a VM is enough to get into Visual Studio, the little pauses began to drive me crazy. I went out and bought a 240 GB Corsair Force GT and couldn't be happier.

Onto my main reason for writing this post - data binding!

I ran into an issue when attempting to bind a HTTP POST to a complex type. The type I was attempting to bind to was QuestionModel, as defined below.

public class QuestionModel
{
 public string Question;
 public List<AnswerModel> Answers;
 public int CorrectAnswer;
 public string Section;
}

public class AnswerModel
{
 public int Id;
 public ChemicalModel ChemicalModel;
}

public class ChemicalModel
{
 public string SMILESString;
}

Binding to the base QuestionModel variables was no problem, this.Bind() worked great. The problem came about in binding to the List of AnswerModels - namely, in that binding simply didn't happen. After a lot of searching, I came across a StackOverflow post regarding binding to types with complex children. It looks like the proper way to handle this situation is to write a custom model binder. One alternative I did not investigate was POSTing a JSON object.

Rather than do things the right way, I chose to apply some ugly duck tape.

Post["/add"] = o =>
 {
  // Doesn't bind much
  var question = this.Bind<QuestionModel>();

  // Bind the rest
  for (int i = 0; i < 20; i++) // Bind a max of 20 children
  {
   bool hitSomething = false;
   if (Request.Form["Answers[" + i + "].AnswerString"] != null) // Bind the AnswerString property
   {
    while (question.Answers.Count <= i) // If the index doesn't exist yet, create it and every index before
     question.Answers.Add(new AnswerModel());
    question.Answers[i].AnswerString = Request.Form["Answers[" + i + "].AnswerString"];
    hitSomething = true;
   }

   if (Request.Form["Answers[" + i + "].SMILESString"] != null) // Bind the SMILESString property
   {
    while (question.Answers.Count < i - 1)
     question.Answers.Add(new AnswerModel());
    question.Answers[i].ChemicalModel =
     new ChemicalModel(Request.Form["Answers[" + i + "].SMILESString"]);
    hitSomething = true;
   }
   if (!hitSomething)
    break;
  }

  DocumentSession.Store(question);
  DocumentSession.SaveChanges();

  Model.question = question;
  return View["ShowQuestion", Model];
 };

This is going to come back to bite me shortly, but it is adequate for the time being. I suppose the moral of the story today is that if you find yourself needing a model binder that is capable of binding to array-indexed names, bother me - hopefully I'll have written it by the time you read this.

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