An example of a many-to-many relationship in a database are blog post tags. A blog post can have many tags and a tag can have many blog posts. So how do you do this in PetaPoco? I’ve added tag support to my PetaPoco A Simple Web App project on GitHub so I’ll explain what I did.

To start with you need to create a link table in your database which will stitch the article and tag tables together. So I created the articleTag table to contain lists of paired article ids and tag ids. This is all the info we need to persist this relationship in the database.

So if we have an article with 5 tags you’ll end up with 5 records in articleTag like this :

Creating these records is a one-liner if you have the article and tag ids :

_database.Execute(
            "insert into articleTag values(@0, @1)",
             article.Id, tagId);

There is a bit more complexity when it comes to reading article pocos and their tags. There are two ways of doing it :

The naughty+1 way

You could do this :

1. Load the articles without the tags.
2. Loop through each article and retrieve its tags
3. Assign the list of tags to each article in the loop.

This is the N+1 problem and it doesn’t scale well. The more articles you’re loading the more database round trips you’ll make and the slower your app will run. Nobody should recommend this approach, but let’s see it anyway :

public Article RetrieveById( int articleId)
{
	var article = _database.Query(
		"select * from article " +
		"join author on author.id = article.author_id " +
		"where article.id=@0 " +
		"order by article.date desc", articleId)
			.Single
<article>();

 var tags = _database.Fetch(
 "select * from tag " +
 "join articleTag on articleTag.tagId = tag.id " +
 "and articleTag.articleId=@0", articleId);

 if( tags != null) article.Tags = tags;

 return article;
}

This example is just for one article. Imagine it if we were pulling back 50 articles. That would be 51 database round trips when all we really need is 1.

The right way

public Article RetrieveById( int articleId)
{
	return _database.Fetch(
		new ArticleRelator().Map,
		"select * from article " +
		"join author on author.id = article.author_id " +
		"left outer join articleTag on " +
				"articleTag.articleId = article.id " +
		"left outer join tag on tag.id=articleTag.tagId " +
		"where article.id=@0 ", articleId).Single();
}

Here I am pulling back a dataset that includes the article record, the author record and the tag records. You can tell this is a many-to-many relationship by the double joining first on the articleTag then on tag itself. The results that come back to PetaPoco look like this :

As you can see there is a fair bit of duplication here and this is a trade off you will want to think carefully about. The trade off is between the number of database round trips (number of queries) and result set efficiency (network traffic from the sql server to the web server (or service layer server)). It is best to have as few database round trips as possible. But on the other hand it is better to have lean result sets too. I’m sticking with the right way.

If I was writing a real blogging app I would think long and hard about a single joined query like this because the body content, which could be thousands of bytes, would be returned as many times as there are tags against the blog post. I would almost certainly use a stored procedure to return multiple result sets so there is only one database round trip. However typical non-blogging datasets won’t contain such unlimited text data eg; orders and order lines. So there’s no problem.

Document databases suit this type of arrangement. The tags would be embedded in the article document and would still be indexable.

Stitching the pocos together

See that new ArticleRelator().Map line above? PetaPoco can utilise a relator helper function for multi-poco queries so that each poco is correctly assigned in the data hierarchy. Having the function wrapped in a class instance means it can remember the previous poco in the results.

If you’re using multi-poco queries I urge you to read the PetaPoco documentation on the subject and experiment for an hour or two. All the relator class does is take the pocos coming in from each row in the resultset and stitch them together. It allows me to add each tag to the article as well as assign the author to the article.

Turn around sir

And what about from the other angle? Where we have a tag and we want to know the articles using the tag? This is the essence of many-to-many, there are two contexts.

public Tag RetrieveByTag( string tagName)
{
	return _database.Fetch(
		new TagRelator().Map,
		"select * from tag " +
		"left outer join articleTag on articleTag.tagId = tag.id " +
		"left outer join article on " +
		"article.id = articleTag.articleId " +
		"where tag.tagName=@0 order by tag.tagName asc", tagName)
		.SingleOrDefault();
}

This is an identical process as with retrieving articles but the tables are reversed. One tag has many articles in this context.

More responsibilities

Having many-to-many relationships does add more responsibilities to your app. For example when deleting an author it’s no longer sufficient to just delete the author’s articles followed by the author record. I have to remove the author’s articles’ tags from the articleTag table too otherwise they become data orphans pointing to articles that no longer exist. Add because we’re performing multiple database calls that are all required to succeed (or not at all), we need a transaction. Like this :

public bool Delete( int authorId)
{
	using( var scope = _database.GetTransaction())
	{
		_database.Execute(
			"delete from articleTag where articleTag.articleId in " +
			"(select id from article where article.author_id=@0)",
			authorId);
		_database.Execute( "delete from article where author_id=@0", authorId);
		_database.Execute( "delete from author where id=@0", authorId);

		scope.Complete();
	}

	return true;
}

Adding many-to-many support to PetaPoco – A Simple Web App was fairly painless and should fill a small hole in the internet. I’ve had a few people ask about it and it seemed the natural next step for the project.

Am I doing many-to-many wrongly? Would you do it differently? Let me know so I can learn from you.

I saw a question on Stack Overflow asking how to setup Sam Saffron’s MVC Mini Profiler to work with PetaPoco and realised it would be a good idea to implement the mini profiler into my PetaPoco example app.

The latest release of PetaPoco included a change that made it very easy to integrate the profiler with PetaPoco if you want to. Prior to the latest version you had to hack the PetaPoco.cs file a little bit or implement your own db provider factory.  Pleasingly there is now a virtual function called OnConnectionOpened(), which is invoked by PetaPoco when the database connection object is first created. Being a virtual function you can override it in a derived class and return the connection object wrapped in the MVC Mini Profiler’s ProfiledDbConnection class. Like this :

public class DatabaseWithMVCMiniProfiler : PetaPoco.Database
{
   public DatabaseWithMVCMiniProfiler(IDbConnection connection) : base(connection) { }
   public DatabaseWithMVCMiniProfiler(string connectionStringName) : base(connectionStringName) { }
   public DatabaseWithMVCMiniProfiler(string connectionString, string providerName) : base(connectionString, providerName) { }
   public DatabaseWithMVCMiniProfiler(string connectionString, DbProviderFactory dbProviderFactory) : base(connectionString, dbProviderFactory) { }

   public override IDbConnection OnConnectionOpened(
      IDbConnection connection)
   {
      // wrap the connection with a profiling connection that tracks timings
      return MvcMiniProfiler.Data.ProfiledDbConnection.Get( connection as DbConnection, MiniProfiler.Current);
   }
}

I didn’t want to lose the existing Glimpse profiling I had in place so I made my class derive from DatabaseWithGlimpseProfiling, which is already in my project’s codebase, so you get both methods of profiling in one app.

public class DatabaseWithMVCMiniProfilerAndGlimpse : DatabaseWithGlimpseProfiling

This is turning into a profile demonstration app with a bit of PetaPoco chucked in :

You can download PetaPoco – A Simple Web App from my GitHub repository.

I’ve put together a basic web app using PetaPoco to manage a one-to-many relationship between authors and their articles.

The author poco contains just the author’s name, id and a list of articles the author has written :

public class Author
{
	public int Id {get; set;}
	public string Name {get; set;}

	[ResultColumn]
	public List<Article> Articles {get; set;}

	public Author()
	{
		Articles = new List<Article>();
	}
}

The Author.Articles list is a [ResultColumn] meaning that PetaPoco won’t try and persist it automatically, it’s ignored during inserts and updates. The Article poco has more properties :

public class Article
{
	public int? Id {get; set;}
	public string Title {get; set;}
	public string Body {get; set;}

	[DisplayFormat(DataFormatString = "{0:dd/MM/yyyy}", ApplyFormatInEditMode=true)]
	public DateTime Date {get; set;}

	[ResultColumn]
	public Author Author {get; set;}

	[Column( "author_id")]
	[DisplayName( "Author")]
	public int? AuthorId {get; set;}

	public Article()
	{
		Date = DateTime.Now;
	}
}

The Date property has a date format attribute so that in the \Article\Edit.cshtml view @Html.EditorFor(model => model.Article.Date) will display the date how I want it. The Author property is another ResultColumn only used when displaying the Article, not when updating it. In the ArticleRepository I use the MulltiPoco functionality in PetaPoco to populate the Author when the Article record is retrieved.

Setup

By default the app uses the tempdb at .\SQLEXPRESS. This is defined in the web.config :

<add name="PetaPocoWebTest" connectionString="Data Source=.\SQLEXPRESS;Initial Catalog=tempdb;Integrated Security=True" providerName="System.Data.SqlClient" />

The tempdb is always present in SQL Server and is recreated when the database is stopped and started. You can change this to suit your environment. The app will auto generate the tables for you if they don’t exist. In theory if you have SQL Express running you don’t need to do anything to get started

A few details

There are two repository classes; AuthorRepository and ArticleRepository. These classes are where the PetaPoco work lives. As well as loading the Author the AuthorRepository class also loads in the list of articles by that author. I use a relationship relator callback class as demonstrated in the recent PetaPoco object relationship mapping blog post. I made a slight tweak to handle objects that have no child objects at all otherwise there was always a new() version of Article in the list. The fix is simple :

if( article.Id != int.MinValue)
{
	// Only add this article if the details aren't blank
	_currentAuthor.Articles.Add( article);
}

I tried to get this working with an int? Id, but had type conversion problems in PetaPoco. I’ll investigate that later as it would be useful to have int? Id to indicate a blank record

I’ve used the Glimpse PetaPoco plugin by Schotime (Adam Schroder). Because the Glimpse plugin uses HttpContext.Current.Items all the SQL statements are lost when RedirectToAction() is called from a controller. I got around this by copying the PetaPoco Glimpse data into TempData before a redirect, and from TempData back into the Items collection before the controller executes.

This means you still see update/insert/delete SQL. It is definitely a requirement to perform a RedirectToAction() after such database calls so that it isn’t possible to refresh the screen and re-execute the commands (ie; cause havoc). If the user did refresh the screen (or otherwise repeat the last request) they’d just get the redirect to action and not the request that fired the database action.

I expect this to be a common pattern with Glimpse plugins that use response redirects.

UPDATE
Many thanks to Schotime who showed me that the Remote tab in Glimpse shows you the last 5 requests. All the SQL is there in Glimpse if you look for it. I didn’t realise this when I added the TempData solution but at least using TempData the SQL commands prior to a redirect are visible right on the PetaPoco tab without having to drill down to the previous request, which I kinda like as it’s more intuitive and effortless.

Hope this is useful to someone. You can download the source at GitHub. This is my first project using git so if there are any problems please let me know.

Finally, I’ve cut my finger and I’m angry with my cat.