Backbone.js collections can listen to their models’ changes
I wanted to know if Backbone.js collections can listen to their models’ events. It turns out they can. In your collection you just need to use the same binding mechanism that you would in a view.
initialize:
function() {
this.on("change:name", this.changeName);
this.on("change:age", this.changeAge);
},
Here’s a JSFiddle test :
I looked in the Backbone.js source code and found where it happens :
// Internal method called every time a model in the set fires an event.
// Sets need to update their indexes when models change ids. All other
// events simply proxy through. "add" and "remove" events that originate
// in other collections are ignored.
_onModelEvent : function(ev, model, collection, options) {
if ((ev == 'add' || ev == 'remove') && collection != this) return;
if (ev == 'destroy') {
this._remove(model, options);
}
if (model && ev === 'change:' + model.idAttribute) {
delete this._byId[model.previous(model.idAttribute)];
this._byId[model.id] = model;
}
this.trigger.apply(this, arguments);
}
Many-to-many relationships with PetaPoco
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 :
- Load the articles without the tags.
- Loop through each article and retrieve its tags
- 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.
Using jQuery to display a form validation error
I’m obsessed with best practice form design and usability. There are lots of guidelines about validation messages.
I’ve decided on a way of displaying validation errors and success messages on forms for my current project. I’ve turned this into a jQuery form error plugin on GitHub which provides some quick wins :
- A single function call to display a validation message next to a field.
- A single function call to remove a validation message and optionally displays a success image in its place.
- No additional css is required.
In the GitHub project you’ll find the Index.html which demonstrates a simple form with some validation. Here’s a video of the plugin in action :
How do I use jquery.formError?
First of all you need to know when a form control has invalid data because it’s then that you want to call the plugin. I’ve kept the validation logic in the demo simple so the focus is on the plugin. If you’re using Backbone.js I can recommend the excellent backbone.validation plugin as it has the required valid/invalid callbacks you’ll need.
First you’ll need to include the jQuery.formError.js plugin javascript file. Then, to display a validation error message on an input with an id of “name” :
$("#name").formError(
"Name cannot be greater than 15 characters long");
To remove the validation message when you have successfully revalidated the value :
$("#name").formError( {remove:true});
By default removing the validation message will place a validation success image in place of the error. So you’ll need an icon for this like the one in the demo. To disable this behaviour :
$("#name").formError({
remove:true,
successImage: {enabled:false}
});
The default image url is just “success.gif” which you can easily modify on a per-call basis :
$("#name").formError({
remove:true,
successImage: {src:"img/success.gif"}
});
The plugin also gives an invalid control the css class invalid. I leave it up to you to decide the visual effect .invalid has on the control. In the demo.css file you’ll see that it applies a red border. This css class is removed when you remove the error message.
Why not put the validation message underneath the control?
It’s common for web forms to put their validation messages directly underneath the invalid control. Like this :
I’ve had two problems with this approach :
- The insertion of the message makes the whole form increase in size. This is a visually jarring experience for the user but there is another problem with this.
- If you type in some invalid data and then press the submit button the first even to fire is the control’s change event. If the user has just corrected some data (a common pattern just prior to pressing the submit button) a validation message may be removed from underneath the control thanks to the control’s change handler. The whole form then shrinks a little and the mouse click, whose event fires next, is no longer on the submit button. The button has moved! So you have to click it again. This is an annoying and confusing user experience.
Finally
I did start with the qTip jQuery plugin for these messages but I wanted something simpler whose HTML I could control.