Question

I’m using Entity Framework Code First and whilst I have working code, I’m having to make what are strictly unnecessary database calls in order to process the following update.

I have a simple POCO class for an album with a collection of related tags:

public class Album
{
    public int Id { get; set; }
    public string Title { get; set; }
    public decimal Price { get; set; }
    public virtual IList<Tag> Tags { get; private set; }
}

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

This is updated via an MVC form – with the tags represented by a series of check-boxes.

So when I get to my Update method in the respository, I have an album class populated with a list of tags – with in theory all I need to make the update.

However the only way I could find to get the list of tags to update (to delete any that were previously set but are now unchecked, and to add any that are currently checked) was to retrieve the original Album from the context and update it.

And secondly because in my implementation the Name field of the Tag is marked with [Required], and that in my Album object populated from the form I only have the IDs of the tags, I also have to retrieve each tag before updating.

Here’s my code:

    public void Update(Album album)
    {
        var albumToUpdate = GetById(album.Id);   // - need to retrieve album with tags in order to update tags 
        albumToUpdate.Title = album.Title;
        albumToUpdate.Price = album.Price;
        albumToUpdate.Tags.Clear();

        if (album.Tags != null)
        {
            foreach (var tag in album.Tags)
            {
                var tagToAdd = context.Tags.Find(tag.Id);   // - need to retrieve full details of tag so doesn't fail validation
                albumToUpdate.AddTag(tagToAdd);
            }
        }     
    }

Appreciate any thoughts as to how I could accomodate this with fewer database hits. It’s not a major deal for this particular function (part of a site admin tool) but would like to know I’m doing things the best way.

Answer 1

Your approach – reloading the entity graph from the database and merge the changes manually into it – is correct in my opinion and the best you can do.

Forget for a moment that you use Entity Framework. What would you do if you had to write SQL statements manually? (EF is a wrapper around a SQL statement generator.) You get posted back an object graph – an Album with a list of Tags. How would you decide now which tags you have to write an INSERT, which tags a DELETE and which tags an UPDATE statement for? (I assume that your relationship between Album and Tag is many-to-many, so you write into a join table.) If you don’t know the original state in the database you can’t decide. Does the tag relation exist in the database or not? You have to query the database to find the answer, no matter if you use EF or direct SQL.

I see only two alternatives:

• Track the entity changes yourself. For you MVC web application it would mean that you have to store the original state with the former GET request somewhere, for example in a session state or in hidden input fields in the page. With the POST request you can retrieve then the original state, build and attach the orginal graph and merge changes into it.

• Write a Stored Procedure which takes the album and tag collection and let the SP do the work to create the appropriate SQL statements.

The first way is complicated and has its costs in HTTP payload (hidden input fields) or is depending on a fragile session state. And the second conflicts with why you are using an ORM. Unless you have really serious performance problems or are a SQL master I would not consider a Stored Procedure.