Cloning Reddit to learn ASP.NET 5 - Part 6

In the last post in the series, we used a DTO to minimize the transfered data but we retrieved all that data from the database, shouldn't we have got just what we wanted?

In short the answer is yes, again the potential savings need to be carefully considered as the code might be more cluttered.

This is the query that gets run against the database:

SELECT [x].[SubRedditId], [x].[IsBanned], [x].[IsDefault], [x].[IsPublic], [x].[Name]
FROM [SubReddit] AS [x]
WHERE ([x].[IsDefault] = 1 AND [x].[IsPublic] = 1) AND [x].[IsBanned] = 0

You might be tempted to modify the Repository method like this:

public IEnumerable<SubRedditDTO> GetDefaultSubReddits()
{
            return ctx.SubReddits
                .Where(x => x.IsDefault && x.IsPublic && !x.IsBanned)
                .Cast<SubRedditDTO>();       
}

If you do this, you'll get this error: System.InvalidOperationException:

No coercion operator is defined between types 'Reddit.Models.SubReddit' and 'Reddit.Models.SubRedditDTO'.

We could use AutoMapper here but it seems a bit clunky when you're trying to reduce object size as the definition needs to be a bit like this:

          Mapper.Initialize(config =>
            {
                config.CreateMap<SubReddit, SubRedditDTO>()
                .ForSourceMember(x => x.IsBanned, o => o.Ignore());               
            });

In all honesty I've not read enough about AutoMapping, DTOs and ViewModels to be able to advise on best practices, but in this case it does seem a bit clunky, so instead I tried the following:

       public IEnumerable<SubRedditDTO> GetDefaultSubReddits()
        {
            return ctx.SubReddits
                .Where(x => x.IsDefault && x.IsPublic && !x.IsBanned)
                .Select(x => new SubRedditDTO() { Name=x.Name });
        }

This actually achieves what we wanted, at least in theory, which is reduce the load on the database. I realize that DB performance is a lot more nuanced than I'm presenting it here, but as a rule of thumb the less data retrieved from the database the better. This is the query that is executed against the database:

SELECT [x].[Name]
FROM [SubReddit] AS [x]
WHERE ([x].[IsDefault] = 1 AND [x].[IsPublic] = 1) AND [x].[IsBanned] = 0

This should be better from a DB performance point of view, but how much, it's hard to say.

The last thing I'll tackle in this post is the lack of links on the front page. You might be tempted to simply change the Index.cshtml page to this:

@{
    ViewData["Title"] = "Reddit";
}
@section scripts {
    <script src="~/lib/angular/angular.js"></script>
    <script src="~/js/redditapp.js"></script>
}
<div ng-app="redditApp">
    <div ng-controller="SubRedditListCtrl">
        <ul class="list-inline">
            <li ng-repeat="subReddit in subReddits">
                <a href="http://reddit.com/r/{{subReddit.Name}}">{{subReddit.Name}}</a>                      
            </li>
        </ul>
    </div>
    <hr />
</div>

Unfortunately, that will not always work as expected, all links will not have the subreddit until angular has substituted the value of subReddit.Name, instead we need to use a angular directive:

<a ng-href="http://reddit.com/r/{{subReddit.Name}}">{{subReddit.Name}}</a>