低效的实体框架查询

Question

I have a following foreach statement:

foreach (var articleId in cleanArticlesIds)
{
    var countArt = context.TrackingInformations.Where(x => x.ArticleId == articleId).Count();
    articleDictionary.Add(articleId, countArt);
}

Database looks like this

TrackingInformation(Id, ArticleId --some stuff
Article(Id, --some stuff

what I want to do is to get all the article ids count from TrackingInformations Table. For example:

ArticleId:1 Count:1
ArticleId:2 Count:8
ArticleId:3 Count:5
ArticleId:4 Count:0

so I can have a dictionary<articleId, count>

Context is the Entity Framework DbContext. The problem is that this solution works very slow (there are > 10k articles in db and they should rapidly grow)

Answer 1

Try next query to gather grouped data and them add missing information. You can try to skip Select clause, I don't know if EF can handle ToDictionary in good manner.

If you encounter Select n + 1 problem (huge amount of database requests), you can add ToList() step between Select and ToDictionary , so that all required information will be brought into memory.

This depends all your mapping configuration, environment, so in order to get good performance, you need to play a little bit with different queries. Main approach is to aggregate as much data as possible at database level with few queries.

var articleDictionary = 
    context.TrackingInformations.Where(trackInfo => cleanArticlesIds.Contains(trackInfo.ArticleId))
                                .GroupBy(trackInfo => trackInfo.ArticleId)
                                .Select(grp => new{grp.Key, Count = grp.Count()})
                                .ToDictionary(info => "ArticleId:" + info.Key, 
                                              info => info.Count);

foreach (var missingArticleId in cleanArticlesIds)
{
    if(!articleDictionary.ContainsKey(missingArticleId))
        articleDictionary.add(missingArticleId, 0);
}

Answer 2

If TrackingInformation is a navigatable property of Article, then you can do this:

var result=context.Article.Select(a=>new {a.id,Count=a.TrackingInformation.Count()});

Putting it into a dictionary is simple as well:

var result=context.Article
  .Select(a=>new {a.id,Count=a.TrackingInformation.Count()})
  .ToDictionary(a=>a.id,a=>a.Count);

If TrackingInforation isn't a navigatable property, then you can do:

var result=context.Article.GroupJoin(
          context.TrackingInformation, 
          foo => foo.id,
          bar => bar.id,
          (x,y) => new { id = x.id, Count = y.Count() })
       .ToDictionary(a=>a.id,a=>a.Count);