查询EDMX中的多对多连接表

Question

I am currently facing performance problem whenever I'm accessing navigation property of an entity.

I have three tables: UserCategory , User and UserCategoryUser . There is many-to-many relation between User and UserCategory tables. UserCategoryUser table is the joiner table and it has two columns UserId and UserCategoryId ie the primary keys of User and UserCategory tables. The joiner table UserCategoryUser is used for maintaining the many-to-many relationship between User and UserCategory tables.

I'm using database first approach. Hence in my EDMX, for the User entity I have one navigation property UserCategories . Similarly for the UserCategory entity there is a navigation property Users .

I want to add a user to a usercategory. So before adding, I'm doing a check whether the user is already added to the usercategory or not. In my database I've around 100k user records and only one usercategory. All the users associated with the only usercategory.

I'm accessing the Users navigation property like follows:

var userCategory = Context.UserCategories.FirstOrDefault(uc => uc.UserCategoryId == userCategoryId);

if (userCategory != null)
{
    if (userCategory.Users.Any(u => u.Username == username))
    {
        //Other operations
    }
}

This code was working before but now it is hanging as we have lots of user data. In particular it is hanging for userCategory.Users.Any(u => u.Username == username)) line. I also tried getting the users count but still it is hanging!

userCategory.Users.Count();

I'm not able to do the join query in LINQ to Entity as the joiner table( UserCategoryUser ) is NOT added as an entity in the EDMX.

How to solve this problem? I can to this check using pure SQL or stored procedure but I want to stay away from those for this case.

Answer 1

Your problem is that with lazy loading enabled, the call to userCategory.Users loads all Users into memory. I think, given that there are very few categories, I would do it the other way round. ie get the User then check whether the user's Category collection contains the category being added.

However, if you had a very large number of categories then you could explicitly filter the collection. First turn off lazy loading on the collection (eg by removing the virtual keyword), then try this:

   Context.Entry(userCategory)
              .Collection(uc => uc.Users)
              .Query()
              .Where(u => u.Username == username)//I'd put an index on username too
              .Load();

   if(userCategory.Users.Any())
   {
      //Other operations
   }

Reference: https://msdn.microsoft.com/en-us/data/jj574232.aspx#explicitFilter