How should I expose the total record count and IEn

问题:

I am using EF4 with code first and have a repository for persistence and a service layer that interacts with it. I have a service layer method that calls a IQueryable method on my repository and returns a IEnumerable containing the entities. I also need to return the total record count so I can calculate the paging links.

How should I return both the int and IEnumerable from my service method?

• Use a out parameter on the method for the total row count
• Create a separate class that includes the total row count as a property
• Move the paging LINQ query out of the service layer (expose the IQueryable from the repo on the service layer)
• Create a full separate method on the service layer that does a new query just for count.

All of these should work, but which one is the cleanest?

UPDATE: Here is some clarification of the architecture. If this is wrong, then please tell me better way (eg - do the paging in the presentation layer instead of service layer,etc)

Repo layer:

returns IQueryable of DbSet, abstracts the db access from the presentation layer

Service layer:

does a LINQ query on the IQueryable to filter and just get the page items as needed using skip and take and returns a IEnumerable (going to also set to List on return to avoid any DbContext lifetime issues)

Presentation layer:

Call the method on the Service layer (getPagedResults(filters, pageNumber, pageSize))

From the looks of it I will also need to add a separate method to get the total results. Was hopeing to do this all in one call.

I would prefer not to bring back all the records to presentation and then page... seems inefficient.

回答1:

You can do something like this

public class Repository<TEntity>
{
   public IEnumerable<TEntity> GetCollection(Expression<Func<TEntity, bool>> filter, 
      int pageSize, int pageIndex)
   {
      return YourDbSet.Where(filter).OrderBy(sortExpression).Skip(pageSize * pageIndex).Take(pageSize);
   }

   public int Count(Expression<Func<TEntity, bool>> filter)
   {
      return YourDbSet.Where(filter).Count();
   }
}

Then You can write an extension method to use both of these methods

public static Pagination<TEntity> GetPagination<TEntity>(this Repository<TEntity> repository, 
   Expression<Func<TEntity, bool>> filter, int pageSize, int pageIndex)
{
   var entities = repository.GetCollection(filter, pageSize, pageIndex);
   var count = repository.Count(filter);

   return new Pagination(entities, pageSize, pageIndex + 1, count);
}

This way you can reuse GetCollection and Count methods independently. You can build the where condition dynamically. Take a look at my answer

回答2:

If the Enumerable you are returning contains all the items, I would do a ToList() on it before returning if from the function. (you can then do Count with no cost on it) If the function is returning a sub set of the total (using Skip and take) I would add a seperate function to get the total count.