使用EntityFramework的分页结果列表

Question

I have a simple SQL Server DB containing data for Users and Communities. Any user can be member of any number of communities, so I have 3 tables: Users , Communities and Map , and corresponding POCO C# classes. I use EntityFramework to get the dictionary of all communities and their memebers:

public Dictionary<Community, List<User>> GetData()
{
    var communitiesAndUsers =
        from community in DbContext.Communities
        from map in DbContext.CommunityUserMap.Where(c => c.Community == community )
        select new { Community = community, User = map.User };


    var result = new Dictionary<Community, List<User>>();
    foreach (var communityGroup in communitiesAndUsers.ToList().GroupBy(x => x.Community.Id))
    {
        var community = communityGroup.First().Community;
        var users = communityGroup.Select(g => g.User).ToList();
        result.Add(community, users);
    }

    return result;
}

Now I need to support pagination in this method:

public Dictionary<Community, List<User>> GetData(int page, int communitiesPerPage)

And I have a requirement to do it using 1 SQL query. How do I need to modify my method to support that?

Answer 1

Use Skip and Take to implement the paging. You will also need to define sort criteria ( Orderby ) to get consistent results.

var pagedCommunityGroups = communitiesAndUsers
    .GroupBy(cau => cau.Community.Id)
    .OrderBy(grp => grp.Key)
    .Skip(page * communitiesPerPage) // assume first page has number 0
    .Take(communitiesPerPage)
    .ToList();                       // execute query

Because communitiesAndUsers has not been enumerated before you apply Skip and Take , this will result in a single SQL query (query will be executed on ToList ).