如何仅从导航属性中查询一小部分（不包括该属性中的所有数据）？

Question

I have this small scenario:

var user = await dbContext.Users
        .Include(u => u.Posts)
    .SingleOrDefaultAsync(u => u.Id == userId);

return user
    .SelectMany(u => u.Posts)
    .Skip(someStartIndex)
    .Take(someCount);

The problem with this scenario is that skip and take happen in the memory (after loading a lot of posts from the database into the memory because of issuing Include ). I just want to query small amount of those posts from the database in the first query where I get the user (instead of including the whole posts). In other words, I want somehow to query small amount of the included data, instead of including them all.

How can I achieve that?

PS: In my real code the Posts are not directly under User, but in multiple sub-properties. I just omitted that to keep the code simple, as the idea of how can I include only a part should still the same.

UPDATE

My real code, to have a better understanding of the situation:

public async Task<IEnumerable<Post>> GetPostsFromFollowsAsync(Guid userId, int count, int startIndex)
{
    //TODO rewrite this ugly query!!!
    var user = await dbContext.Users
        .Include(u => u.UserFollowing)
            .ThenInclude(uf => uf.FollowingUser)
                .ThenInclude(u => u.Posts)
                    .ThenInclude(p => p.Writer)
                        .ThenInclude(u => u.Profile)
        .Include(u => u.UserFollowing)
            .ThenInclude(uf => uf.FollowingUser)
                .ThenInclude(u => u.Posts)
                    .ThenInclude(p => p.PostLikes)
        .Include(u => u.UserFollowing)
            .ThenInclude(uf => uf.FollowingUser)
                .ThenInclude(u => u.Posts).
                    ThenInclude(p => p.PostCategories)
                        .ThenInclude(pc => pc.Category)
        .Include(u => u.UserFollowing)
            .ThenInclude(uf => uf.FollowingUser)
                .ThenInclude(u => u.Posts)
                    .ThenInclude(p => p.Comments)
        .SingleOrDefaultAsync(u => u.Id == userId);

    return user
        .UserFollowing
        .Select(uf => uf.FollowingUser)
        .SelectMany(u => u.Posts)
        .Skip(startIndex)
        .Take(count);
}

Answer 1

In this specific scenario you can start the query from Posts and use the inverse navigation property for filtering / additional includes:

var userPosts = await dbContext.Posts
    .Include(p => p.User)
    // other includes ...
    .Where(p => p.User.Id == userId)
    .Skip(someStartIndex)
    .Take(someCount)
    .ToListAsync();

This way the Skip / Take will happen server side.

Update: The actual structure doesn't change the concept. You just need to navigate backwards and change the user Id filter to Any due to many-to-many relationship:

return await dbContext.Posts
    .Include(p => p.Writer) // Parent info
        .ThenInclude(u => u.UserFollowers)
            .ThenInclude(uf => uf.FollowerUser)
    .Include(p => p.Writer) // Child info
        .ThenInclude(u => u.Profile)
    .Include(p => p.PostLikes)
    .Include(p => p.PostCategories)
        .ThenInclude(pc => pc.Category)
    .Include(p => p.Comments)
    .Where(p => p.Writer.UserFollowers.Any(uf => uf.FollowerUser.Id == userId))
    .Skip(startIndex)
    .Take(count)
    .ToListAsync();