N plus one (ARCH203)

Key dotnet.ef.n-plus-one · Severity warning · Category ef

Accessing a related entity on each iteration runs one extra query per row in addition to the main query (the N+1 problem).

Incorrect

var blogs = await _db.Blogs.ToListAsync();
foreach (var blog in blogs)
{
    Console.WriteLine(blog.Posts.Count); // extra query each loop
}

Correct

var blogs = await _db.Blogs
    .Include(b => b.Posts)
    .ToListAsync();
foreach (var blog in blogs)
{
    Console.WriteLine(blog.Posts.Count);
}

Fix

Eager-load the related data once with Include().