How do you fix the N+1 query problem in EF Core?

The trap:

var orders = await db.Orders.ToListAsync();        // 1 query
foreach (var o in orders)
    Console.WriteLine(o.Customer.Email);            // 100 lazy-load queries

Three fixes — pick by query shape:

1. Include — for 1-to-1 or small 1-to-many:

var orders = await db.Orders
    .Include(o => o.Customer)
    .ToListAsync();

One JOIN, one round-trip.

2. Split queries — when including multiple collections:

var orders = await db.Orders
    .Include(o => o.Customer)
    .Include(o => o.LineItems).ThenInclude(li => li.Product)
    .AsSplitQuery()
    .ToListAsync();

Multiple Include on collections causes a Cartesian product (rows ✕ rows). AsSplitQuery runs separate queries and stitches them in memory.

3. Projection — usually the right answer:

var rows = await db.Orders
    .Select(o => new OrderListItem(o.Id, o.Customer.Name, o.LineItems.Count))
    .ToListAsync();

EF generates one efficient SQL with only the columns you need. No tracking, no Cartesian, no surprise lazy-loads.

Detect in dev: enable EF Core SQL logging and look for repeated identical SELECT shapes. In tests, count queries with MiniProfiler and assert an upper bound.