Delete child table records from database using EF Core 7

I have a solution where I am using the repository pattern along with UnitOfWork.

I have a repository for aggregate root (ParentTable) which has a relation with ChildOne, ChildTwo tables.

ParentTable? parentTable = await _unitOfWork.ParentRepository.GetSingleAsync(ParentId);

if (parentTable == null)
    return false;

List<ChildTwo> childrenToDelete = new()
            { 
                new ChildTwo { ChildTwoId = 3, ParentId = parentTable.ParentId }, 
                new ChildTwo { ChildTwoId = 4, ParentId = parentTable.ParentId }
            };
parentTable.Name = "Parent Name Updated";

foreach (var T in childrenToDelete)
    parentTable.ChildTwo.Remove(T);

await _unitOfWork.SaveChangesAsync();

In this code, after calling SaveChangesAsync(), it is updating ParentTable rows, but not deleting ChildTwo rows added to navigation ICollection property.

I've looked various solutions available online but nothing seems to be working, in EF Core 7 there is no Attach() method to mark it deleted.

One more reason not to use Generic Repository patterns...

One option to check for, or enhance your repository method is the option to retrieve related entities with the parent. For instance:

Task<T?> async GetSingleAsync<T>(int id, params string[] includes)
{
    var query = _context.Set<T>()
        .WHere(x => x.Id == id);
    foreach(var include in includes)
        query = query.Include(include);

    return await query.SingleOrDefaultAsync();
}

Alternatively this could be done with a params of Expressions, but either way has limitations on how to handle reference depth like supporting Include and ThenInclude for instance. With a method like the above available you could use:

ParentTable? parentTable = await _unitOfWork.ParentRepository.GetSingleAsync(ParentId, "ChildTwo");
if (parentTable == null)
    return false;
 
 foreach(var childId in childIdsToRemove)
 {
     var child = parentTable.ChildTwo.FirstOrDefault(x => x.Id == childId);
     if (child != null)
         parentTable.ChildTwo.Remove(child);
 }
 
  _unitOfWork.SaveChanges();

When modifying an entity's related children, you essentially want to either eager load the collection or explicitly load the collection. As a quick hack you can Attach a stub and remove items that way but I do not recommend using that approach for child entities as you will always need to check if the DbContext happens to be tracking a reference for each item before attaching it otherwise you will get an seemingly intermittent/situational exception when it encounters that scenario.

This is a good example of why I don't recommend using a Generic or per-Entity Repository pattern with EF. If the code had just used an injected DbContext, or a repository pattern that exposed IQueryable, the solution would be as simple as adding the .Include(x => x.ChildTwos) or with EF Core you could even Just load the items to delete using:

var childrenToDelete = await _context.Entry(parent)
   .Collection(x => x.ChildTwo)
   .Query()
   .Where(c => childIdsToDelete.Contains(c.Id))
   .ToListAsync();

foreach(var child in childrenToDelete)
    parent.ChildTwo.Remove(child);

That explicitly loads only the child entities from the parent that were found in the ID list rather than eager loading a Cartesian Product of all children like Include() would do.