Fake implementation of DbSet<T> for unit testing

FakeDbSet.cs

public class FakeDbSet<T> : DbSet<T>, IQueryable
    where T : class
{
    private readonly Func<T, object[], bool> _find;
    readonly ObservableCollection<T> _items;
    private IQueryable<T> _query;

    public FakeDbSet(Func<T, object[], bool> find = null)
    {
        _find = find;
        _items = new ObservableCollection<T>();
        _query = _items.AsQueryable();
    }

    public override ObservableCollection<T> Local { get { return _items; } }
    IQueryProvider IQueryable.Provider { get { return _query.Provider; } }
    Expression IQueryable.Expression { get { return _query.xpression; } }

    public override IEnumerable<T> AddRange(IEnumerable<T> entities)
    {
        foreach (var entity in entities)
        {
            _items.Add(entity);
        }
        return entities;
    }

    public override T Add(T item)
    {
        _items.Add(item);
        return item;
    }

    public override IEnumerable<T> RemoveRange(IEnumerable<T> entities)
    {
        foreach (var entity in entities)
        {
            _items.Remove(entity);
        }
        return entities;
    }

    public override T Remove(T item)
    {
        _items.Remove(item);
        return item;
    }

    public override T Attach(T item)
    {
        _items.Add(item);
        return item;
    }

    public override T Create()
    {
        return Activator.CreateInstance<T>();
    }

    public override TDerivedEntity Create<TDerivedEntity>()
    {
        return Activator.CreateInstance<TDerivedEntity>();
    }

    public override T Find(params object[] keyValues)
    {
        if (_find == null)
        {
            throw new NotSupportedException();
        }
        return _items.SingleOrDefault(e => _find(e, keyValues));
    }
}