Creating a group by parameter for the get method in a generic repository for entity framework

I am using the repository pattern with Entity Framework as described in this article: repository pattern with Entity Framework In the part where the GenericRepository is described (Generic Repository) there is a method which is used to get entities from the database set called Get. It has an orderBy but no groupBy. I am wondering how one might implement a groupBy in the same manner as the orderBy so that you can specify which field to group by dynamically on the entity.

What I have come up with is this:

Func<IQueryable<TEntity>, IGrouping<string, TEntity>> groupBy = null

and then in the method code it should be used something like this:

if(groupBy != null)
{
    query = groupBy(query).ToList();
}

But this is not compiling since the IGrouping is not queryable. Does someone know how to point me in the right direction or has a solution to this?

Edit: The reason for doing this instead of using groupby on the returned list is for performance reasons. I want the groupby to be sent as an sql statement to the database and resolved there.

Grouping has no sense without projection. So you have to define new method which returns IEnumerable with new type.

I have added sample of such method. Also removed includeProperties because EF Core ignores Includes during grouping.

Usage sample:

_orderRepostory
   .GetGrouped(e => e.UserId, g => new { UserId = g.Key, Count = g.Count()});

And implementation:

 public class GenericRepository<TEntity> where TEntity : class
 {
    ... // other code

    public virtual IEnumerable<TResult> GetGrouped<TKey, TResult>(
        Expression<Func<TEntity, TKey>> groupingKey,
        Expression<Func<IGrouping<TKey, TEntity>, TResult>> resultSelector,
        Expression<Func<TEntity, bool>>? filter = null)
    {
        var query = dbSet.AsQueryable();

        if (filter != null)
        {
            query = query.Where(filter);
        }

        return query.GroupBy(groupingKey).Select(resultSelector);
    }
 }