How to GroupBy nothing in Dynamic Linq Core

Question

I'm creating some dynamic linq statement for user to decide what to group by. However, when the user input nothing I'd like the query to group by 'nothing', as in just output the query result as if there was no groupby. Something similar to this answer here but using Dynamic Linq Core.

var query = db.II_POLICY_AND_COVERAGE
                    .Where(x => (x.date>= settings.StartDate && x.date<= settings.EndDate))
                    .GroupBy(user_groupby_input_string);

                if (user_defined_calc_method.Equals("Total"))
                {
                    query = query.Select("new(Key as name, Sum(money_column) as value)");
                }
                else if (user_defined_calc_method.Equals("Count"))
                {
                    query = query.Select("new(Key as name, Count() as value)");
                }

For example, if user_groupby_input_string is "gender", the query will groupby the gender column. If user_groupby_input_string is "" or null, the query will ignore the groupby line and just do sum or count the whole dataset.

Answer 1

This case is specially handled by LINQ Translator when you GroupBy by constant, for example 1. LINQ Translator will remove grouping and make simple plain SQL with aggregation functions.

Answer 2

I didn't look properly and you're using it EF and this may or may not work without modifications.

You could try to do it yourself:

Step 1. Create the 'dynamic' grouper.

class X
{
    public string A { get; set; }
    public int B { get; set; }

    public static Func<X, object> GetGroupBy(string name)
        => name?.ToLower() switch
        {

            "a" => x => x.A,
            "b" => x => x.B,
            _ => x => 1 // or x.GetHashCode() if used for objects in memory
        };

    // public override int GetHashCode() => HashCode.Combine(A, B);
}

Step 2. Use it

var groups = .GroupBy(X.GetGroupBy(query))

Step 3. Test

var arr = new[] { new X { A = "x", B = 1 }, new X { A = "x", B = 2 }, new X { A = "", B = 1 }, new X { A = "", B = 3 } };

foreach (var query in new[]{ "a", "B", "", null, "7" })
{
    var queryStr = query != null ? $"'{query}'" : "null";
    Console.WriteLine($"Query: {queryStr}");
    var groups = arr.GroupBy(X.GetGroupBy(query));

    foreach (var g in groups)
    {
        var elements = string.Join(",", g.Select(x => $"'{x.A}':{x.B}"));
        Console.WriteLine($"\tGroup: '{g.Key}'\tElements: {elements}");
    }
}

Query: 'a'
    Group: 'x'  Elements: 'x':1,'x':2
    Group: ''   Elements: '':1,'':3
Query: 'B'
    Group: '1'  Elements: 'x':1,'':1
    Group: '2'  Elements: 'x':2
    Group: '3'  Elements: '':3
Query: ''
    Group: '1'  Elements: 'x':1,'x':2,'':1,'':3
Query: null
    Group: '1'  Elements: 'x':1,'x':2,'':1,'':3
Query: '7'
    Group: '1'  Elements: 'x':1,'x':2,'':1,'':3

With _ => x => x.GetHashCode() this would be:

Query: 'a'
    Group: 'x'  Elements: 'x':1,'x':2
    Group: ''   Elements: '':1,'':3
Query: 'B'
    Group: '1'  Elements: 'x':1,'':1
    Group: '2'  Elements: 'x':2
    Group: '3'  Elements: '':3
Query: ''
    Group: '94073102'   Elements: 'x':1
    Group: '142666244'  Elements: 'x':2
    Group: '186235014'  Elements: '':1
    Group: '-46300365'  Elements: '':3
Query: null
    Group: '94073102'   Elements: 'x':1
    Group: '142666244'  Elements: 'x':2
    Group: '186235014'  Elements: '':1
    Group: '-46300365'  Elements: '':3
Query: '7'
    Group: '94073102'   Elements: 'x':1
    Group: '142666244'  Elements: 'x':2
    Group: '186235014'  Elements: '':1
    Group: '-46300365'  Elements: '':3