LINQ group by date - include empty days WITHOUT using join

Question

Using C#, NHibernate, NHibernate to LINQ. Using NHibernate to LINQ, I do not have the JOIN functionality. I can not use QueryOver either.

I have a LINQ query that co

Answer 1

There's a great way to do it by implementing an extension method for IEnumerable> (the type that is returned from GroupBy). In my humble opinion it's the best approach for a number of reasons:

• Doesn't recalculate the result set being filled (.ToList() also accomplishes that, but at the price of eager evaluation),
• Keeps lazy evaluation (deferred execution) of GroupBy,
• One-liners friendly (can be chained inside a fluent LINQ query),
• Generic, reusable,
• Elegant, easy to read.

Implementation

public static IEnumerable> Fill(this IEnumerable> groups, IEnumerable filling)
{
    List keys = filling.ToList();

    foreach (var g in groups)
    {
        if(keys.Contains(g.Key))
            keys.Remove(g.Key);

        yield return g;
    }

    foreach(var k in keys)
        yield return new EmptyGrouping(k);
}



class EmptyGrouping : List, IGrouping
{
    public TKey Key { get; set; }

    public EmptyGrouping(TKey key)
    {
        this.Key = key;
    }
}

Sample usage

Random rand = new Random();

var results = Enumerable.Repeat(0, 5)                    // Give us five
    .Select(i => rand.Next(100))                         // Random numbers 0 - 99
    .GroupBy(r => r.Dump("Calculating group for:") / 10) // Group by tens (0, 10, 20, 30, 40...)
    .Fill(Enumerable.Range(0, 10))                       // Fill aby missing tens
    .OrderBy(g => g.Key);                                // Sort

@"Anything above = eager evaluations.
Anything below = lazy evaluations.".Dump();

results.Dump();

Sample output

(Five integers on top are printed from inside the query when it's being evaluated. As you can see, there was only one calculation pass).