/// <summary>
/// Groups a collection of documents given a grouping criteria,
/// and returns a dictionary of listed document groups with keys having the different values of the grouping criteria.
/// </summary>
/// <typeparam name="TDocument">The type representing a Document.</typeparam>
/// <typeparam name="TGroupKey">The type of the grouping criteria.</typeparam>
/// <typeparam name="TProjection">The type of the projected group.</typeparam>
/// <param name="groupingCriteria">The grouping criteria.</param>
/// <param name="groupProjection">The projected group result.</param>
/// <param name="partitionKey">The partition key of your document, if any.</param>
public virtual List <TProjection> GroupBy <TDocument, TGroupKey, TProjection>(
Expression <Func <TDocument, TGroupKey> > groupingCriteria,
Expression <Func <IGrouping <TGroupKey, TDocument>, TProjection> > groupProjection,
string partitionKey = null)
where TDocument : IDocument <TKey>
where TProjection : class, new()
{
return(MongoDbReader.GroupBy <TDocument, TGroupKey, TProjection, TKey>(groupingCriteria, groupProjection, partitionKey));
}