mongodb-generic-repository/MongoDbGenericRepository/BaseMongoDbRepository.Maths.cs

using MongoDB.Driver;
using MongoDB.Driver.Linq;
using MongoDbGenericRepository.Models;
using System;
using System.Linq;
using System.Linq.Expressions;
using System.Threading.Tasks;

namespace MongoDbGenericRepository
{
    /// <summary>
    /// The base Repository, it is meant to be inherited from by your custom custom MongoRepository implementation.
    /// Its constructor must be given a connection string and a database name.
    /// </summary>
    public abstract partial class BaseMongoRepository : ReadOnlyMongoRepository, IBaseMongoRepository
    {
        /// <summary>
        /// Sums the values of a selected field for a given filtered collection of documents.
        /// </summary>
        /// <typeparam name="TDocument">The type representing a Document.</typeparam>
        /// <param name="filter">A LINQ expression filter.</param>
        /// <param name="selector">The field you want to sum.</param>
        /// <param name="partitionKey">The partition key of your document, if any.</param>
        public virtual async Task<decimal> SumByAsync<TDocument>(Expression<Func<TDocument, bool>> filter,
                                                       Expression<Func<TDocument, decimal>> selector,
                                                       string partitionKey = null)
                                                       where TDocument : IDocument
        {
            var collection = string.IsNullOrEmpty(partitionKey) ? GetCollection<TDocument>() : GetCollection<TDocument>(partitionKey);

            return await collection.AsQueryable()
                                   .Where(filter)
                                   .SumAsync(selector);
        }

    }
}