diff --git a/MongoDbGenericRepository/BaseMongoDbRepository.Maths.cs b/MongoDbGenericRepository/BaseMongoDbRepository.Maths.cs
new file mode 100644
index 0000000..bbc6552
--- /dev/null
+++ b/MongoDbGenericRepository/BaseMongoDbRepository.Maths.cs
@@ -0,0 +1,37 @@
+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);
+        }
+
+    }
+}
\ No newline at end of file
diff --git a/MongoDbGenericRepository/BaseMongoDbRepository.cs b/MongoDbGenericRepository/BaseMongoDbRepository.cs
index 7a3a76e..2a3f07e 100644
--- a/MongoDbGenericRepository/BaseMongoDbRepository.cs
+++ b/MongoDbGenericRepository/BaseMongoDbRepository.cs
@@ -822,7 +822,7 @@ namespace MongoDbGenericRepository
         /// </summary>
         /// <typeparam name="TDocument">The type representing a Document.</typeparam>
         /// <typeparam name="TProjection">The type representing the model you want to project to.</typeparam>
-        /// <param name="filter"></param>
+        /// <param name="filter">A LINQ expression filter.</param>
         /// <param name="projection">The projection expression.</param>
         /// <param name="partitionKey">An optional partition key.</param>
         public virtual async Task<TProjection> ProjectOneAsync<TDocument, TProjection>(Expression<Func<TDocument, bool>> filter, Expression<Func<TDocument, TProjection>> projection, string partitionKey = null)
@@ -840,7 +840,7 @@ namespace MongoDbGenericRepository
         /// <typeparam name="TDocument">The type representing a Document.</typeparam>
         /// <typeparam name="TKey">The type of the primary key for a Document.</typeparam>
         /// <typeparam name="TProjection">The type representing the model you want to project to.</typeparam>
-        /// <param name="filter"></param>
+        /// <param name="filter">A LINQ expression filter.</param>
         /// <param name="projection">The projection expression.</param>
         /// <param name="partitionKey">An optional partition key.</param>
         public virtual async Task<TProjection> ProjectOneAsync<TDocument, TProjection, TKey>(Expression<Func<TDocument, bool>> filter, Expression<Func<TDocument, TProjection>> projection, string partitionKey = null)
@@ -849,8 +849,8 @@ namespace MongoDbGenericRepository
             where TProjection : class
         {
             return await HandlePartitioned<TDocument, TKey>(partitionKey).Find(filter)
-                                                                   .Project(projection)
-                                                                   .FirstOrDefaultAsync();
+                                                                         .Project(projection)
+                                                                         .FirstOrDefaultAsync();
         }
 
         /// <summary>
@@ -858,7 +858,7 @@ namespace MongoDbGenericRepository
         /// </summary>
         /// <typeparam name="TDocument">The type representing a Document.</typeparam>
         /// <typeparam name="TProjection">The type representing the model you want to project to.</typeparam>
-        /// <param name="filter"></param>
+        /// <param name="filter">A LINQ expression filter.</param>
         /// <param name="projection">The projection expression.</param>
         /// <param name="partitionKey">An optional partition key.</param>
         public virtual TProjection ProjectOne<TDocument, TProjection>(Expression<Func<TDocument, bool>> filter, Expression<Func<TDocument, TProjection>> projection, string partitionKey = null)
@@ -876,7 +876,7 @@ namespace MongoDbGenericRepository
         /// <typeparam name="TDocument">The type representing a Document.</typeparam>
         /// <typeparam name="TKey">The type of the primary key for a Document.</typeparam>
         /// <typeparam name="TProjection">The type representing the model you want to project to.</typeparam>
-        /// <param name="filter"></param>
+        /// <param name="filter">A LINQ expression filter.</param>
         /// <param name="projection">The projection expression.</param>
         /// <param name="partitionKey">An optional partition key.</param>
         public virtual TProjection ProjectOne<TDocument, TProjection, TKey>(Expression<Func<TDocument, bool>> filter, Expression<Func<TDocument, TProjection>> projection, string partitionKey = null)
@@ -885,8 +885,8 @@ namespace MongoDbGenericRepository
             where TProjection : class
         {
             return HandlePartitioned<TDocument, TKey>(partitionKey).Find(filter)
-                                                             .Project(projection)
-                                                             .FirstOrDefault();
+                                                                   .Project(projection)
+                                                                   .FirstOrDefault();
         }
 
         /// <summary>
@@ -894,7 +894,7 @@ namespace MongoDbGenericRepository
         /// </summary>
         /// <typeparam name="TDocument">The type representing a Document.</typeparam>
         /// <typeparam name="TProjection">The type representing the model you want to project to.</typeparam>
-        /// <param name="filter"></param>
+        /// <param name="filter">A LINQ expression filter.</param>
         /// <param name="projection">The projection expression.</param>
         /// <param name="partitionKey">An optional partition key.</param>
         public virtual async Task<List<TProjection>> ProjectManyAsync<TDocument, TProjection>(Expression<Func<TDocument, bool>> filter, Expression<Func<TDocument, TProjection>> projection, string partitionKey = null)
@@ -912,7 +912,7 @@ namespace MongoDbGenericRepository
         /// <typeparam name="TDocument">The type representing a Document.</typeparam>
         /// <typeparam name="TKey">The type of the primary key for a Document.</typeparam>
         /// <typeparam name="TProjection">The type representing the model you want to project to.</typeparam>
-        /// <param name="filter"></param>
+        /// <param name="filter">A LINQ expression filter.</param>
         /// <param name="projection">The projection expression.</param>
         /// <param name="partitionKey">An optional partition key.</param>
         public virtual async Task<List<TProjection>> ProjectManyAsync<TDocument, TProjection, TKey>(Expression<Func<TDocument, bool>> filter, Expression<Func<TDocument, TProjection>> projection, string partitionKey = null)
@@ -930,7 +930,7 @@ namespace MongoDbGenericRepository
         /// </summary>
         /// <typeparam name="TDocument">The type representing a Document.</typeparam>
         /// <typeparam name="TProjection">The type representing the model you want to project to.</typeparam>
-        /// <param name="filter"></param>
+        /// <param name="filter">A LINQ expression filter.</param>
         /// <param name="projection">The projection expression.</param>
         /// <param name="partitionKey">An optional partition key.</param>
         public virtual List<TProjection> ProjectMany<TDocument, TProjection>(Expression<Func<TDocument, bool>> filter, Expression<Func<TDocument, TProjection>> projection, string partitionKey = null)
@@ -996,7 +996,7 @@ namespace MongoDbGenericRepository
         /// <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="filter"></param>
+        /// <param name="filter">A LINQ expression filter.</param>
         /// <param name="selector">The grouping criteria.</param>
         /// <param name="projection">The projected group result.</param>
         /// <param name="partitionKey">The partition key of your document, if any.</param>
@@ -1021,7 +1021,7 @@ namespace MongoDbGenericRepository
         /// Asynchronously returns a paginated list of the documents matching the filter condition.
         /// </summary>
         /// <typeparam name="TDocument">The type representing a Document.</typeparam>
-        /// <param name="filter"></param>
+        /// <param name="filter">A LINQ expression filter.</param>
         /// <param name="skipNumber">The number of documents you want to skip. Default value is 0.</param>
         /// <param name="takeNumber">The number of documents you want to take. Default value is 50.</param>
         /// <param name="partitionKey">An optional partition key.</param>
@@ -1036,7 +1036,7 @@ namespace MongoDbGenericRepository
         /// </summary>
         /// <typeparam name="TDocument">The type representing a Document.</typeparam>
         /// <typeparam name="TKey">The type of the primary key for a Document.</typeparam>
-        /// <param name="filter"></param>
+        /// <param name="filter">A LINQ expression filter.</param>
         /// <param name="skipNumber">The number of documents you want to skip. Default value is 0.</param>
         /// <param name="takeNumber">The number of documents you want to take. Default value is 50.</param>
         /// <param name="partitionKey">An optional partition key.</param>
@@ -1053,7 +1053,7 @@ namespace MongoDbGenericRepository
         /// GetAndUpdateOne with filter
         /// </summary>
         /// <typeparam name="TDocument">The type representing a Document.</typeparam>
-        /// <param name="filter"></param>
+        /// <param name="filter">A LINQ expression filter.</param>
         /// <param name="update"></param>
         /// <param name="options"></param>
         /// <returns></returns>
@@ -1067,7 +1067,7 @@ namespace MongoDbGenericRepository
         /// </summary>
         /// <typeparam name="TDocument">The type representing a Document.</typeparam>
         /// <typeparam name="TKey">The type of the primary key for a Document.</typeparam>
-        /// <param name="filter"></param>
+        /// <param name="filter">A LINQ expression filter.</param>
         /// <param name="update"></param>
         /// <param name="options"></param>
         /// <returns></returns>