first step to split function into separate classes for better reuse and easier maintenance.

MongoDbGenericRepository/DataAccess/Base/DataAccessBase.cs

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+using MongoDB.Driver;
+using MongoDB.Driver.Linq;
+using MongoDbGenericRepository.Models;
+using System;
+using System.Linq;
+using System.Linq.Expressions;
+
+namespace MongoDbGenericRepository.DataAccess.Base
+{
+    public class DataAccessBase
+    {
+        protected IMongoDbContext MongoDbContext;
+
+        public DataAccessBase(IMongoDbContext mongoDbContext)
+        {
+            MongoDbContext = mongoDbContext;
+        }
+
+        #region Utility Methods
+
+        protected virtual IMongoQueryable<TDocument> GetQuery<TDocument, TKey>(Expression<Func<TDocument, bool>> filter, string partitionKey = null)
+            where TDocument : IDocument<TKey>
+            where TKey : IEquatable<TKey>
+        {
+            return GetCollection<TDocument, TKey>(partitionKey).AsQueryable().Where(filter);
+        }
+
+        /// <summary>
+        /// Gets a collections for a potentially partitioned document type.
+        /// </summary>
+        /// <typeparam name="TDocument">The document type.</typeparam>
+        /// <typeparam name="TKey">The type of the primary key.</typeparam>
+        /// <param name="document">The document.</param>
+        /// <returns></returns>
+        protected virtual IMongoCollection<TDocument> HandlePartitioned<TDocument, TKey>(TDocument document)
+            where TDocument : IDocument<TKey>
+            where TKey : IEquatable<TKey>
+        {
+            if (document is IPartitionedDocument)
+            {
+                return GetCollection<TDocument, TKey>(((IPartitionedDocument)document).PartitionKey);
+            }
+            return GetCollection<TDocument, TKey>();
+        }
+
+        /// <summary>
+        /// Gets a collections for the type TDocument with a partition key.
+        /// </summary>
+        /// <typeparam name="TDocument">The document type.</typeparam>
+        /// <typeparam name="TKey">The type of the primary key.</typeparam>
+        /// <param name="partitionKey">The collection partition key.</param>
+        /// <returns></returns>
+        protected virtual IMongoCollection<TDocument> GetCollection<TDocument, TKey>(string partitionKey = null)
+            where TDocument : IDocument<TKey>
+            where TKey : IEquatable<TKey>
+        {
+            return MongoDbContext.GetCollection<TDocument>(partitionKey);
+        }
+
+        /// <summary>
+        /// Gets a collections for a potentially partitioned document type.
+        /// </summary>
+        /// <typeparam name="TDocument">The document type.</typeparam>
+        /// <typeparam name="TKey">The type of the primary key.</typeparam>
+        /// <param name="partitionKey">The collection partition key.</param>
+        /// <returns></returns>
+        protected virtual IMongoCollection<TDocument> HandlePartitioned<TDocument, TKey>(string partitionKey)
+            where TDocument : IDocument<TKey>
+            where TKey : IEquatable<TKey>
+        {
+            if (!string.IsNullOrEmpty(partitionKey))
+            {
+                return GetCollection<TDocument, TKey>(partitionKey);
+            }
+            return GetCollection<TDocument, TKey>();
+        }
+
+        /// <summary>
+        /// Converts a LINQ expression of TDocument, TValue to a LINQ expression of TDocument, object
+        /// </summary>
+        /// <typeparam name="TDocument">The document type.</typeparam>
+        /// <typeparam name="TValue">The type of the value.</typeparam>
+        /// <param name="expression">The expression to convert</param>
+        protected virtual Expression<Func<TDocument, object>> ConvertExpression<TDocument, TValue>(Expression<Func<TDocument, TValue>> expression)
+        {
+            var param = expression.Parameters[0];
+            Expression body = expression.Body;
+            var convert = Expression.Convert(body, typeof(object));
+            return Expression.Lambda<Func<TDocument, object>>(convert, param);
+        }
+
+        /// <summary>
+        /// Maps a IndexCreationOptions object to a MongoDB.Driver.CreateIndexOptions object
+        /// </summary>
+        /// <param name="indexCreationOptions">The options for creating an index.</param>
+        /// <returns></returns>
+        protected virtual CreateIndexOptions MapIndexOptions(IndexCreationOptions indexCreationOptions)
+        {
+            return new CreateIndexOptions
+            {
+                Unique = indexCreationOptions.Unique,
+                TextIndexVersion = indexCreationOptions.TextIndexVersion,
+                SphereIndexVersion = indexCreationOptions.SphereIndexVersion,
+                Sparse = indexCreationOptions.Sparse,
+                Name = indexCreationOptions.Name,
+                Min = indexCreationOptions.Min,
+                Max = indexCreationOptions.Max,
+                LanguageOverride = indexCreationOptions.LanguageOverride,
+                ExpireAfter = indexCreationOptions.ExpireAfter,
+                DefaultLanguage = indexCreationOptions.DefaultLanguage,
+                BucketSize = indexCreationOptions.BucketSize,
+                Bits = indexCreationOptions.Bits,
+                Background = indexCreationOptions.Background,
+                Version = indexCreationOptions.Version
+            };
+        }
+
+
+        /// <summary>
+        /// Gets the minimum value of a property in a mongodb collections that is satisfying the filter.
+        /// </summary>
+        /// <typeparam name="TDocument">The document type.</typeparam>
+        /// <typeparam name="TKey">The type of the primary key.</typeparam>
+        /// <typeparam name="TValue">The type of the value used to order the query.</typeparam>
+        /// <param name="filter">A LINQ expression filter.</param>
+        /// <param name="minValueSelector">A property selector to order by ascending.</param>
+        /// <param name="partitionKey">An optional partition key.</param>
+        protected virtual IFindFluent<TDocument, TDocument> GetMinMongoQuery<TDocument, TKey, TValue>(Expression<Func<TDocument, bool>> filter, Expression<Func<TDocument, TValue>> minValueSelector, string partitionKey = null)
+                    where TDocument : IDocument<TKey>
+                    where TKey : IEquatable<TKey>
+        {
+            return GetCollection<TDocument, TKey>(partitionKey).Find(Builders<TDocument>.Filter.Where(filter))
+                                                               .SortBy(ConvertExpression(minValueSelector))
+                                                               .Limit(1);
+        }
+
+        /// <summary>
+        /// Gets the minimum value of a property in a mongodb collections that is satisfying the filter.
+        /// </summary>
+        /// <typeparam name="TDocument">The document type.</typeparam>
+        /// <typeparam name="TKey">The type of the primary key.</typeparam>
+        /// <typeparam name="TValue">The type of the value used to order the query.</typeparam>
+        /// <param name="filter">A LINQ expression filter.</param>
+        /// <param name="maxValueSelector">A property selector to order by descending.</param>
+        /// <param name="partitionKey">An optional partition key.</param>
+        protected virtual IFindFluent<TDocument, TDocument> GetMaxMongoQuery<TDocument, TKey, TValue>(Expression<Func<TDocument, bool>> filter, Expression<Func<TDocument, TValue>> maxValueSelector, string partitionKey = null)
+                    where TDocument : IDocument<TKey>
+                    where TKey : IEquatable<TKey>
+        {
+            return GetCollection<TDocument, TKey>(partitionKey).Find(Builders<TDocument>.Filter.Where(filter))
+                                                               .SortByDescending(ConvertExpression(maxValueSelector))
+                                                               .Limit(1);
+        }
+
+
+        #endregion
+    }
+}