feature: extension method on entity repository to process each command with the previous entity

useful for transaction subscribers

Author: the-avid-engineer

src/EntityDb.Common.Tests/Extensions/IEntityRepositoryExtensions.cs

@@ -0,0 +1,53 @@
+using EntityDb.Abstractions.Entities;
+using EntityDb.Abstractions.Transactions;
+using System;
+using System.Linq;
+using System.Threading.Tasks;
+
+namespace EntityDb.Common.Tests.Extensions
+{
+    /// <summary>
+    ///     Extensions on <see cref="IEntityRepository{TEntity}"/>
+    /// </summary>
+    public static class EntityRepositoryExtensions
+    {
+        public static async Task ForEachCommandWithPreviousEntity<TEntity>
+        (
+            this IEntityRepository<TEntity> entityRepository,
+            ITransaction<TEntity> transaction,
+            Action<TEntity> processor
+        )
+        {
+            var entityTaskDictionary = transaction.Commands
+                .GroupBy(transactionCommand => transactionCommand.EntityId)
+                .ToDictionary
+                (
+                    group => group.Key,
+                    group =>
+                    {
+                        var firstCommand = group.First();
+
+                        return entityRepository.GetAtVersion(firstCommand.EntityId, firstCommand.EntityVersionNumber - 1);
+                    }
+                );
+
+            await Task.WhenAll(entityTaskDictionary.Values);
+
+            var entityDictionary = entityTaskDictionary
+                .ToDictionary
+                (
+                    x => x.Key,
+                    x => x.Value.Result
+                );
+
+            foreach (var transactionCommand in transaction.Commands)
+            {
+                var previousEntity = entityDictionary[transactionCommand.EntityId];
+                
+                processor.Invoke(previousEntity);
+
+                entityDictionary[transactionCommand.EntityId] = transactionCommand.Command.Reduce(previousEntity);
+            }
+        }
+    }
+}