Refactored.

src/Volo.Abp/Volo/Abp/Uow/ChildUnitOfWork.cs

@@ -79,6 +79,11 @@ namespace Volo.Abp.Uow
             return _parent.FindDatabaseApi(key);
         }
 
+        public void AddDatabaseApi(string key, IDatabaseApi api)
+        {
+            _parent.AddDatabaseApi(key, api);
+        }
+
         public IDatabaseApi GetOrAddDatabaseApi(string key, Func<IDatabaseApi> factory)
         {
             return _parent.GetOrAddDatabaseApi(key, factory);
@@ -94,6 +99,11 @@ namespace Volo.Abp.Uow
             _parent.AddTransactionApi(key, api);
         }
 
+        public ITransactionApi GetOrAddTransactionApi(string key, Func<ITransactionApi> factory)
+        {
+            return _parent.GetOrAddTransactionApi(key, factory);
+        }
+
         public void Dispose()
         {
 

src/Volo.Abp/Volo/Abp/Uow/IDatabaseApiContainer.cs

@@ -9,6 +9,8 @@ namespace Volo.Abp.Uow
         [CanBeNull]
         IDatabaseApi FindDatabaseApi([NotNull] string key);
 
+        void AddDatabaseApi([NotNull] string key, [NotNull] IDatabaseApi api);
+
         [NotNull]
         IDatabaseApi GetOrAddDatabaseApi([NotNull] string key, [NotNull] Func<IDatabaseApi> factory);
     }

src/Volo.Abp/Volo/Abp/Uow/ITransactionApiContainer.cs

@@ -1,4 +1,5 @@
-using JetBrains.Annotations;
+using System;
+using JetBrains.Annotations;
 
 namespace Volo.Abp.Uow
 {
@@ -8,5 +9,8 @@ namespace Volo.Abp.Uow
         ITransactionApi FindTransactionApi([NotNull] string key);
 
         void AddTransactionApi([NotNull] string key, [NotNull] ITransactionApi api);
+
+        [NotNull]
+        ITransactionApi GetOrAddTransactionApi([NotNull] string key, [NotNull] Func<ITransactionApi> factory);
     }
 }

src/Volo.Abp/Volo/Abp/Uow/UnitOfWork.cs

@@ -2,7 +2,6 @@
 using System.Collections.Generic;
 using System.Threading;
 using System.Threading.Tasks;
-using JetBrains.Annotations;
 using Microsoft.Extensions.Options;
 using Volo.Abp.DependencyInjection;
 
@@ -115,6 +114,19 @@ namespace Volo.Abp.Uow
             return _databaseApis.GetOrDefault(key);
         }
 
+        public void AddDatabaseApi(string key, IDatabaseApi api)
+        {
+            Check.NotNull(key, nameof(key));
+            Check.NotNull(api, nameof(api));
+
+            if (_databaseApis.ContainsKey(key))
+            {
+                throw new AbpException("There is already a database API in this unit of work with given key: " + key);
+            }
+
+            _databaseApis.Add(key, api);
+        }
+
         public IDatabaseApi GetOrAddDatabaseApi(string key, Func<IDatabaseApi> factory)
         {
             Check.NotNull(key, nameof(key));
@@ -137,12 +149,20 @@ namespace Volo.Abp.Uow
 
             if (_transactionApis.ContainsKey(key))
             {
-                throw new AbpException("There is already a transaction API in this unit of work!");
+                throw new AbpException("There is already a transaction API in this unit of work with given key: " + key);
             }
 
             _transactionApis.Add(key, api);
         }
 
+        public ITransactionApi GetOrAddTransactionApi(string key, Func<ITransactionApi> factory)
+        {
+            Check.NotNull(key, nameof(key));
+            Check.NotNull(factory, nameof(factory));
+
+            return _transactionApis.GetOrAdd(key, factory);
+        }
+
         protected virtual void OnCompleted()
         {
             Completed.InvokeSafely(this);