Revised multi tenant connection string resolver.

src/Volo.Abp.MultiTenancy/Volo/Abp/Data/MultiTenancy/ITenantConnectionStringStore.cs

@@ -5,6 +5,9 @@ namespace Volo.Abp.Data.MultiTenancy
     public interface ITenantConnectionStringStore
     {
         [CanBeNull]
-        string GetConnectionStringOrNull([NotNull] string tenantId, [CanBeNull] string connStringName);
+        string GetDefaultConnectionStringOrNull([NotNull] string tenantId);
+
+        [CanBeNull]
+        string GetConnectionStringOrNull([NotNull] string tenantId, [NotNull] string connStringName);
     }
 }

src/Volo.Abp.MultiTenancy/Volo/Abp/Data/MultiTenancy/MultiTenantConnectionStringResolver.cs

@@ -1,10 +1,10 @@
 using Microsoft.Extensions.Options;
 using Volo.Abp.MultiTenancy;
 using Volo.DependencyInjection;
+using Volo.ExtensionMethods.Collections.Generic;
 
 namespace Volo.Abp.Data.MultiTenancy
 {
-    //TODO: It would be better to use composition over inheritance on connection string resolve progress!
     //TODO: Create a replace service registration convention, instead of custom registration in AbpMultiTenancyModule?
 
     [DisableConventionalRegistration]
@@ -26,20 +26,40 @@ namespace Volo.Abp.Data.MultiTenancy
         public override string Resolve(string connectionStringName = null)
         {
             var tenant = _multiTenancyManager.CurrentTenant;
+
+            //No current tenant, fallback to default logic
             if (tenant == null)
             {
                 return base.Resolve(connectionStringName);
             }
 
-            var connectionString = _tenantConnectionStringStore.GetConnectionStringOrNull(tenant.Id, connectionStringName);
-            if (connectionString == null)
+            //Requesting default connection string
+            if (connectionStringName == null)
             {
-                return base.Resolve(connectionStringName);
+                return _tenantConnectionStringStore.GetDefaultConnectionStringOrNull(tenant.Id) ??
+                       Options.ConnectionStrings.Default;
             }
 
-            //TODO: If given tenant did not specified a connectionStringName specific connection string, then use the default connection string for connectionStringName, not tenant's default database
+            //Requesting specific connection string
+            var connString = _tenantConnectionStringStore.GetConnectionStringOrNull(tenant.Id, connectionStringName);
+            if (connString != null)
+            {
+                return connString;
+            }
+
+            /* Requested a specific connection string, but it's not specified for the tenant.
+             * - If it's specified in options, use it.
+             * - If not, use tenant's default conn string.
+             */
+                   
+            var connStringInOptions = Options.ConnectionStrings.GetOrDefault(connectionStringName);
+            if (connStringInOptions != null)
+            {
+                return connStringInOptions;
+            }
 
-            return connectionString;
+            return _tenantConnectionStringStore.GetDefaultConnectionStringOrNull(tenant.Id) ??
+                   Options.ConnectionStrings.Default;
         }
     }
 }

src/Volo.Abp.MultiTenancy/Volo/Abp/Data/MultiTenancy/NullTenantConnectionStringStore.cs

@@ -4,6 +4,12 @@ namespace Volo.Abp.Data.MultiTenancy
 {
     public sealed class NullTenantConnectionStringStore : ITenantConnectionStringStore, ISingletonDependency
     {
+        public string GetDefaultConnectionStringOrNull(string tenantId)
+        {
+            //No tenant specific connection string by default
+            return null;
+        }
+
         public string GetConnectionStringOrNull(string tenantId, string connStringName)
         {
             //No tenant specific connection string by default

src/Volo.Abp/Volo/Abp/Data/DefaultConnectionStringResolver.cs

@@ -7,11 +7,11 @@ namespace Volo.Abp.Data
 {
     public class DefaultConnectionStringResolver : IConnectionStringResolver, ITransientDependency
     {
-        private readonly DbConnectionOptions _options;
+        protected DbConnectionOptions Options { get; }
 
         public DefaultConnectionStringResolver(IOptionsSnapshot<DbConnectionOptions> options)
         {
-            _options = options.Value;
+            Options = options.Value;
         }
 
         public virtual string Resolve(string connectionStringName = null)
@@ -19,7 +19,7 @@ namespace Volo.Abp.Data
             //Get module specific value if provided
             if (!connectionStringName.IsNullOrEmpty())
             {
-                var moduleConnString = _options.ConnectionStrings.GetOrDefault(connectionStringName);
+                var moduleConnString = Options.ConnectionStrings.GetOrDefault(connectionStringName);
                 if (!moduleConnString.IsNullOrEmpty())
                 {
                     return moduleConnString;
@@ -27,7 +27,7 @@ namespace Volo.Abp.Data
             }
             
             //Get default value
-            return _options.ConnectionStrings.Default;
+            return Options.ConnectionStrings.Default;
         }
     }
 }

src/Volo.Abp/Volo/Abp/Data/IConnectionStringResolver.cs

@@ -1,7 +1,10 @@
-namespace Volo.Abp.Data
+using JetBrains.Annotations;
+
+namespace Volo.Abp.Data
 {
     public interface IConnectionStringResolver
     {
+        [NotNull]
         string Resolve(string connectionStringName = null);
     }
 }

test/Volo.Abp.MultiTenancy.Tests/Volo/Abp/Data/MultiTenancy/MultiTenantConnectionStringResolver_Tests.cs

@@ -3,6 +3,7 @@ using Microsoft.Extensions.DependencyInjection.Extensions;
 using Microsoft.Extensions.Options;
 using Shouldly;
 using Volo.Abp.MultiTenancy;
+using Volo.ExtensionMethods.Collections.Generic;
 using Xunit;
 
 namespace Volo.Abp.Data.MultiTenancy
@@ -73,22 +74,12 @@ namespace Volo.Abp.Data.MultiTenancy
 
             public string GetConnectionStringOrNull(string tenantId, string connStringName)
             {
-                if (connStringName != null)
-                {
-                    if (_options.Value.ConnectionStrings.ContainsKey(tenantId + "#" + connStringName))
-                    {
-                        return _options.Value.ConnectionStrings[tenantId + "#" + connStringName];
-                    }
-                }
-                else
-                {
-                    if (_options.Value.ConnectionStrings.ContainsKey(tenantId + "#Default"))
-                    {
-                        return _options.Value.ConnectionStrings[tenantId + "#Default"];
-                    }
-                }
+                return _options.Value.ConnectionStrings.GetOrDefault(tenantId + "#" + connStringName);
+            }
 
-                return null;
+            public string GetDefaultConnectionStringOrNull(string tenantId)
+            {
+                return _options.Value.ConnectionStrings.GetOrDefault(tenantId + "#Default");
             }
         }
     }