JasperFx · jeremydmiller · Aug 1, 2026 · Aug 1, 2026
diff --git a/src/Marten.Testing/Harness/TenantDatabases.cs b/src/Marten.Testing/Harness/TenantDatabases.cs
@@ -0,0 +1,53 @@
+#nullable enable
+using System.Threading.Tasks;
+using Npgsql;
+using Weasel.Postgresql;
+using Weasel.Postgresql.Migrations;
+
+namespace Marten.Testing.Harness
+{
+    /// <summary>
+    /// Provisions per-tenant databases on the test PostgreSQL server for
+    /// database-per-tenant multi-tenancy tests. Databases are created once and
+    /// reused across runs (creation is expensive); callers are responsible for
+    /// resetting schemas or data inside them.
+    /// </summary>
+    public static class TenantDatabases
+    {
+        /// <summary>
+        /// Ensure the named database exists on the server behind
+        /// <paramref name="conn"/> (an open connection to the master test
+        /// database) and return a connection string targeting it.
+        /// </summary>
+        public static async Task<string> CreateIfNotExistsAsync(NpgsqlConnection conn, string databaseName)
+        {
+            if (!await conn.DatabaseExists(databaseName))
+            {
+                try
+                {
+                    await new DatabaseSpecification().BuildDatabase(conn, databaseName);
+                }
+                catch (PostgresException e) when (e.SqlState is PostgresErrorCodes.DuplicateDatabase
+                                                      or PostgresErrorCodes.UniqueViolation)
+                {
+                    // The test assemblies can run once per target framework, concurrently;
+                    // the check-then-create above is not atomic, so the loser of that race
+                    // sees the database it wanted already there.
+                }
+            }
+
+            return ConnectionStringFor(databaseName);
+        }
+
+        /// <summary>
+        /// A connection string for the named database on the test server.
+        /// </summary>
+        public static string ConnectionStringFor(string databaseName)
+        {
+            return new NpgsqlConnectionStringBuilder(ConnectionSource.ConnectionString)
+            {
+                Database = databaseName
+            }.ConnectionString;
+        }
+    }
+}
diff --git a/src/MultiTenancyTests/DocumentStore_IMartenStorage_implementation.cs b/src/MultiTenancyTests/DocumentStore_IMartenStorage_implementation.cs
@@ -16,25 +16,15 @@
 
 namespace MultiTenancyTests;
 
-[CollectionDefinition("multi-tenancy", DisableParallelization = true)]
+[Collection("multi-tenancy")]
 public class DocumentStore_IMartenStorage_implementation : IAsyncLifetime
 {
     private IHost _host;
     private IDocumentStore theStore;
 
-    private async Task<string> CreateDatabaseIfNotExists(NpgsqlConnection conn, string databaseName)
+    private static async Task<string> CreateDatabaseIfNotExists(NpgsqlConnection conn, string databaseName)
     {
-        var builder = new NpgsqlConnectionStringBuilder(ConnectionSource.ConnectionString);
-
-        var exists = await conn.DatabaseExists(databaseName);
-        if (!exists)
-        {
-            await new DatabaseSpecification().BuildDatabase(conn, databaseName);
-        }
-
-        builder.Database = databaseName;
-
-        var connectionString = builder.ConnectionString;
+        var connectionString = await TenantDatabases.CreateIfNotExistsAsync(conn, databaseName);
 
         await SchemaUtils.DropSchema(connectionString, "multi_tenancy");
         await SchemaUtils.DropSchema(connectionString, "mt_events");

diff --git a/src/MultiTenancyTests/MultiTenancyCollection.cs b/src/MultiTenancyTests/MultiTenancyCollection.cs
@@ -0,0 +1,9 @@
+using Xunit;
+
+namespace MultiTenancyTests;
+
+// The single definition of the "multi-tenancy" collection. The classes in this
+// collection all provision real per-tenant databases against the shared test
+// server, so they must never run concurrently with each other.
+[CollectionDefinition("multi-tenancy", DisableParallelization = true)]
+public class MultiTenancyCollection;
diff --git a/src/MultiTenancyTests/MultiTenancyTests.csproj b/src/MultiTenancyTests/MultiTenancyTests.csproj
@@ -116,6 +116,9 @@
         <Compile Include="..\Marten.Testing\Harness\StoreContext.cs">
             <Link>Harness\StoreContext.cs</Link>
         </Compile>
+        <Compile Include="..\Marten.Testing\Harness\TenantDatabases.cs">
+            <Link>Harness\TenantDatabases.cs</Link>
+        </Compile>
         <Compile Include="..\Marten.Testing\Harness\StoreFixture.cs">
             <Link>Harness\StoreFixture.cs</Link>
         </Compile>

diff --git a/src/MultiTenancyTests/SingleServerMultiTenancyTests.cs b/src/MultiTenancyTests/SingleServerMultiTenancyTests.cs
@@ -13,7 +13,7 @@
 
 namespace MultiTenancyTests;
 
-[CollectionDefinition("multi-tenancy", DisableParallelization = true)]
+[Collection("multi-tenancy")]
 public class SingleServerMultiTenancyTests: IAsyncLifetime
 {
     private DefaultNpgsqlDataSourceFactory dataSourceFactory = new();

diff --git a/src/MultiTenancyTests/projection_statuses_per_database.cs b/src/MultiTenancyTests/projection_statuses_per_database.cs
@@ -67,8 +67,8 @@ public async ValueTask InitializeAsync()
         await using var conn = new NpgsqlConnection(ConnectionSource.ConnectionString);
         await conn.OpenAsync();
         await conn.DropSchemaAsync(MasterSchema);
-        var tenantA = await CreateDatabaseIfNotExists(conn, TenantA);
-        var tenantB = await CreateDatabaseIfNotExists(conn, TenantB);
+        var tenantA = await TenantDatabases.CreateIfNotExistsAsync(conn, TenantA);
+        var tenantB = await TenantDatabases.CreateIfNotExistsAsync(conn, TenantB);
         await conn.CloseAsync();
 
         // Each run starts from sequence 0 so the assertions below are exact.
@@ -170,27 +170,6 @@ private static long SequenceFor(System.Collections.Generic.IReadOnlyList<Project
     private static long HeadFor(System.Collections.Generic.IReadOnlyList<ProjectionStatus> statuses)
         => statuses.Single(x => x.ProjectionName == nameof(WidgetTally)).Shards.Single().EventStoreSequence;
 
-    private static async Task<string> CreateDatabaseIfNotExists(NpgsqlConnection conn, string databaseName)
-    {
-        var builder = new NpgsqlConnectionStringBuilder(ConnectionSource.ConnectionString);
-        if (!await conn.DatabaseExists(databaseName))
-        {
-            try
-            {
-                await new DatabaseSpecification().BuildDatabase(conn, databaseName);
-            }
-            catch (PostgresException e) when (e.SqlState is PostgresErrorCodes.DuplicateDatabase
-                                                  or PostgresErrorCodes.UniqueViolation)
-            {
-                // The test assembly runs once per target framework, concurrently; the check-then-create above
-                // is not atomic, so the loser of that race sees the database it wanted already there.
-            }
-        }
-
-        builder.Database = databaseName;
-        return builder.ConnectionString;
-    }
-
     private static async Task DropSchemaAsync(string connectionString)
     {
         await using var conn = new NpgsqlConnection(connectionString);

diff --git a/src/MultiTenancyTests/using_bucketed_database_sharding_and_document_partitioning.cs b/src/MultiTenancyTests/using_bucketed_database_sharding_and_document_partitioning.cs
@@ -26,7 +26,7 @@
 
 namespace MultiTenancyTests;
 
-[CollectionDefinition("multi-tenancy", DisableParallelization = true)]
+[Collection("multi-tenancy")]
 public class using_bucketed_database_sharding_and_document_partitioning: IAsyncLifetime
 {
     private const int NumberOfPartitions = 4;
@@ -40,20 +40,6 @@ public class using_bucketed_database_sharding_and_document_partitioning: IAsyncL
     private static readonly string TenantBeta = "tenant_beta";
     private static readonly string TenantGamma = "tenant_gamma";
 
-    private async Task<string> CreateDatabaseIfNotExists(NpgsqlConnection conn, string databaseName)
-    {
-        var builder = new NpgsqlConnectionStringBuilder(ConnectionSource.ConnectionString);
-
-        var exists = await conn.DatabaseExists(databaseName);
-        if (!exists)
-        {
-            await new DatabaseSpecification().BuildDatabase(conn, databaseName);
-        }
-
-        builder.Database = databaseName;
-        return builder.ConnectionString;
-    }
-
     public async ValueTask InitializeAsync()
     {
         await using var conn = new NpgsqlConnection(ConnectionSource.ConnectionString);
@@ -64,7 +50,7 @@ public async ValueTask InitializeAsync()
 
         foreach (var name in _dbNames)
         {
-            _connectionStrings[name] = await CreateDatabaseIfNotExists(conn, name);
+            _connectionStrings[name] = await TenantDatabases.CreateIfNotExistsAsync(conn, name);
         }
 
         _registry = BucketRegistry.EvenlySpreadOver(_dbNames);

diff --git a/src/MultiTenancyTests/using_master_table_multi_tenancy.cs b/src/MultiTenancyTests/using_master_table_multi_tenancy.cs
@@ -22,21 +22,6 @@ namespace MultiTenancyTests;
 
 public class master_table_multi_tenancy_independent_auto_create
 {
-    private async Task<string> CreateDatabaseIfNotExists(NpgsqlConnection conn, string databaseName)
-    {
-        var builder = new NpgsqlConnectionStringBuilder(ConnectionSource.ConnectionString);
-
-        var exists = await conn.DatabaseExists(databaseName);
-        if (!exists)
-        {
-            await new DatabaseSpecification().BuildDatabase(conn, databaseName);
-        }
-
-        builder.Database = databaseName;
-
-        return builder.ConnectionString;
-    }
-
     [Fact]
     public async Task can_still_create_own_tables()
     {
@@ -45,10 +30,10 @@ public async Task can_still_create_own_tables()
 
         await conn.DropSchemaAsync("tenants", TestContext.Current.CancellationToken);
 
-        var tenant1ConnectionString = await CreateDatabaseIfNotExists(conn, "tenant1");
-        var tenant2ConnectionString = await CreateDatabaseIfNotExists(conn, "tenant2");
-        var tenant3ConnectionString = await CreateDatabaseIfNotExists(conn, "tenant3");
-        var tenant4ConnectionString = await CreateDatabaseIfNotExists(conn, "tenant4");
+        var tenant1ConnectionString = await TenantDatabases.CreateIfNotExistsAsync(conn, "tenant1");
+        var tenant2ConnectionString = await TenantDatabases.CreateIfNotExistsAsync(conn, "tenant2");
+        var tenant3ConnectionString = await TenantDatabases.CreateIfNotExistsAsync(conn, "tenant3");
+        var tenant4ConnectionString = await TenantDatabases.CreateIfNotExistsAsync(conn, "tenant4");
 
         await conn.CloseAsync();
 
@@ -86,7 +71,7 @@ public async Task can_still_create_own_tables()
 }
 
 
-[CollectionDefinition("multi-tenancy", DisableParallelization = true)]
+[Collection("multi-tenancy")]
 public class master_table_multi_tenancy_seeding : IAsyncLifetime
 {
     private IHost _host;
@@ -96,21 +81,6 @@ public class master_table_multi_tenancy_seeding : IAsyncLifetime
     private string tenant3ConnectionString;
     private string tenant4ConnectionString;
 
-    private async Task<string> CreateDatabaseIfNotExists(NpgsqlConnection conn, string databaseName)
-    {
-        var builder = new NpgsqlConnectionStringBuilder(ConnectionSource.ConnectionString);
-
-        var exists = await conn.DatabaseExists(databaseName);
-        if (!exists)
-        {
-            await new DatabaseSpecification().BuildDatabase(conn, databaseName);
-        }
-
-        builder.Database = databaseName;
-
-        return builder.ConnectionString;
-    }
-
     public async ValueTask InitializeAsync()
     {
         await using var conn = new NpgsqlConnection(ConnectionSource.ConnectionString);
@@ -119,10 +89,10 @@ public async ValueTask InitializeAsync()
         await conn.DropSchemaAsync("tenants");
 
 
-        tenant1ConnectionString = await CreateDatabaseIfNotExists(conn, "tenant1");
-        tenant2ConnectionString = await CreateDatabaseIfNotExists(conn, "tenant2");
-        tenant3ConnectionString = await CreateDatabaseIfNotExists(conn, "tenant3");
-        tenant4ConnectionString = await CreateDatabaseIfNotExists(conn, "tenant4");
+        tenant1ConnectionString = await TenantDatabases.CreateIfNotExistsAsync(conn, "tenant1");
+        tenant2ConnectionString = await TenantDatabases.CreateIfNotExistsAsync(conn, "tenant2");
+        tenant3ConnectionString = await TenantDatabases.CreateIfNotExistsAsync(conn, "tenant3");
+        tenant4ConnectionString = await TenantDatabases.CreateIfNotExistsAsync(conn, "tenant4");
 
         _host = await Host.CreateDefaultBuilder()
             .ConfigureServices(services =>
@@ -199,7 +169,7 @@ public async Task can_be_immediately_using_tenant_databases_that_were_seeded()
 
 }
 
-[CollectionDefinition("multi-tenancy", DisableParallelization = true)]
+[Collection("multi-tenancy")]
 public class using_master_table_multi_tenancy : IAsyncLifetime
 {
     private IHost _host;
@@ -209,21 +179,6 @@ public class using_master_table_multi_tenancy : IAsyncLifetime
     private string tenant3ConnectionString;
     private string tenant4ConnectionString;
 
-    private async Task<string> CreateDatabaseIfNotExists(NpgsqlConnection conn, string databaseName)
-    {
-        var builder = new NpgsqlConnectionStringBuilder(ConnectionSource.ConnectionString);
-
-        var exists = await conn.DatabaseExists(databaseName);
-        if (!exists)
-        {
-            await new DatabaseSpecification().BuildDatabase(conn, databaseName);
-        }
-
-        builder.Database = databaseName;
-
-        return builder.ConnectionString;
-    }
-
     public async ValueTask InitializeAsync()
     {
         await using var conn = new NpgsqlConnection(ConnectionSource.ConnectionString);
@@ -232,10 +187,10 @@ public async ValueTask InitializeAsync()
         await conn.DropSchemaAsync("tenants");
 
 
-        tenant1ConnectionString = await CreateDatabaseIfNotExists(conn, "tenant1");
-        tenant2ConnectionString = await CreateDatabaseIfNotExists(conn, "tenant2");
-        tenant3ConnectionString = await CreateDatabaseIfNotExists(conn, "tenant3");
-        tenant4ConnectionString = await CreateDatabaseIfNotExists(conn, "tenant4");
+        tenant1ConnectionString = await TenantDatabases.CreateIfNotExistsAsync(conn, "tenant1");
+        tenant2ConnectionString = await TenantDatabases.CreateIfNotExistsAsync(conn, "tenant2");
+        tenant3ConnectionString = await TenantDatabases.CreateIfNotExistsAsync(conn, "tenant3");
+        tenant4ConnectionString = await TenantDatabases.CreateIfNotExistsAsync(conn, "tenant4");
 
         _host = await Host.CreateDefaultBuilder()
             .ConfigureServices(services =>
@@ -282,10 +237,10 @@ public async Task run_describe()
         await conn.DropSchemaAsync("tenants", TestContext.Current.CancellationToken);
 
 
-        tenant1ConnectionString = await CreateDatabaseIfNotExists(conn, "tenant1");
-        tenant2ConnectionString = await CreateDatabaseIfNotExists(conn, "tenant2");
-        tenant3ConnectionString = await CreateDatabaseIfNotExists(conn, "tenant3");
-        tenant4ConnectionString = await CreateDatabaseIfNotExists(conn, "tenant4");
+        tenant1ConnectionString = await TenantDatabases.CreateIfNotExistsAsync(conn, "tenant1");
+        tenant2ConnectionString = await TenantDatabases.CreateIfNotExistsAsync(conn, "tenant2");
+        tenant3ConnectionString = await TenantDatabases.CreateIfNotExistsAsync(conn, "tenant3");
+        tenant4ConnectionString = await TenantDatabases.CreateIfNotExistsAsync(conn, "tenant4");
 
 
         var result = await Host.CreateDefaultBuilder()