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Schema Feature Extensions

New in Marten 2.4.0 is the ability to add additional features with custom database schema objects that simply plug into Marten's [schema management facilities)[/schema/migrations). The key abstraction is the IFeatureSchema interface from the Weasel.Core library.

TODO -- should link to Weasel. And document Weasel I suppose. Ugh.

Not to worry though, Marten comes with a base class that makes it a bit simpler to build out new features. Here's a very simple example that defines a custom table with one column:

public class FakeStorage : FeatureSchemaBase
{
    private readonly StoreOptions _options;

    public FakeStorage(StoreOptions options) : base("fake", options.Advanced.Migrator)
    {
        _options = options;
    }

    protected override IEnumerable<ISchemaObject> schemaObjects()
    {
        var table = new Table(new DbObjectName(_options.DatabaseSchemaName, "mt_fake_table"));
        table.AddColumn("name", "varchar");

        yield return table;
    }
}

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Now, to actually apply this feature to your Marten applications, use this syntax:

var store = DocumentStore.For(_ =>
{
    // Creates a new instance of FakeStorage and
    // passes along the current StoreOptions
    _.Storage.Add<FakeStorage>();

    // or

    _.Storage.Add(new FakeStorage(_));
});

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Do note that when you use the Add<T>() syntax, Marten will pass along the current StoreOptions to the constructor function if there is a constructor with that signature. Otherwise, it uses the no-arg constructor.

While you can directly implement the ISchemaObject interface for something Marten doesn't already support, it's probably far easier to just configure one of the existing implementations shown in the following sections.

  • Table
  • Function
  • Sequence

Table

Postgresql tables can be modeled with the Table class as shown in this example from the event store inside of Marten:

internal class EventsTable: Table
{
    public EventsTable(EventGraph events): base(new DbObjectName(events.DatabaseSchemaName, "mt_events"))
    {
        AddColumn(new EventTableColumn("seq_id", x => x.Sequence)).AsPrimaryKey();
        AddColumn(new EventTableColumn("id", x => x.Id)).NotNull();
        AddColumn(new StreamIdColumn(events));

        AddColumn(new EventTableColumn("version", x => x.Version)).NotNull();
        AddColumn<EventJsonDataColumn>();
        AddColumn<EventTypeColumn>();
        AddColumn(new EventTableColumn("timestamp", x => x.Timestamp))
            .NotNull().DefaultValueByString("(now())");

        AddColumn<TenantIdColumn>();

        AddColumn<DotNetTypeColumn>().AllowNulls();

        AddIfActive(events.Metadata.CorrelationId);
        AddIfActive(events.Metadata.CausationId);
        AddIfActive(events.Metadata.Headers);

        if (events.TenancyStyle == TenancyStyle.Conjoined)
        {
            ForeignKeys.Add(new ForeignKey("fkey_mt_events_stream_id_tenant_id")
            {
                ColumnNames = new string[]{"stream_id", TenantIdColumn.Name},
                LinkedNames = new string[]{"id", TenantIdColumn.Name},
                LinkedTable = new DbObjectName(events.DatabaseSchemaName, "mt_streams")
            });

            Indexes.Add(new IndexDefinition("pk_mt_events_stream_and_version")
            {
                IsUnique = true,
                Columns = new string[]{"stream_id", TenantIdColumn.Name, "version"}
            });
        }
        else
        {
            ForeignKeys.Add(new ForeignKey("fkey_mt_events_stream_id")
            {
                ColumnNames = new string[]{"stream_id"},
                LinkedNames = new string[]{"id"},
                LinkedTable = new DbObjectName(events.DatabaseSchemaName, "mt_streams"),
                OnDelete = CascadeAction.Cascade
            });

            Indexes.Add(new IndexDefinition("pk_mt_events_stream_and_version")
            {
                IsUnique = true,
                Columns = new string[]{"stream_id", "version"}
            });
        }

        Indexes.Add(new IndexDefinition("pk_mt_events_id_unique")
        {
            Columns = new string[]{"id"},
            IsUnique = true
        });

        AddColumn<IsArchivedColumn>();
    }

    internal IList<IEventTableColumn> SelectColumns()
    {
        var columns = new List<IEventTableColumn>();
        columns.AddRange(Columns.OfType<IEventTableColumn>());

        var data = columns.OfType<EventJsonDataColumn>().Single();
        var typeName = columns.OfType<EventTypeColumn>().Single();
        var dotNetTypeName = columns.OfType<DotNetTypeColumn>().Single();

        columns.Remove(data);
        columns.Insert(0, data);
        columns.Remove(typeName);
        columns.Insert(1, typeName);
        columns.Remove(dotNetTypeName);
        columns.Insert(2, dotNetTypeName);

        return columns;
    }

    private void AddIfActive(MetadataColumn column)
    {
        if (column.Enabled)
        {
            AddColumn(column);
        }
    }
}

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Function

Postgresql functions can be managed by creating a subclass of the Function base class as shown below from the big "append event" function in the event store:

// TODO: Add sample

Sequence

Postgresql sequences can be managed with this usage:

var sequence = new Sequence(new DbObjectName(DatabaseSchemaName, "mt_events_sequence"))
{
    Owner = eventsTable.Identifier,
    OwnerColumn = "seq_id"
};

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