Document Identity ​

Besides being serializable, Marten's only other requirement for a .Net type to be a document is the existence of an identifier field or property that Marten can use as the primary key for the document type. The Id can be either a public field or property, and the name must be either id or Id or ID. As of this time, Marten supports these Id types:

1. String. It might be valuable to use a natural key as the identifier, especially if it is valuable within the Identity Map feature of Marten Db. In this case, the user will be responsible for supplying the identifier.
2. Guid. If the id is a Guid, Marten will assign a new value for you when you persist the document for the first time if the id is empty. And for the record, it's pronounced "gwid".
3. CombGuid is a sequential Guid algorithm. It can improve performance over the default Guid as it reduces fragmentation of the PK index.
4. Int or Long. As of right now, Marten uses a HiLo generator approach to assigning numeric identifiers by document type. Marten may support Postgresql sequences or star-based algorithms as later alternatives.
5. When the ID member of a document is not settable or not public a NoOpIdGeneration strategy is used. This ensures that Marten does not set the ID itself, so the ID should be generated manually.
6. A Custom ID generator strategy is used to implement the ID generation strategy yourself.

Marten by default uses the identity value set on documents and only assigns one in case it has no value (Guid.Empty, 0, string.Empty etc).

You can see some example id usages below:

public class Division
{
    // String property as Id
    public string Id { get; set; }
}

public class Category
{
    // Guid's work, fields too
    public Guid Id;
}

public class Invoice
{
    // int's and long's can be the Id
    // "id" is accepted
    public int id { get; set; }
}

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Overriding the Choice of Id Property/Field ​

If you really want to, or you're migrating existing document types from another document database, Marten provides the [Identity] attribute to force Marten to use a property or field as the identifier that doesn't match the "id" or "Id" or "ID" convention:

public class NonStandardDoc
{
    [Identity]
    public string Name;
}

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The identity property or field can also be configured through StoreOptions by using the Schema to obtain a document mapping:

storeOptions.Schema.For<OverriddenIdDoc>().Identity(x => x.Name);

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Guid Identifiers ​

To use CombGuid generation you should enable it when configuring the document store. This defines that the CombGuid generation strategy will be used for all the documents types.

options.Policies.ForAllDocuments(m =>
{
    if (m.IdType == typeof(Guid))
    {
        m.IdStrategy = new CombGuidIdGeneration();
    }
});

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It is also possible use the SequentialGuid id generation algorithm for a specific document type.

options.Schema.For<UserWithGuid>().IdStrategy(new CombGuidIdGeneration());

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Sequential Identifiers with Hilo ​

The Hilo sequence generation can be customized with either global defaults or document type-specific overrides. By default, the Hilo sequence generation in Marten increments by 1 and uses a "maximum lo" number of 1000.

To set different global defaults, use the StoreOptions.HiloSequenceDefaults property like this sample:

var store = DocumentStore.For(_ =>
{
    _.Advanced.HiloSequenceDefaults.MaxLo = 55;
    _.Connection(ConnectionSource.ConnectionString);
    _.DatabaseSchemaName = "sequences";
});

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It's also possible to use one sequence with multiple document types by specifying the same "sequence name".

var store = DocumentStore.For(_ =>
{
    _.Advanced.HiloSequenceDefaults.SequenceName = "Entity";
    _.Connection(ConnectionSource.ConnectionString);

    _.DatabaseSchemaName = "sequences";
});

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To override the Hilo configuration for a specific document type, you can decorate the document type with the [HiloSequence] attribute as in this example:

[HiloSequence(MaxLo = 66, SequenceName = "Entity")]
public class OverriddenHiloDoc
{
    public int Id { get; set; }
}

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You can also use the MartenRegistry fluent interface to override the Hilo configuration for a document type as in this example:

var store = DocumentStore.For(_ =>
{
    // Overriding the Hilo settings for the document type "IntDoc"
    _.Schema.For<IntDoc>()
        .HiloSettings(new HiloSettings {MaxLo = 66});

    _.Connection(ConnectionSource.ConnectionString);

    _.DatabaseSchemaName = "sequences";
});

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Set the HiLo Identifier Floor ​

Marten 1.2 adds a convenience method to reset the "floor" of the Hilo sequence for a single document type:

var store = DocumentStore.For(opts =>
{
    opts.Connection(ConnectionSource.ConnectionString);
    opts.DatabaseSchemaName = "sequences";
});

// Resets the minimum Id number for the IntDoc document
// type to 2500
await store.Tenancy.Default.Database.ResetHiloSequenceFloor<IntDoc>(2500);

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This functionality was added specifically to aid in importing data from an existing data source. Do note that this functionality simply guarantees that all new IDs assigned for the document type will be higher than the new floor. It is perfectly possible, and even likely, that there will be some gaps in the id sequence.

String Identity ​

If you use a document type with a string identity member, you will be responsible for supplying the identity value to Marten on any object passed to any storage API like IDocumentSession.Store(). You can choose to use the Identity Key option for automatic identity generation as shown in the next section.

Identity Key ​

Let's say you have a document type with a string for the identity member like this one:

public class DocumentWithStringId
{
    public string Id { get; set; }
}

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You can use the "identity key" option for identity generation that would create string values of the pattern [type alias]/[sequence] where the type alias is typically the document class name in all lower case and the sequence is a HiLo sequence number.

You can opt into the identity key strategy for identity and even override the document alias name with this syntax:

var store = DocumentStore.For(opts =>
{
    opts.Connection("some connection string");
    opts.Schema.For<DocumentWithStringId>()
        .UseIdentityKey()
        .DocumentAlias("doc");
});

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Custom Identity Strategies ​

A custom ID generator strategy should implement IIdGeneration.

public class CustomIdGeneration : IIdGeneration
{
    public IEnumerable<Type> KeyTypes { get; } = new Type[] {typeof(string)};

    public bool RequiresSequences { get; } = false;
    public void GenerateCode(GeneratedMethod assign, DocumentMapping mapping)
    {
        var document = new Use(mapping.DocumentType);
        assign.Frames.Code($"_setter({{0}}, \"newId\");", document);
        assign.Frames.Code($"return {{0}}.{mapping.IdMember.Name};", document);
    }

}

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The Build() method should return the actual IdGenerator<T> for the document type, where T is the type of the Id field.

For more advances examples you can have a look at existing ID generator: HiloIdGeneration, CombGuidGenerator and the IdentityKeyGeneration,

To use custom id generation you should enable it when configuring the document store. This defines that the strategy will be used for all the documents types.

options.Policies.ForAllDocuments(m =>
{
    if (m.IdType == typeof(string))
    {
        m.IdStrategy = new CustomIdGeneration();
    }
});

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It is also possible define a custom id generation algorithm for a specific document type.

options.Schema.For<UserWithString>().IdStrategy(new CustomIdGeneration());

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