Checking Document Existence

Sometimes you only need to know whether a document with a given id exists in the database, without actually loading and deserializing the full document. Marten provides the CheckExistsAsync API for this purpose, which issues a lightweight SELECT EXISTS(...) query against PostgreSQL. This avoids the overhead of JSON deserialization and object materialization, making it significantly more efficient than loading the document just to check if it's there.

Usage

CheckExistsAsync<T> is available on IQuerySession (and therefore also on IDocumentSession). It supports all identity types: Guid, int, long, string, and strongly-typed identifiers.

[Fact]
public async Task check_exists_by_object_id()
{
    var doc = new GuidDoc { Id = Guid.NewGuid() };
    theSession.Store(doc);
    await theSession.SaveChangesAsync();

    // Use the object overload for dynamic id types
    var exists = await theSession.CheckExistsAsync<GuidDoc>((object)doc.Id);
    exists.ShouldBeTrue();
}

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Supported Identity Types

Id Type	Supported
Guid	Yes
int	Yes
long	Yes
string	Yes
object	Yes (for dynamic id types)
Strong-typed ids (Vogen, record structs, etc.)	Yes (via object overload)

Batched Queries

CheckExists<T> is also available as part of batched queries, allowing you to check existence of multiple documents in a single round-trip to the database:

[Fact]
public async Task check_exists_in_batch_by_guid_id()
{
    var doc = new GuidDoc { Id = Guid.NewGuid() };
    theSession.Store(doc);
    await theSession.SaveChangesAsync();

    var batch = theSession.CreateBatchQuery();
    var existsHit = batch.CheckExists<GuidDoc>(doc.Id);
    var existsMiss = batch.CheckExists<GuidDoc>(Guid.NewGuid());
    await batch.Execute();

    (await existsHit).ShouldBeTrue();
    (await existsMiss).ShouldBeFalse();
}

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Behavior Notes

• Returns true if the document exists, false otherwise.
• Respects soft-delete filters: if a document type uses soft deletes, a soft-deleted document will return false.
• Respects multi-tenancy: the check is scoped to the current session's tenant.
• Does not load the document into the identity map or trigger any deserialization.