Initial Baseline Data

Seed reference data when the application starts.

public class SeedCountries : IInitialData
{
    public async Task Populate(IDocumentStore store, CancellationToken token)
    {
        await using var session = store.LightweightSession();

        session.Store(new Country { Id = "no", Name = "Norway" });
        session.Store(new Country { Id = "se", Name = "Sweden" });

        await session.SaveChangesAsync(token);
    }
}

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builder.Services.AddFisher(opts =>
{
    opts.Connection("Data Source=app.db");
    opts.InitialData.Add(new SeedCountries());
})
.ApplyAllDatabaseChangesOnStartup()   // must come first
.SeedInitialDataOnStartup();

Order matters, and Fisher enforces it

WARNING

SeedInitialDataOnStartup() refuses to be registered before ApplyAllDatabaseChangesOnStartup().

Hosted services start in registration order, so the other way round writes to tables that do not exist yet — and that presents as no such table, which names the table and not the mistake.

There is no "already seeded" marker

TIP

Deliberately. A seeder that upserts by a known id is idempotent for free, which is what every useful seeder does — and a marker table would be a table nobody asked for holding a claim Fisher cannot verify. Both siblings say the same.

So write your seeders to be safely re-runnable:

// Good: an upsert by a known id
session.Store(new Country { Id = "no", Name = "Norway" });

// Bad: unconditional inserts with generated ids
session.Insert(new Country { Id = Guid.NewGuid().ToString(), Name = "Norway" });

Seeding without the host

foreach (var seeder in store.Options.InitialData)
{
    await seeder.Populate(store, token);
}

Multi-tenancy

An IInitialData gets the store, so it decides which tenants it seeds:

public async Task Populate(IDocumentStore store, CancellationToken token)
{
    foreach (var tenantId in _tenants)
    {
        await using var session = store.LightweightSession(tenantId);
        session.Store(new Country { Id = "no", Name = "Norway" });
        await session.SaveChangesAsync(token);
    }
}

Under database-per-tenant that is a session per file, so each seeds its own database.