db-subclass-to-dto/tests/Db.Tests/AfterPersistenceTests.cs

using Domain;
using static TestData.ClientBuilder;
using static TestData.OrderBuilder;

namespace Db.Tests;

/// <summary>The fake <see cref="DbContext"/> stores data-only records, never domain objects; Save maps state into rows (insert or refresh), the graph flattened to ids.</summary>
public class AfterPersistenceTests
{
    [Fact]
    public void Save_maps_domain_state_into_data_records()
    {
        var db = new DbContext();
        var order = ValidOrder().WithDescription("one").WithTotal(150_00).Build();
        var client = ValidClient().WithName("Acme").WithOrder(order).Build();

        db.Save(client);
        db.Save(order);

        var clientRow = Assert.Single(db.Clients);
        Assert.Equal(client.Id, clientRow.Id);
        Assert.Equal("Acme", clientRow.Name);

        var orderRow = Assert.Single(db.Orders);
        Assert.Equal(order.Id, orderRow.Id);
        Assert.Equal("one", orderRow.Description);
        Assert.Equal(150_00, orderRow.TotalInCents);
        Assert.Equal(client.Id, orderRow.ClientId);   // the graph is flattened to a key
    }

    [Fact]
    public void An_order_without_client_stores_an_empty_client_id()
    {
        var db = new DbContext();
        var order = ValidOrder().WithDescription("orphan").Build();

        db.Save(order);

        Assert.Equal(Guid.Empty, Assert.Single(db.Orders).ClientId);
    }

    [Fact]
    public void Saving_again_refreshes_the_row_instead_of_duplicating_it()
    {
        var db = new DbContext();
        var order = ValidOrder().WithDescription("one").WithTotal(150_00).Build();
        var client = ValidClient().WithName("Acme").WithOrder(order).Build();
        db.Save(client);
        db.Save(order);

        order.Description = "one (edited)";
        order.TotalInCents = OrderMoney.From(99_00);
        client.Name = "Acme GmbH";
        db.Save(order);
        db.Save(client);

        // One row per saved object, refreshed with the *current* state:
        var orderRow = Assert.Single(db.Orders);
        Assert.Equal(order.Id, orderRow.Id);
        Assert.Equal("one (edited)", orderRow.Description);
        Assert.Equal(99_00, orderRow.TotalInCents);
        Assert.Equal(client.Id, orderRow.ClientId);

        var clientRow = Assert.Single(db.Clients);
        Assert.Equal("Acme GmbH", clientRow.Name);
    }

    [Fact]
    public void Rows_are_reachable_through_Set_and_Find()
    {
        var db = new DbContext();
        var order = ValidOrder().WithDescription("one").WithTotal(150_00).Build();
        var client = ValidClient().WithName("Acme").WithOrder(order).Build();
        db.Save(client);
        db.Save(order);

        Assert.Single(db.Set<ClientData>());
        Assert.Single(db.Set<OrderData>());

        var found = db.Find<OrderData>(row => row.Description == "one");
        Assert.Equal(order.Id, found!.Id);
        Assert.Null(db.Find<OrderData>(row => row.Description == "no such order"));
    }

    // Read-back: LoadOrder / LoadClient reassemble domain objects from the stored rows.

    [Fact]
    public void Loading_an_order_restores_its_state_and_its_client()
    {
        var db = new DbContext();
        var order = ValidOrder().WithDescription("one").WithTotal(150_00).Build();
        var client = ValidClient().WithName("Acme").WithOrder(order).Build();
        db.Save(client);
        db.Save(order);

        var loaded = db.LoadOrder(order.Id);

        Assert.NotNull(loaded);
        Assert.Equal(order.Id, loaded!.Id);
        Assert.Equal("one", loaded.Description);
        Assert.Equal(150_00, loaded.TotalInCents.Cents);

        // The graph is reassembled: the loaded order knows its client again,
        // and the client's collection holds the loaded order.
        Assert.NotNull(loaded.Client);
        Assert.Equal("Acme", loaded.Client!.Name);
        Assert.Same(loaded, Assert.Single(loaded.Client.Orders));
    }

    [Fact]
    public void Loading_a_client_restores_it_with_all_of_its_orders()
    {
        var db = new DbContext();
        var first = ValidOrder().WithDescription("one").WithTotal(150_00).Build();
        var second = ValidOrder().WithDescription("two").WithTotal(250_00).Build();
        var client = ValidClient().WithName("Acme").WithOrder(first).WithOrder(second).Build();
        db.Save(client);
        db.Save(first);
        db.Save(second);

        var loaded = db.LoadClient(client.Id);

        Assert.NotNull(loaded);
        Assert.Equal("Acme", loaded!.Name);
        Assert.Equal(2, loaded.Orders.Count);
        Assert.All(loaded.Orders, o => Assert.Same(loaded, o.Client));
    }

    [Fact]
    public void Loading_an_unknown_id_returns_null()
    {
        var db = new DbContext();

        Assert.Null(db.LoadOrder(Guid.NewGuid()));
        Assert.Null(db.LoadClient(Guid.NewGuid()));
    }

    // Tracking: explicit Save, distinct ids; a saved object keeps no reference to the context.
    [Fact]
    public void DbContext_saves_plain_domain_objects_and_tracks_them()
    {
        var db = new DbContext();
        var order = ValidOrder().WithDescription("one").Build();
        var client = ValidClient().WithName("Acme").WithOrder(order).Build();

        Assert.Equal(Guid.Empty, client.Id);
        db.Save(client);
        db.Save(order);

        Assert.NotEqual(Guid.Empty, client.Id);
        Assert.NotEqual(Guid.Empty, order.Id);
        Assert.NotEqual(client.Id, order.Id);
        Assert.True(db.IsTracked(client));
        Assert.True(db.IsTracked(order));
        Assert.Equal(2, db.Tracked.Count);
    }

    [Fact]
    public void Saving_twice_does_not_reassign_the_id()
    {
        var db = new DbContext();
        var client = ValidClient().WithName("Acme").Build();
        db.Save(client);
        var id = client.Id;

        db.Save(client);

        Assert.Equal(id, client.Id);
        Assert.Single(db.Tracked);
    }

    [Fact]
    public void A_saved_domain_object_keeps_no_reference_to_the_context()
    {
        var db = new DbContext();
        var client = ValidClient().WithName("Acme").Build();
        db.Save(client);

        // Saved (the id was assigned by the context) ...
        Assert.NotEqual(Guid.Empty, client.Id);
        Assert.True(db.IsTracked(client));

        // No way back to the context through the domain object (see DomainStaysPlainTests): the only "receipt" is the plain Id value.
        Assert.Equal(db.Tracked.Single(t => t.Id == client.Id).Id, client.Id);
    }
}