db-subclass-to-dto/tests/BeforeAfter.Tests/BeforeTests.cs

using Before;

namespace BeforeAfter.Tests;

/// <summary>
/// Tests for the BEFORE situation (Yak 01): DB-backed domain classes that
/// inherit <see cref="DbBase"/> and leak their <see cref="DbContext"/>.
/// </summary>
public class BeforeTests
{
    // ---------------------------------------------------------------------
    // (1) Inheritance: the domain classes ARE DB-backed (they derive from
    //     DbBase). This is the "before" shape Yak 02 will undo.
    // ---------------------------------------------------------------------
    [Fact]
    public void Client_and_Order_derive_from_DbBase()
    {
        Assert.True(typeof(DbBase).IsAssignableFrom(typeof(Client)));
        Assert.True(typeof(DbBase).IsAssignableFrom(typeof(Order)));
    }

    // ---------------------------------------------------------------------
    // (2) Fresh entities are unsaved: their Id is still Guid.Empty.
    // ---------------------------------------------------------------------
    [Theory]
    [InlineData(typeof(Client))]
    [InlineData(typeof(Order))]
    public void New_entities_have_an_empty_id(Type type)
    {
        var entity = (DbBase)Activator.CreateInstance(type)!;
        Assert.Equal(Guid.Empty, entity.Id);
    }

    // ---------------------------------------------------------------------
    // (3) Save() fakes EF's SaveChanges: it assigns fresh, distinct Guids to
    //     entities that do not have one yet.
    // ---------------------------------------------------------------------
    [Fact]
    public void Save_assigns_fresh_distinct_ids()
    {
        var db = new DbContext();
        var acme = new Client { Name = "Acme" };
        var globex = new Client { Name = "Globex" };
        db.Attach(acme);
        db.Attach(globex);

        Assert.Equal(2, db.Save());

        Assert.NotEqual(Guid.Empty, acme.Id);
        Assert.NotEqual(Guid.Empty, globex.Id);
        Assert.NotEqual(acme.Id, globex.Id);
    }

    // ---------------------------------------------------------------------
    // (4) A saved entity is registered with its context, reachable via the
    //     active-record back-reference.
    // ---------------------------------------------------------------------
    [Fact]
    public void Saved_entity_is_registered_with_its_context()
    {
        var db = new DbContext();
        var client = new Client { Name = "Acme" };
        db.Attach(client);
        db.Save();

        Assert.Same(db, client.DbContext);
        Assert.True(db.IsTracked(client));
    }

    // ---------------------------------------------------------------------
    // (5) Navigation fix-up: adding an order to client.Orders wires the
    //     order.Client back-reference.
    // ---------------------------------------------------------------------
    [Fact]
    public void Adding_an_order_sets_the_client_back_reference()
    {
        var client = new Client { Name = "Acme" };
        var order = new Order { Description = "first order" };

        client.Orders.Add(order);

        Assert.Same(client, order.Client);
        Assert.Same(order, client.Orders[0]);
        Assert.Single(client.Orders);
    }

    // ---------------------------------------------------------------------
    // (6) THE LEAK: from a plain Client you can reach its DbContext.
    //     This is exactly what the "after" situation removes: a ClientDto has
    //     no DbContext to reach, so a consumer can never touch the persistence
    //     layer through it.
    // ---------------------------------------------------------------------
    [Fact]
    public void A_client_can_reach_its_DbContext__the_leak()
    {
        var db = new DbContext();
        var client = new Client { Name = "Acme" };
        db.Attach(client);
        db.Save();

        // Reaching the persistence layer *through* the domain object:
        Assert.NotNull(client.DbContext);
        Assert.Same(db, client.DbContext);
        Assert.True(client.DbContext!.IsTracked(client));
    }

    // ---------------------------------------------------------------------
    // (7) The WebApp consumes the domain classes directly — including a method
    //     that rides the leak.
    // ---------------------------------------------------------------------
    [Fact]
    public void WebApp_works_on_the_domain_classes_directly()
    {
        var app = new WebApp();
        var db = new DbContext();

        var client = new Client { Name = "Acme" };
        var order = new Order { Description = "order one" };
        client.Orders.Add(order);
        db.Attach(client);
        db.Attach(order);
        db.Save();

        // ShowClient consumes Client and its Orders directly:
        var view = app.ShowClient(client);
        Assert.Contains("Acme", view);
        Assert.Contains("order one", view);

        // IsPersisted rides the leak (client.DbContext) — and it is true here
        // because the client was saved through the context:
        Assert.True(app.IsPersisted(client));
    }

    // A companion check: a client that was never attached/saved exposes no
    // context yet, so the leak is dormant until the entity meets a context.
    [Fact]
    public void A_bare_client_has_no_context_until_saved()
    {
        var app = new WebApp();
        var client = new Client { Name = "Nobody" };

        Assert.Null(client.DbContext);
        Assert.False(app.IsPersisted(client));
    }
}