From b8d90c08afdcf7f7dd01d87afc58c069f495c1fa Mon Sep 17 00:00:00 2001
From: JKuijperM <jkmontero17@gmail.com>
Date: Tue, 22 Sep 2026 08:52:30 +0200
Subject: [PATCH] test: cobertura de traspasos

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
---
 .../expenses/tests/test_categories.py         |  20 +
 .../expenses/tests/test_transfer_model.py     | 977 ++++++++++++++++++
 .../expenses/tests/test_transfers.py          |  17 +-
 3 files changed, 1010 insertions(+), 4 deletions(-)
 create mode 100644 expenses_manager/expenses/tests/test_transfer_model.py

diff --git a/expenses_manager/expenses/tests/test_categories.py b/expenses_manager/expenses/tests/test_categories.py
index 6c9bf74..33a6329 100644
--- a/expenses_manager/expenses/tests/test_categories.py
+++ b/expenses_manager/expenses/tests/test_categories.py
@@ -101,6 +101,26 @@ def test_category_delete_without_expenses_succeeds(auth_client, user):
     assert not Category.objects.filter(pk=cat.pk).exists()
 
 
+def test_system_category_cannot_be_deleted(auth_client, user):
+    """Las categorías de traspaso y comisión las pone la app, no el usuario."""
+    cat = Category.objects.create(name="Traspaso", owner=user, is_system=True)
+
+    response = auth_client.post(reverse('category_delete', args=[cat.pk]))
+
+    assert response.status_code == 302
+    assert Category.objects.filter(pk=cat.pk).exists()
+    messages = [str(m) for m in get_messages(response.wsgi_request)]
+    assert any("traspasos" in m for m in messages)
+
+
+def test_system_category_has_no_delete_link_in_the_list(auth_client, user):
+    cat = Category.objects.create(name="Traspaso", owner=user, is_system=True)
+
+    response = auth_client.get(reverse('category_list'))
+
+    assert reverse('category_delete', args=[cat.pk]) not in response.content.decode()
+
+
 def test_category_list_view_returns_tree_ordered_with_depth(auth_client, user):
     root_a = Category.objects.create(name="A raiz", owner=user)
     Category.objects.create(name="B raiz", owner=user)
diff --git a/expenses_manager/expenses/tests/test_transfer_model.py b/expenses_manager/expenses/tests/test_transfer_model.py
new file mode 100644
index 0000000..68bc266
--- /dev/null
+++ b/expenses_manager/expenses/tests/test_transfer_model.py
@@ -0,0 +1,977 @@
+"""El modelo Transfer: orquesta las dos patas de un traspaso.
+
+Transfer no sustituye al Expense de salida y al Income de entrada: los crea, los
+vincula y los mantiene sincronizados. De ahí que aquí no se compruebe ningún
+método de saldo nuevo, sino que los de siempre siguen dando el número correcto
+porque las patas siguen siendo un gasto y un ingreso reales.
+
+El punto más delicado, y el que más tests tiene: la comisión va con
+is_transfer=False, porque es dinero que sale del patrimonio de verdad y sí tiene
+que contar en los análisis.
+"""
+
+import importlib
+from datetime import date
+from decimal import Decimal
+from urllib.parse import parse_qs, urlparse
+
+import pytest
+from django.apps import apps as django_apps
+from django.core.exceptions import ValidationError
+from django.urls import reverse
+
+from expenses.models import Account, Category, Expense, Income, Transfer
+
+pytestmark = pytest.mark.django_db
+
+
+@pytest.fixture
+def savings(user):
+    return Account.objects.create(
+        owner=user, name="Ahorro", initial_balance=Decimal("0"), active=True
+    )
+
+
+def make_transfer(user, from_account, to_account, amount="400", on=None, fee=None):
+    transfer = Transfer(
+        owner=user,
+        date=on or date(2024, 5, 11),
+        amount=Decimal(amount),
+        from_account=from_account,
+        to_account=to_account,
+        fee_amount=Decimal(fee) if fee is not None else None,
+    )
+    transfer.sync()
+    return transfer
+
+
+# --------------------------------------------------------------------------
+# Creación: las dos patas
+# --------------------------------------------------------------------------
+
+
+def test_creating_a_transfer_builds_and_links_both_legs(user, account, savings):
+    transfer = make_transfer(user, account, savings)
+
+    assert transfer.expense_leg is not None
+    assert transfer.income_leg is not None
+
+    assert transfer.expense_leg.account == account
+    assert transfer.expense_leg.amount == Decimal("400")
+    assert transfer.expense_leg.date == transfer.date
+    assert transfer.expense_leg.owner == user
+    assert transfer.expense_leg.is_transfer is True
+
+    assert transfer.income_leg.account == savings
+    assert transfer.income_leg.amount == Decimal("400")
+    assert transfer.income_leg.date == transfer.date
+    assert transfer.income_leg.owner == user
+    assert transfer.income_leg.is_transfer is True
+
+    # El vínculo se ve desde las dos puntas.
+    assert transfer.expense_leg.transfer == transfer
+    assert transfer.income_leg.transfer == transfer
+
+
+def test_legs_use_the_system_transfer_category(user, account, savings):
+    transfer = make_transfer(user, account, savings)
+
+    leg_category = transfer.expense_leg.category
+    assert leg_category.slug == "traspaso"
+    assert leg_category.is_system is True
+    assert leg_category.owner == user
+
+
+def test_two_transfers_share_the_same_system_category(user, account, savings):
+    first = make_transfer(user, account, savings)
+    second = make_transfer(user, account, savings, on=date(2024, 6, 11))
+
+    assert first.expense_leg.category_id == second.expense_leg.category_id
+    assert Category.objects.filter(owner=user, slug="traspaso").count() == 1
+
+
+def test_both_balances_move(user, account, savings):
+    account.initial_balance = Decimal("1000")
+    account.save()
+
+    make_transfer(user, account, savings)
+
+    assert account.current_balance() == Decimal("600")
+    assert savings.current_balance() == Decimal("400")
+
+
+def test_transfer_does_not_change_the_total_patrimony(user, account, savings):
+    account.initial_balance = Decimal("1000")
+    account.save()
+
+    make_transfer(user, account, savings)
+
+    total = account.current_balance() + savings.current_balance()
+    assert total == Decimal("1000")
+
+
+# --------------------------------------------------------------------------
+# La comisión: un gasto real, sin is_transfer
+# --------------------------------------------------------------------------
+
+
+def test_fee_creates_a_third_expense_that_is_not_a_transfer(user, account, savings):
+    transfer = make_transfer(user, account, savings, fee="3")
+
+    assert transfer.fee_expense is not None
+    assert transfer.fee_expense.amount == Decimal("3")
+    assert transfer.fee_expense.account == account
+    assert transfer.fee_expense.date == transfer.date
+    # El punto donde es más fácil equivocarse: la comisión NO es un traspaso.
+    assert transfer.fee_expense.is_transfer is False
+    assert transfer.fee_expense.category.slug == "comision"
+    assert transfer.fee_expense.category.is_system is True
+
+
+def test_fee_is_charged_to_the_origin_account(user, account, savings):
+    account.initial_balance = Decimal("1000")
+    account.save()
+
+    make_transfer(user, account, savings, fee="3")
+
+    # El origen paga el traspaso y la comisión; el destino solo recibe el traspaso.
+    assert account.current_balance() == Decimal("597")
+    assert savings.current_balance() == Decimal("400")
+
+
+def test_dashboard_counts_the_fee_but_not_the_legs(auth_client, user, account, savings):
+    make_transfer(user, account, savings, fee="3", on=date(2024, 5, 11))
+
+    response = auth_client.get(reverse("dashboard"), {"year": 2024})
+
+    assert response.context["kpi_total"] == Decimal("3")
+    assert response.context["kpi_count"] == 1
+
+    rows = {row["category__name"]: row["total"] for row in response.context["by_category"]}
+    assert rows == {"Comisión": Decimal("3")}
+
+
+def test_without_fee_the_dashboard_sees_nothing(auth_client, user, account, savings):
+    make_transfer(user, account, savings, on=date(2024, 5, 11))
+
+    response = auth_client.get(reverse("dashboard"), {"year": 2024})
+
+    assert response.context["kpi_total"] == Decimal("0")
+    assert response.context["kpi_count"] == 0
+
+
+# --------------------------------------------------------------------------
+# Edición: sincronizar las patas
+# --------------------------------------------------------------------------
+
+
+def test_editing_the_amount_updates_both_legs(user, account, savings):
+    transfer = make_transfer(user, account, savings)
+
+    transfer.amount = Decimal("250")
+    transfer.sync()
+
+    transfer.expense_leg.refresh_from_db()
+    transfer.income_leg.refresh_from_db()
+    assert transfer.expense_leg.amount == Decimal("250")
+    assert transfer.income_leg.amount == Decimal("250")
+
+    assert account.current_balance() == Decimal("-250")
+    assert savings.current_balance() == Decimal("250")
+
+
+def test_editing_the_date_updates_both_legs(user, account, savings):
+    transfer = make_transfer(user, account, savings)
+
+    transfer.date = date(2024, 8, 1)
+    transfer.sync()
+
+    transfer.expense_leg.refresh_from_db()
+    transfer.income_leg.refresh_from_db()
+    assert transfer.expense_leg.date == date(2024, 8, 1)
+    assert transfer.income_leg.date == date(2024, 8, 1)
+
+
+def test_editing_the_accounts_moves_the_legs(user, account, savings):
+    transfer = make_transfer(user, account, savings)
+    third = Account.objects.create(
+        owner=user, name="Bolsillo", initial_balance=Decimal("0"), active=True
+    )
+
+    transfer.from_account = savings
+    transfer.to_account = third
+    transfer.sync()
+
+    transfer.expense_leg.refresh_from_db()
+    transfer.income_leg.refresh_from_db()
+    assert transfer.expense_leg.account == savings
+    assert transfer.income_leg.account == third
+    assert account.current_balance() == Decimal("0")
+
+
+def test_editing_does_not_create_new_legs(user, account, savings):
+    transfer = make_transfer(user, account, savings)
+    expense_id, income_id = transfer.expense_leg_id, transfer.income_leg_id
+
+    transfer.amount = Decimal("250")
+    transfer.sync()
+
+    assert transfer.expense_leg_id == expense_id
+    assert transfer.income_leg_id == income_id
+    assert Expense.objects.count() == 1
+    assert Income.objects.count() == 1
+
+
+def test_adding_a_fee_on_edit_creates_it(user, account, savings):
+    transfer = make_transfer(user, account, savings)
+    assert transfer.fee_expense is None
+
+    transfer.fee_amount = Decimal("3")
+    transfer.sync()
+
+    assert transfer.fee_expense is not None
+    assert transfer.fee_expense.is_transfer is False
+    assert Expense.objects.count() == 2
+
+
+def test_removing_the_fee_on_edit_deletes_it(user, account, savings):
+    transfer = make_transfer(user, account, savings, fee="3")
+    fee_id = transfer.fee_expense_id
+
+    transfer.fee_amount = None
+    transfer.sync()
+
+    assert transfer.fee_expense is None
+    assert not Expense.objects.filter(pk=fee_id).exists()
+
+    transfer.refresh_from_db()
+    assert transfer.fee_expense_id is None
+
+
+def test_editing_the_fee_amount_updates_the_expense(user, account, savings):
+    transfer = make_transfer(user, account, savings, fee="3")
+
+    transfer.fee_amount = Decimal("5")
+    transfer.sync()
+
+    transfer.fee_expense.refresh_from_db()
+    assert transfer.fee_expense.amount == Decimal("5")
+    assert Expense.objects.count() == 2
+
+
+# --------------------------------------------------------------------------
+# Borrado
+# --------------------------------------------------------------------------
+
+
+def test_deleting_the_transfer_deletes_both_legs(user, account, savings):
+    transfer = make_transfer(user, account, savings)
+
+    transfer.delete()
+
+    assert Expense.objects.count() == 0
+    assert Income.objects.count() == 0
+    assert Transfer.objects.count() == 0
+
+
+def test_deleting_the_transfer_deletes_the_fee_too(user, account, savings):
+    transfer = make_transfer(user, account, savings, fee="3")
+
+    transfer.delete()
+
+    assert Expense.objects.count() == 0
+    assert Income.objects.count() == 0
+    assert Transfer.objects.count() == 0
+
+
+def test_deleting_the_transfer_restores_both_balances(user, account, savings):
+    account.initial_balance = Decimal("1000")
+    account.save()
+    transfer = make_transfer(user, account, savings, fee="3")
+
+    transfer.delete()
+
+    assert account.current_balance() == Decimal("1000")
+    assert savings.current_balance() == Decimal("0")
+
+
+# --------------------------------------------------------------------------
+# Validación y atomicidad
+# --------------------------------------------------------------------------
+
+
+def test_same_account_on_both_sides_is_rejected(user, account):
+    transfer = Transfer(
+        owner=user,
+        date=date(2024, 5, 11),
+        amount=Decimal("400"),
+        from_account=account,
+        to_account=account,
+    )
+
+    with pytest.raises(ValidationError) as error:
+        transfer.full_clean()
+
+    assert "to_account" in error.value.message_dict
+
+
+def test_different_accounts_pass_validation(user, account, savings):
+    transfer = Transfer(
+        owner=user,
+        date=date(2024, 5, 11),
+        amount=Decimal("400"),
+        from_account=account,
+        to_account=savings,
+    )
+
+    # A pelo y sin excluir nada: las patas son blank=True porque aún no existen.
+    transfer.full_clean()  # no levanta
+
+
+def test_a_failed_leg_leaves_nothing_behind(user, account, savings, monkeypatch):
+    """Atomicidad: si la segunda pata falla, la primera no queda suelta."""
+
+    def boom(self):
+        raise RuntimeError("fallo al construir la pata de entrada")
+
+    monkeypatch.setattr(Transfer, "_income_name", boom)
+
+    transfer = Transfer(
+        owner=user,
+        date=date(2024, 5, 11),
+        amount=Decimal("400"),
+        from_account=account,
+        to_account=savings,
+    )
+
+    with pytest.raises(RuntimeError):
+        transfer.sync()
+
+    assert Expense.objects.count() == 0
+    assert Income.objects.count() == 0
+    assert Transfer.objects.count() == 0
+
+
+# --------------------------------------------------------------------------
+# Las patas no se editan por separado
+# --------------------------------------------------------------------------
+
+
+def test_expense_leg_cannot_be_edited_on_its_own(auth_client, user, account, savings):
+    transfer = make_transfer(user, account, savings)
+
+    response = auth_client.get(
+        reverse("expense_edit", args=[transfer.expense_leg_id]), follow=True
+    )
+
+    assert response.redirect_chain[0][0] == reverse("transfer_edit", args=[transfer.pk])
+
+
+def test_expense_leg_cannot_be_deleted_on_its_own(auth_client, user, account, savings):
+    transfer = make_transfer(user, account, savings)
+
+    auth_client.post(reverse("expense_delete", args=[transfer.expense_leg_id]))
+
+    assert Expense.objects.filter(pk=transfer.expense_leg_id).exists()
+
+
+def test_income_leg_cannot_be_deleted_on_its_own(auth_client, user, account, savings):
+    transfer = make_transfer(user, account, savings)
+
+    auth_client.post(reverse("income_delete", args=[transfer.income_leg_id]))
+
+    assert Income.objects.filter(pk=transfer.income_leg_id).exists()
+
+
+def test_the_fee_expense_cannot_be_edited_on_its_own(auth_client, user, account, savings):
+    transfer = make_transfer(user, account, savings, fee="3")
+
+    response = auth_client.get(
+        reverse("expense_edit", args=[transfer.fee_expense_id]), follow=True
+    )
+
+    assert response.redirect_chain[0][0] == reverse("transfer_edit", args=[transfer.pk])
+
+
+def test_a_normal_expense_is_still_editable(auth_client, user, account, category):
+    expense = Expense.objects.create(
+        owner=user,
+        account=account,
+        category=category,
+        amount=Decimal("10"),
+        date=date(2024, 5, 11),
+    )
+
+    response = auth_client.get(reverse("expense_edit", args=[expense.pk]))
+
+    assert response.status_code == 200
+    assert expense.transfer is None
+
+
+def test_the_listing_links_a_leg_to_its_transfer(auth_client, user, account, savings):
+    transfer = make_transfer(user, account, savings)
+
+    response = auth_client.get(reverse("expense_list"), {"year": 2024})
+    html = response.content.decode()
+
+    assert reverse("transfer_edit", args=[transfer.pk]) in html
+    assert reverse("expense_delete", args=[transfer.expense_leg_id]) not in html
+
+
+@pytest.mark.parametrize("listing", ["expense_list", "income_list"])
+def test_no_template_comment_leaks_into_the_listing(
+    auth_client, user, account, savings, listing
+):
+    """Los {# #} de Django son de una sola línea: partidos en dos se imprimen."""
+    make_transfer(user, account, savings)
+
+    response = auth_client.get(reverse(listing), {"year": 2024})
+
+    assert "{#" not in response.content.decode()
+
+
+# --------------------------------------------------------------------------
+# Vistas CRUD
+# --------------------------------------------------------------------------
+
+
+def test_create_view_builds_the_three_legs(auth_client, user, account, savings):
+    response = auth_client.post(
+        reverse("transfer_create"),
+        {
+            "date": "2024-05-11",
+            "amount": "400",
+            "from_account": account.id,
+            "to_account": savings.id,
+            "fee_amount": "3",
+        },
+    )
+
+    assert response.status_code == 302
+    transfer = Transfer.objects.get(owner=user)
+    assert transfer.expense_leg.is_transfer is True
+    assert transfer.income_leg.is_transfer is True
+    assert transfer.fee_expense.is_transfer is False
+
+
+def test_create_view_without_fee(auth_client, user, account, savings):
+    auth_client.post(
+        reverse("transfer_create"),
+        {
+            "date": "2024-05-11",
+            "amount": "400",
+            "from_account": account.id,
+            "to_account": savings.id,
+            "fee_amount": "",
+        },
+    )
+
+    transfer = Transfer.objects.get(owner=user)
+    assert transfer.fee_amount is None
+    assert transfer.fee_expense is None
+    assert Expense.objects.count() == 1
+
+
+def test_create_view_rejects_the_same_account_twice(auth_client, user, account):
+    response = auth_client.post(
+        reverse("transfer_create"),
+        {
+            "date": "2024-05-11",
+            "amount": "400",
+            "from_account": account.id,
+            "to_account": account.id,
+        },
+    )
+
+    assert response.status_code == 200
+    assert Transfer.objects.count() == 0
+    assert "to_account" in response.context["form"].errors
+
+
+def test_edit_view_syncs_the_legs(auth_client, user, account, savings):
+    transfer = make_transfer(user, account, savings)
+
+    response = auth_client.post(
+        reverse("transfer_edit", args=[transfer.pk]),
+        {
+            "date": "2024-05-11",
+            "amount": "250",
+            "from_account": account.id,
+            "to_account": savings.id,
+            "fee_amount": "",
+        },
+    )
+
+    assert response.status_code == 302
+    transfer.expense_leg.refresh_from_db()
+    transfer.income_leg.refresh_from_db()
+    assert transfer.expense_leg.amount == Decimal("250")
+    assert transfer.income_leg.amount == Decimal("250")
+
+
+def test_delete_view_removes_everything(auth_client, user, account, savings):
+    transfer = make_transfer(user, account, savings, fee="3")
+
+    response = auth_client.post(reverse("transfer_delete", args=[transfer.pk]))
+
+    assert response.status_code == 302
+    assert Transfer.objects.count() == 0
+    assert Expense.objects.count() == 0
+    assert Income.objects.count() == 0
+
+
+def test_delete_view_needs_post(auth_client, user, account, savings):
+    transfer = make_transfer(user, account, savings)
+
+    response = auth_client.get(reverse("transfer_delete", args=[transfer.pk]))
+
+    assert response.status_code == 200
+    assert Transfer.objects.count() == 1
+
+
+def test_list_view_only_shows_your_own(auth_client, user, account, savings, django_user_model):
+    make_transfer(user, account, savings)
+
+    other = django_user_model.objects.create_user(username="otro", password="x")
+    other_account = Account.objects.create(
+        owner=other, name="Suya", initial_balance=Decimal("0"), active=True
+    )
+    other_savings = Account.objects.create(
+        owner=other, name="Suya 2", initial_balance=Decimal("0"), active=True
+    )
+    make_transfer(other, other_account, other_savings)
+
+    response = auth_client.get(reverse("transfer_list"))
+
+    assert [t.owner for t in response.context["transfers"]] == [user]
+
+
+def test_form_only_offers_your_active_accounts(auth_client, user, account, savings):
+    Account.objects.create(
+        owner=user, name="Cerrada", initial_balance=Decimal("0"), active=False
+    )
+
+    response = auth_client.get(reverse("transfer_create"))
+    form = response.context["form"]
+
+    names = {acc.name for acc in form.fields["from_account"].queryset}
+    assert names == {account.name, savings.name}
+    assert names == {acc.name for acc in form.fields["to_account"].queryset}
+
+
+def test_transfer_edit_view_is_scoped_by_owner(auth_client, django_user_model):
+    other = django_user_model.objects.create_user(username="otro", password="x")
+    other_account = Account.objects.create(
+        owner=other, name="Suya", initial_balance=Decimal("0"), active=True
+    )
+    other_savings = Account.objects.create(
+        owner=other, name="Suya 2", initial_balance=Decimal("0"), active=True
+    )
+    transfer = make_transfer(other, other_account, other_savings)
+
+    response = auth_client.get(reverse("transfer_edit", args=[transfer.pk]))
+
+    assert response.status_code == 404
+
+
+# --------------------------------------------------------------------------
+# El atajo de la casilla: del gasto o del ingreso al traspaso
+# --------------------------------------------------------------------------
+
+
+def _query(url):
+    return parse_qs(urlparse(url).query)
+
+
+def test_the_expense_form_carries_its_data_to_the_transfer_form(
+    auth_client, user, account, category
+):
+    """La cuenta de un gasto entra como origen: el dinero sale de ahí."""
+    response = auth_client.post(
+        reverse("expense_create"),
+        {
+            "date": "2024-05-11",
+            "amount": "400",
+            "description": "",
+            "category": category.id,
+            "account": account.id,
+            "is_transfer": "on",
+        },
+    )
+
+    assert response.status_code == 302
+    assert urlparse(response.url).path == reverse("transfer_create")
+    assert _query(response.url) == {
+        "date": ["2024-05-11"],
+        "amount": ["400"],
+        "from_account": [str(account.id)],
+    }
+
+
+def test_the_income_form_carries_its_account_as_destination(auth_client, user, savings):
+    response = auth_client.post(
+        reverse("income_create"),
+        {
+            "account": savings.id,
+            "name": "Traspaso",
+            "amount": "400",
+            "date": "2024-05-11",
+            "is_transfer": "on",
+        },
+    )
+
+    assert _query(response.url) == {
+        "date": ["2024-05-11"],
+        "amount": ["400"],
+        "to_account": [str(savings.id)],
+    }
+
+
+def test_a_half_filled_expense_still_reaches_the_transfer_form(
+    auth_client, user, account
+):
+    """Sin categoría el gasto no valida, pero el salto no depende de que valide:
+    se lleva lo que haya y lo que falte se queda en blanco."""
+    response = auth_client.post(
+        reverse("expense_create"),
+        {"date": "", "amount": "400", "account": account.id, "is_transfer": "on"},
+    )
+
+    assert response.status_code == 302
+    assert _query(response.url) == {
+        "amount": ["400"],
+        "from_account": [str(account.id)],
+    }
+    assert not Expense.objects.filter(owner=user).exists()
+
+
+def test_the_transfer_form_prefills_from_the_url(auth_client, user, account, savings):
+    response = auth_client.get(
+        reverse("transfer_create"),
+        {
+            "date": "2024-05-11",
+            "amount": "400",
+            "from_account": account.id,
+            "to_account": savings.id,
+        },
+    )
+
+    assert response.context["form"].initial == {
+        "date": date(2024, 5, 11),
+        "amount": Decimal("400"),
+        "from_account": account.id,
+        "to_account": savings.id,
+    }
+
+
+@pytest.mark.parametrize(
+    "params",
+    [
+        {"date": "11/05/2024"},
+        {"date": "2024-13-45"},
+        {"amount": "cuatrocientos"},
+        {"amount": "NaN"},
+        {"amount": "Infinity"},
+        {"from_account": "primera"},
+        {"to_account": "999999"},
+    ],
+)
+def test_an_invalid_url_param_is_ignored_and_does_not_break(auth_client, user, params):
+    """La URL la escribe cualquiera: lo que no se entienda sale en blanco."""
+    response = auth_client.get(reverse("transfer_create"), params)
+
+    assert response.status_code == 200
+    assert response.context["form"].initial == {}
+
+
+def test_an_account_of_another_user_is_not_prefilled(
+    auth_client, user, django_user_model
+):
+    intruder = django_user_model.objects.create_user(username="otro", password="x")
+    foreign = Account.objects.create(
+        owner=intruder, name="Ajena", initial_balance=Decimal("0"), active=True
+    )
+
+    response = auth_client.get(reverse("transfer_create"), {"from_account": foreign.id})
+
+    assert response.context["form"].initial == {}
+
+
+def test_an_inactive_account_is_not_prefilled(auth_client, user):
+    """Las cuentas desactivadas no están en el desplegable: preseleccionar una
+    dejaría el formulario con un valor que no se puede enviar."""
+    closed = Account.objects.create(
+        owner=user, name="Cerrada", initial_balance=Decimal("0"), active=False
+    )
+
+    response = auth_client.get(reverse("transfer_create"), {"to_account": closed.id})
+
+    assert response.context["form"].initial == {}
+
+
+# --------------------------------------------------------------------------
+# La casilla de traspaso no se ofrece al editar
+# --------------------------------------------------------------------------
+
+
+def test_the_checkbox_is_offered_when_creating_an_expense(auth_client, user):
+    response = auth_client.get(reverse("expense_create"))
+
+    assert "id_is_transfer" in response.content.decode()
+
+
+def test_the_checkbox_is_offered_when_creating_an_income(auth_client, user):
+    response = auth_client.get(reverse("income_create"))
+
+    assert "id_is_transfer" in response.content.decode()
+
+
+@pytest.mark.parametrize(
+    "view_name,account_param",
+    [("expense_create", "from_account"), ("income_create", "to_account")],
+)
+def test_the_create_form_tells_the_javascript_where_to_jump(
+    auth_client, user, view_name, account_param
+):
+    """El JS no sabe nada de traspasos: el destino y el papel de la cuenta se los
+    da la plantilla en data-*. Renombrarlos rompería el salto inmediato sin que
+    nada se queje, porque el POST seguiría redirigiendo igual.
+    """
+    response = auth_client.get(reverse(view_name))
+    html = response.content.decode()
+
+    assert f'data-transfer-url="{reverse("transfer_create")}"' in html
+    assert f'data-transfer-account-param="{account_param}"' in html
+    assert "transfer_redirect.js" in html
+
+
+def test_the_checkbox_is_hidden_when_editing_an_expense(
+    auth_client, user, account, category
+):
+    """Editar un gasto no es convertirlo en traspaso: la casilla no pinta nada."""
+    expense = Expense.objects.create(
+        owner=user,
+        account=account,
+        category=category,
+        amount=Decimal("10"),
+        date=date(2024, 5, 11),
+    )
+
+    response = auth_client.get(reverse("expense_edit", args=[expense.pk]))
+
+    assert "id_is_transfer" not in response.content.decode()
+
+
+def test_the_checkbox_is_hidden_when_editing_an_income(auth_client, user, account):
+    income = Income.objects.create(
+        owner=user,
+        account=account,
+        name="Nómina",
+        amount=Decimal("10"),
+        date=date(2024, 5, 11),
+    )
+
+    response = auth_client.get(reverse("income_edit", args=[income.pk]))
+
+    assert "id_is_transfer" not in response.content.decode()
+
+
+def test_posting_is_transfer_while_editing_has_no_effect(
+    auth_client, user, account, category
+):
+    """El campo no está en el formulario, así que mandarlo a mano tampoco cuela."""
+    expense = Expense.objects.create(
+        owner=user,
+        account=account,
+        category=category,
+        amount=Decimal("10"),
+        date=date(2024, 5, 11),
+    )
+
+    auth_client.post(
+        reverse("expense_edit", args=[expense.pk]),
+        {
+            "date": "2024-05-11",
+            "amount": "10",
+            "description": "",
+            "category": category.id,
+            "account": account.id,
+            "is_transfer": "on",
+        },
+    )
+
+    expense.refresh_from_db()
+    assert expense.is_transfer is False
+
+
+def test_a_leg_shows_its_transfer_link_and_no_checkbox(
+    auth_client, user, account, savings
+):
+    """Una pata sigue mandando al traspaso: ni casilla, ni edición suelta."""
+    transfer = make_transfer(user, account, savings)
+
+    listing = auth_client.get(reverse("expense_list"), {"year": 2024})
+    html = listing.content.decode()
+
+    assert reverse("transfer_edit", args=[transfer.pk]) in html
+    assert "id_is_transfer" not in html
+
+    edit = auth_client.get(
+        reverse("expense_edit", args=[transfer.expense_leg_id]), follow=True
+    )
+
+    assert edit.redirect_chain[0][0] == reverse("transfer_edit", args=[transfer.pk])
+    assert "id_is_transfer" not in edit.content.decode()
+
+
+# --------------------------------------------------------------------------
+# Migración de datos: emparejar los traspasos históricos
+# --------------------------------------------------------------------------
+
+
+def link_legacy_transfers():
+    """Lanza la función de la migración 0015 contra los modelos reales."""
+    migration = importlib.import_module("expenses.migrations.0015_link_legacy_transfers")
+    migration.link_legacy_transfers(django_apps, None)
+
+
+def legacy_pair(user, from_account, to_account, category, amount, on):
+    expense = Expense.objects.create(
+        owner=user,
+        account=from_account,
+        category=category,
+        amount=Decimal(amount),
+        date=on,
+        is_transfer=True,
+    )
+    income = Income.objects.create(
+        owner=user,
+        account=to_account,
+        name="Traspaso",
+        amount=Decimal(amount),
+        date=on,
+        is_transfer=True,
+    )
+    return expense, income
+
+
+def test_migration_pairs_an_unambiguous_legacy_transfer(
+    user, account, savings, category
+):
+    expense, income = legacy_pair(
+        user, account, savings, category, "200", date(2024, 5, 11)
+    )
+
+    link_legacy_transfers()
+
+    transfer = Transfer.objects.get()
+    assert transfer.expense_leg_id == expense.pk
+    assert transfer.income_leg_id == income.pk
+    assert transfer.from_account_id == account.pk
+    assert transfer.to_account_id == savings.pk
+    assert transfer.amount == Decimal("200")
+    assert transfer.date == date(2024, 5, 11)
+    assert transfer.fee_amount is None
+
+
+def test_migration_leaves_ambiguous_candidates_alone(user, account, savings, category):
+    # Dos gastos y dos ingresos con el mismo importe y fecha: no hay forma de
+    # saber cuál va con cuál, así que no se toca ninguno.
+    legacy_pair(user, account, savings, category, "200", date(2024, 5, 11))
+    legacy_pair(user, account, savings, category, "200", date(2024, 5, 11))
+
+    link_legacy_transfers()
+
+    assert Transfer.objects.count() == 0
+    assert Expense.objects.filter(is_transfer=True).count() == 2
+    assert Income.objects.filter(is_transfer=True).count() == 2
+
+
+def test_migration_leaves_orphans_alone(user, account, category):
+    Expense.objects.create(
+        owner=user,
+        account=account,
+        category=category,
+        amount=Decimal("200"),
+        date=date(2024, 5, 11),
+        is_transfer=True,
+    )
+
+    link_legacy_transfers()
+
+    assert Transfer.objects.count() == 0
+    assert Expense.objects.filter(is_transfer=True).count() == 1
+
+
+def test_migration_ignores_normal_movements(user, account, savings, category):
+    Expense.objects.create(
+        owner=user,
+        account=account,
+        category=category,
+        amount=Decimal("200"),
+        date=date(2024, 5, 11),
+    )
+    Income.objects.create(
+        owner=user,
+        account=savings,
+        name="Nómina",
+        amount=Decimal("200"),
+        date=date(2024, 5, 11),
+    )
+
+    link_legacy_transfers()
+
+    assert Transfer.objects.count() == 0
+
+
+def test_migration_skips_a_pair_on_the_same_account(user, account, category):
+    legacy_pair(user, account, account, category, "200", date(2024, 5, 11))
+
+    link_legacy_transfers()
+
+    assert Transfer.objects.count() == 0
+
+
+def test_migration_does_not_cross_owners(user, account, category, django_user_model):
+    other = django_user_model.objects.create_user(username="otro", password="x")
+    other_account = Account.objects.create(
+        owner=other, name="Suya", initial_balance=Decimal("0"), active=True
+    )
+
+    Expense.objects.create(
+        owner=user,
+        account=account,
+        category=category,
+        amount=Decimal("200"),
+        date=date(2024, 5, 11),
+        is_transfer=True,
+    )
+    Income.objects.create(
+        owner=other,
+        account=other_account,
+        name="Traspaso",
+        amount=Decimal("200"),
+        date=date(2024, 5, 11),
+        is_transfer=True,
+    )
+
+    link_legacy_transfers()
+
+    assert Transfer.objects.count() == 0
+
+
+def test_migration_is_idempotent(user, account, savings, category):
+    legacy_pair(user, account, savings, category, "200", date(2024, 5, 11))
+
+    link_legacy_transfers()
+    link_legacy_transfers()
+
+    assert Transfer.objects.count() == 1
+
+
+def test_migration_does_not_touch_legs_of_an_existing_transfer(user, account, savings):
+    make_transfer(user, account, savings)
+
+    link_legacy_transfers()
+
+    assert Transfer.objects.count() == 1
diff --git a/expenses_manager/expenses/tests/test_transfers.py b/expenses_manager/expenses/tests/test_transfers.py
index 33f42c3..86727c3 100644
--- a/expenses_manager/expenses/tests/test_transfers.py
+++ b/expenses_manager/expenses/tests/test_transfers.py
@@ -203,7 +203,12 @@ def test_account_list_balance_counts_the_transfer(auth_client, user, account, ca
 # Formularios
 # --------------------------------------------------------------------------
 
-def test_expense_form_can_mark_a_transfer(auth_client, user, account, category):
+# Marcar la casilla ya no guarda un movimiento marcado: reconduce al formulario de
+# traspaso, que crea las dos patas. El detalle está en test_transfer_model.py.
+
+def test_marking_the_expense_form_as_transfer_creates_nothing(
+    auth_client, user, account, category
+):
     response = auth_client.post(
         reverse("expense_create"),
         {
@@ -217,10 +222,13 @@ def test_expense_form_can_mark_a_transfer(auth_client, user, account, category):
     )
 
     assert response.status_code == 302
-    assert Expense.objects.get(owner=user).is_transfer is True
+    assert reverse("transfer_create") in response.url
+    assert not Expense.objects.filter(owner=user).exists()
 
 
-def test_income_form_can_mark_a_transfer(auth_client, user, account):
+def test_marking_the_income_form_as_transfer_creates_nothing(
+    auth_client, user, account
+):
     response = auth_client.post(
         reverse("income_create"),
         {
@@ -233,7 +241,8 @@ def test_income_form_can_mark_a_transfer(auth_client, user, account):
     )
 
     assert response.status_code == 302
-    assert Income.objects.get(owner=user).is_transfer is True
+    assert reverse("transfer_create") in response.url
+    assert not Income.objects.filter(owner=user).exists()
 
 
 def test_expense_defaults_to_not_a_transfer(user, account, category):