pytest-orm-boundaries

💡 Even if you control your imports, boundaries can still leak through the ORM.

pytest-orm-boundaries is a pytest plugin that reports ORM queries crossing your DDD aggregate boundaries.

Currently works with Django ORM, SQLAlchemy is in roadmap.

In domain-driven design, an aggregate is a consistency boundary: code in one aggregate should not reach into the internals of another. Django's __ relation lookups make it easy to cross those boundaries silently:

# Purchase and Client belong to different aggregates — this query couples them.
Purchase.objects.get(client__name="John")

pytest-orm-boundaries watches the ORM activity exercised by your test suite and reports access that crosses a configured boundary - through __ lookups, select_related, prefetch_related, subqueries, or hand-written .raw() SQL.

Install

Install the plugin in your Django project:

pip install pytest-orm-boundaries

pytest discovers the plugin automatically.

The plugin uses the Django version already installed by your project. If you are installing into an environment without Django and want pip to install it too, use the optional extra:

pip install "pytest-orm-boundaries[django]"

Configure

Declare your aggregates and their Django models in boundaries.toml at the project root (or point at the file with --boundaries-config / the boundaries_config ini option):

[aggregates.client]
models = ["bookshop.Client"]

[aggregates.book]
models = ["bookshop.Book"]

[aggregates.purchase]
models = ["bookshop.Purchase", "bookshop.PurchaseLine"]

Models are written as app_label.Model. Models not listed in any aggregate are not checked. Without a config file the plugin emits a warning and runs no checks.

What it catches

The plugin flags executed ORM access that spans more than one configured group. In the DDD example below, each crossing couples the purchase and client aggregates:

• __ relation lookups:

  Purchase.objects.get(client__name="John")
  

• select_related:

  Purchase.objects.select_related("client")
  

• Subqueries - a table reached through a subquery still counts:

  berlin_clients = Client.objects.filter(city="Berlin").values("id")
  Purchase.objects.filter(client_id__in=berlin_clients)
  

• Hand-written .raw() SQL:

  Purchase.objects.raw(
      "SELECT p.id FROM bookshop_purchase p "
      "JOIN bookshop_client c ON p.client_id = c.id"
  )
  

• Bare cursor.execute() - the same join reached through a raw cursor.

• prefetch_related:

  Purchase.objects.prefetch_related("client")
  

Queries that don't actually join across the boundary are not flagged - for example a foreign-key lookup by id, which Django resolves without a join:

Purchase.objects.filter(client_id=42)     # reads one table
Purchase.objects.filter(client__pk=42)    # Django trims the join

The report

At the end of the run, the plugin prints one grouped entry per offending place:

====================== orm-boundaries: boundary crossings ======================
1 place(s) in your code crossed aggregate boundaries, affecting 1 test(s):

bookshop/reports.py:13
    crossed aggregates: client ↔ purchase
    models: bookshop.Client, bookshop.Purchase
    1 test(s) affected:
      test_purchases.py::test_list_purchases_with_client

orm-boundaries: FAILED - 1 boundary crossing(s), run exits non-zero.

Each entry names the aggregates the query crossed and the models it joined. Places are ordered by how many tests they affect. Pass -v to see every affected test (otherwise the list is capped at 5 per place).

Allow and ignore

CQRS read models may cross aggregate boundaries by design, while existing application code may contain crossings you want to fix over time. [allow] and [ignore] let you tell the plugin which is which:

• [allow] - the crossing is intentional. Use it for code that is meant to span aggregates, such as CQRS read models. An allowed crossing is suppressed and never reported.
• [ignore] - the crossing is known debt you plan to fix. It is suppressed and the plugin reports the entry when its matching code runs without a crossing.

[allow]
files = [
    "app/read_models/sales.py",
]

[ignore]
files = [
    "app/billing.py",
    "app/legacy/*",
]

Each entry is a glob (fnmatch), resolved relative to pytest's root directory and matched against either:

• the file that issues the query, or
• the test file.

An [ignore] whose matching code runs through the whole suite without crossing a boundary is stale, so the plugin lists it for removal:

======================= orm-boundaries: stale ignores ========================
These [ignore] entries matched files that ran without crossing a boundary. Remove them from boundaries.toml:
  - app/billing.py

[allow] entries are never reported this way. If a file sits in both sections, the allow wins and its [ignore] entry shows up as stale to remove.

A note on Django internals

Catching prefetch_related relies on Django internals that come with no stability promise, so new Django releases may require compatibility updates.

Known gaps (on the project roadmap)

• lazy attribute access (e.g. purchase.client);
• related-manager reads/writes (client.purchases.all(), client.purchases.create(...));
• a direct query on another aggregate's model, e.g. Client.objects.get(...) written inside purchase code.

Status

Alpha - testing basic version.