Fix bulk_create not setting M2M on FK-related objects by benaduo · Pull Request #583 · model-bakers/model_bakery

benaduo · 2026-03-10T15:23:35Z

Summary

Fixes baker.make() with _bulk_create=True not applying M2M relationships
when the M2M field is on a FK-related object, specified via double-underscore
syntax (e.g. home__dogs=[dog])
Adds HomeOwner test model and a regression test reproducing the exact
scenario from the issue

Motivation

When a M2M value is passed as fk_field__m2m_field=[...], baker stores the
value in the sub-baker's m2m_dict during prepare(), but _handle_m2m() is
gated behind if _commit: and never executes in the bulk_create path.
_save_related_objs() only saves FK rows without touching M2M, and the two
existing M2M loops in bulk_create() only scan the top-level model's own
fields — neither handles the fk__m2m kwarg pattern. The result is that
home.dogs.set([dog]) is never called and the M2M relation stays empty.

Changes

model_bakery/baker.py: import FieldDoesNotExist from
django.core.exceptions; add FK-nested M2M handling loop at the end of
bulk_create() — parses kwargs for fk_field__m2m_field patterns, validates
field types, guards against custom through models, and calls .set() on the
FK object for each bulk-created entry
tests/generic/models.py: add HomeOwner model with ForeignKey to Home
tests/test_baker.py: add test_create_through_foreign_key_field to
TestCreateM2MWhenBulkCreate

Testing

pytest tests/test_baker.py::TestCreateM2MWhenBulkCreate -v

Author

Benjamin Aduo (@benaduo)

Summary by CodeRabbit

New Features
- Enhanced bulk_create to support setting many-to-many relationships on related objects via double-underscore syntax.
Tests
- Added a regression test validating M2M relations on foreign-key–related objects are populated after bulk creation.
- Added a test model to support the new regression scenario.

coderabbitai · 2026-03-10T15:24:08Z

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📥 Commits

Reviewing files that changed from the base of the PR and between e0919e0 and 5574857.

📒 Files selected for processing (3)

model_bakery/baker.py
tests/generic/models.py
tests/test_baker.py

✅ Files skipped from review due to trivial changes (1)

tests/test_baker.py

🚧 Files skipped from review as they are similar to previous changes (2)

tests/generic/models.py
model_bakery/baker.py

📝 Walkthrough

Walkthrough

Adds a second post-processing pass in bulk_create to apply many-to-many relations specified via single-level double-underscore kwargs on foreign-key-related objects (e.g., home__dogs=[...]), with validations for field types and auto-created through models.

Changes

Cohort / File(s)	Summary
Core M2M handling `model_bakery/baker.py`	Imported `FieldDoesNotExist` and added a second-pass in `bulk_create` that parses single-level `fk__m2m` kwargs, validates FK and M2M fields, skips custom through models, and sets M2M relations on the FK-related objects after bulk creation.
Test models `tests/generic/models.py`	Added `HomeOwner` model with `home = models.ForeignKey(Home, on_delete=models.CASCADE)` to exercise FK→M2M scenarios.
Regression test `tests/test_baker.py`	Added `test_create_through_foreign_key_field` verifying bulk_create with `_bulk_create=True` can set `home__dogs=[dog]` for each created `HomeOwner`.

Sequence Diagram

sequenceDiagram
    participant User
    participant Baker as baker.bulk_create
    participant ORM as Django ORM
    participant Through as Through Model

    User->>Baker: bulk_create(entries, home__dogs=[dog], _bulk_create=True)
    Baker->>Baker: prepare entries, persist FK fields
    Baker->>ORM: bulk_create(prepared_entries)
    ORM-->>Baker: created objects
    Baker->>Baker: existing per-entry M2M handling
    Baker->>Baker: parse single-level `fk__m2m` kwargs
    loop per created object with fk__m2m
        Baker->>Baker: validate FK is FK/OneToOne and m2m exists
        Baker->>Through: set m2m on fk_obj (if through.auto_created)
        Through-->>Baker: relation established
    end
    Baker-->>User: return created entries

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~22 minutes

Possibly related PRs

Fix _bulk_create for ManyToManyField with custom through models (#477) #570: Adjusts through-model FK name resolution in bulk_create M2M handling; closely related to through-model logic modified here.

Poem

🐰 I hopped through code to stitch a thread,
FK to M2M now gently spread,
One underscore, then just one more,
Bulk-created bonds we now restore.
Tiny checks keep pathways neat — hooray, repeat! 🥕

🚥 Pre-merge checks | ✅ 2 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 66.67% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately describes the main change: fixing bulk_create to handle M2M relationships on FK-related objects via double-underscore syntax, which is the core problem addressed in this PR.

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@benaduo

When baker.make() is called with _bulk_create=True and a M2M field is specified via double-underscore FK lookup syntax (e.g. home__dogs=[dog]), the M2M relationship was never applied to the saved FK object. Root cause: during the prepare() phase, baker correctly stores M2M values in the sub-baker's m2m_dict, but _handle_m2m() is gated behind `if _commit:` and therefore never executes in the bulk_create path. Subsequently, _save_related_objs() only saves FK rows and makes no M2M calls. The two existing M2M loops in bulk_create() scan only the top-level model's direct and reverse M2M fields, neither of which covers the fk__m2m kwarg pattern. The net result is that home.dogs.set([dog]) is never called, leaving the M2M relation empty in the database. Fix: after all entries have been bulk-created and FK objects saved, inspect the kwargs dict for double-underscore patterns of the form fk_field__m2m_field. For each pattern where the prefix resolves to a ForeignKey or OneToOneField on the current model and the suffix resolves to a ManyToManyField on the related model, call .set() on the FK object for every created entry. Changes: - model_bakery/baker.py: add FK-nested M2M handling loop at the end of bulk_create(), after the existing reverse-relation M2M loop - tests/generic/models.py: add HomeOwner model with ForeignKey to Home, providing the minimal fixture required to reproduce the reported scenario - tests/test_baker.py: add test_create_through_foreign_key_field to TestCreateM2MWhenBulkCreate, reproducing the exact scenario from the issue (baker.make(HomeOwner, home__dogs=[dog], _quantity=10, _bulk_create=True)) Fixes model-bakers#490 Author: Benjamin Aduo (@benaduo)

@benaduo

When baker.make() is called with _bulk_create=True and a M2M field is specified via double-underscore FK lookup syntax (e.g. home__dogs=[dog]), the M2M relationship was never applied to the saved FK object. Root cause: during the prepare() phase, baker correctly stores M2M values in the sub-baker's m2m_dict, but _handle_m2m() is gated behind `if _commit:` and therefore never executes in the bulk_create path. Subsequently, _save_related_objs() only saves FK rows and makes no M2M calls. The two existing M2M loops in bulk_create() scan only the top-level model's direct and reverse M2M fields, neither of which covers the fk__m2m kwarg pattern. The net result is that home.dogs.set([dog]) is never called, leaving the M2M relation empty in the database. Fix: after all entries have been bulk-created and FK objects saved, inspect the kwargs dict for double-underscore patterns of the form fk_field__m2m_field. For each pattern where the prefix resolves to a ForeignKey or OneToOneField on the current model and the suffix resolves to a ManyToManyField on the related model, call .set() on the FK object for every created entry. Only auto-created through models are supported; fields using a custom through= model are skipped since they require explicit through instance creation with extra field values baker cannot infer. FieldDoesNotExist is caught specifically (not bare Exception) so that unexpected errors still surface rather than being silently swallowed. Changes: - model_bakery/baker.py: import FieldDoesNotExist from django.core.exceptions; add FK-nested M2M handling loop at the end of bulk_create() with specific exception handling and auto-created through model guard - tests/generic/models.py: add HomeOwner model with ForeignKey to Home, providing the minimal fixture required to reproduce the reported scenario - tests/test_baker.py: add test_create_through_foreign_key_field to TestCreateM2MWhenBulkCreate, reproducing the exact scenario from the issue (baker.make(HomeOwner, home__dogs=[dog], _quantity=10, _bulk_create=True)) Fixes model-bakers#490 Author: Benjamin Aduo (@benaduo)

amureki

Hey @benaduo !

Thank you for opening this PR and fighting against linters and typing checks. I understand that this experience could be improved and will try to address things there.

Regarding the change, very well done with handling all the different cases and scenarios here, good guardrails!

I noted a couple of things we could improve before moving forward.

amureki · 2026-03-20T16:10:15Z

tests/test_baker.py

+        h1, h2 = models.HomeOwner.objects.all()[:2]
+        assert list(h1.home.dogs.all()) == list(h2.home.dogs.all()) == [dog]


What stops us from doing a full assert here?

Suggested change

h1, h2 = models.HomeOwner.objects.all()[:2]

assert list(h1.home.dogs.all()) == list(h2.home.dogs.all()) == [dog]

for owner in models.HomeOwner.objects.all():

assert list(owner.home.dogs.all()) == [dog]

amureki · 2026-03-20T16:25:56Z

model_bakery/baker.py

+        for entry in created_entries:
+            fk_obj = getattr(entry, fk_field_name, None)
+            if fk_obj is not None:
+                getattr(fk_obj, m2m_field_name).set(kwarg_value)


How about we consider optimizing this via bulk_create?

This would be less readable than a simple set(); however, we clearly reduce the number of queries used.

Suggested change

for entry in created_entries:

fk_obj = getattr(entry, fk_field_name, None)

if fk_obj is not None:

getattr(fk_obj, m2m_field_name).set(kwarg_value)

through_rows = []

for entry in created_entries:

fk_obj = getattr(entry, fk_field_name, None)

if fk_obj is not None:

through_rows.extend(

through_model(

**{

related_m2m.m2m_field_name(): fk_obj,

related_m2m.m2m_reverse_field_name(): obj,

}

)

for obj in kwarg_value

)

if through_rows:

through_model.objects.bulk_create(through_rows)

We can potentially extend the test with self.assertNumQueries to see its stats.

amureki · 2026-03-20T16:27:56Z

model_bakery/baker.py

+    # set many-to-many relations on FK-related objects specified via double-underscore
+    # syntax (e.g. home__dogs=[dog]). During prepare() the M2M values are stored in
+    # the sub-baker's m2m_dict but _handle_m2m() is never called because commit=False.
+    # _save_related_objs() only persists FK objects without touching M2M, so we must
+    # apply them here after all entries have been bulk-created and FK objects saved.


Could we revise this comment?
I appreciate you going the extra mile to document things, but it's hard to read here. :(

Suggested change

# set many-to-many relations on FK-related objects specified via double-underscore

# syntax (e.g. home__dogs=[dog]). During prepare() the M2M values are stored in

# the sub-baker's m2m_dict but _handle_m2m() is never called because commit=False.

# _save_related_objs() only persists FK objects without touching M2M, so we must

# apply them here after all entries have been bulk-created and FK objects saved.

# set M2M on FK-related objects (e.g. `home__dogs=[dog]`)

# `_handle_m2m()` is skipped during `prepare()` since `commit=False`,

# and `_save_related_objs()` only persists FK rows without touching M2M.

Something like this is more readable?

benaduo mentioned this pull request Mar 10, 2026

Add regression test for bulk_create with M2M field using explicit related_name #582

Merged

benaduo added 2 commits March 19, 2026 16:25

benaduo force-pushed the fix/bulk-create-fk-nested-m2m branch from e0919e0 to 5574857 Compare March 19, 2026 15:26

amureki reviewed Mar 20, 2026

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Fix bulk_create not setting M2M on FK-related objects#583

Fix bulk_create not setting M2M on FK-related objects#583
benaduo wants to merge 2 commits intomodel-bakers:mainfrom
benaduo:fix/bulk-create-fk-nested-m2m

benaduo commented Mar 10, 2026 •

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		h1, h2 = models.HomeOwner.objects.all()[:2]
		assert list(h1.home.dogs.all()) == list(h2.home.dogs.all()) == [dog]

-        for entry in created_entries:
-            fk_obj = getattr(entry, fk_field_name, None)
-            if fk_obj is not None:
-                getattr(fk_obj, m2m_field_name).set(kwarg_value)
+        through_rows = []
+        for entry in created_entries:
+            fk_obj = getattr(entry, fk_field_name, None)
+            if fk_obj is not None:
+                through_rows.extend(
+                    through_model(
+                        **{
+                            related_m2m.m2m_field_name(): fk_obj,
+                            related_m2m.m2m_reverse_field_name(): obj,
+                        }
+                    )
+                    for obj in kwarg_value
+                )
+        if through_rows:
+            through_model.objects.bulk_create(through_rows)

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benaduo commented Mar 10, 2026 •

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