Fix ContentReplicaRouter to allow relations across default/replica

[YasenT]

ContentReplicaRouter didn't implement allow_relation, so Django fell back to its default same-database check. Domains read during content app requests come from "replica", but pulp_maven's path_index builds a new, unsaved Artifact (db=None) and assigns that domain to it. Since "replica" != None, Django raised:

ValueError: Cannot assign "<Domain: ...>": the current database
router prevents this relation.

"replica" mirrors "default", so relations between objects loaded from either database are always safe.

Summary by Sourcery

Permit safe relations across mirrored default and replica databases in the content database router.

Bug Fixes:

• Allow relations between objects from the replica, default, and unassigned databases to prevent content request failures when assigning replica-read objects to new instances.

Tests:

• Add coverage confirming the database router permits cross-database and unassigned-object relations.

[sourcery-ai]

Reviewer's guide (collapsed on small PRs)

Reviewer's Guide

The router now explicitly permits all model relations, avoiding Django’s same-database rejection when replica-loaded domains are assigned to new unsaved objects, with parameterized tests covering replica, default, and unset database states.

Sequence diagram for replica-read relation assignment

sequenceDiagram
    participant ContentApp
    participant Router as ContentReplicaRouter
    participant Replica
    participant Artifact
    participant Domain

    ContentApp->>Replica: db_for_read(Domain)
    Replica-->>ContentApp: Domain(db=replica)
    ContentApp->>Artifact: Create unsaved Artifact(db=None)
    ContentApp->>Router: allow_relation(Artifact, Domain)
    Router-->>ContentApp: True
    ContentApp->>Artifact: Assign Domain relation

Loading

File-Level Changes

Change	Details	Files
Allow relations between objects regardless of database assignment because the replica mirrors the default database.	Add an allow_relation method that unconditionally returns True. Document the safety rationale for replica/default and unsaved-object relations.	pulp_service/pulp_service/app/database_router.py
Add coverage for cross-database and unsaved-object relation handling.	Parameterize router tests across replica, default, and unset database states. Verify allow_relation returns True for each supported combination.	pulp_service/pulp_service/tests/unit/test_database_router.py

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[sourcery-ai]

Hey - I've found 1 issue

Prompt for AI Agents

Please address the comments from this code review:

## Individual Comments

### Comment 1
<location path="pulp_service/pulp_service/app/database_router.py" line_range="38" />
<code_context>
     def db_for_write(self, model, **hints):
         return "default"

+    def allow_relation(self, obj1, obj2, **hints):
+        # "replica" mirrors "default", so relations between objects loaded from
+        # either database (or not yet assigned to one) are always safe. Without
+        # this, Django's default same-db check rejects assigning a replica-read
+        # object (e.g. a Domain) to a new, unsaved instance (db=None), such as
+        # when pulp_maven builds an Artifact during a content-app read.
+        return True
+
     def allow_migrate(self, db, app_label, model_name=None, **hints):
</code_context>
<issue_to_address>
**issue (broader_impact):** `allow_relation` returns `True` for every pair of objects, including objects loaded from an unrelated database alias such as `other`. When such an object is assigned to an object saved on `default`, Django accepts the relation and the resulting foreign key references the other database's row ID, raising an integrity error when absent or linking to the wrong row when IDs collide.

**Triggers:** When callers explicitly load or construct an object whose `_state.db` is an alias other than `default`, `replica`, or `None`.

**Suggested fix:** Return `True` only when both database states are in the supported `default`/`replica`/`None` set, and return `None` otherwise so Django's normal same-database check applies.

```suggestion
        if {obj1._state.db, obj2._state.db} <= {"default", "replica", None}:
            return True
        return None
```
</issue_to_address>

Sourcery assessment

Needs a human reviewer. 1 finding to address first, and if the router permits an object from an unintended database, Django could persist an incorrect foreign-key or many-to-many association because cross-database constraints are not enforced. Reverting stops the new behavior, but any bad associations already written would need to be identified and repaired or recomputed.

Blocking findings: pulp_service/pulp_service/app/database_router.py:38

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[sourcery-ai]

issue (broader_impact): allow_relation returns True for every pair of objects, including objects loaded from an unrelated database alias such as other. When such an object is assigned to an object saved on default, Django accepts the relation and the resulting foreign key references the other database's row ID, raising an integrity error when absent or linking to the wrong row when IDs collide.

Triggers: When callers explicitly load or construct an object whose _state.db is an alias other than default, replica, or None.

Suggested fix: Return True only when both database states are in the supported default/replica/None set, and return None otherwise so Django's normal same-database check applies.

Suggested change

	return True
	if {obj1._state.db, obj2._state.db} <= {"default", "replica", None}:
	return True
	return None