Fix multivector config lost in CollectionConfig.to_dict()

[Shah91n]

_VectorIndexConfig.to_dict() serialized multi-vector settings as "multiVector": {"aggregation": ..., "encoding": {...}}. The server expects "multivector": {"enabled": true, "aggregation": ..., "muvera": {...}}.

Why

recreating a collection from export_config().to_dict() with create_from_dict() silently builds regular vector indexes instead of multi-vector ones. Depending on the server, inserting multi-vectors then fails, or the inserts succeed and multi-vector search fails with provided vector is a multi vector but vector index supports regular vectors. Found while migrating a ColBERT-style collection between clusters.

###Change

emit multivector with enabled: true, and muvera when an encoding is set, the same way quantizers are already handled. Adds a round-trip test with and without MUVERA.

Test

Verified against Weaviate 1.40.0. I exported a collection with a plain multi-vector, a MUVERA multi-vector and a regular vector, recreated it on a second instance and copied 200 objects. Before the fix, both multi-vector indexes came back as regular and all inserts failed. After the fix, both are preserved and every vector's search returns the right object.

[orca-security-eu]

Orca Security Scan Summary

Status	Check	Issues by priority
Passed	Infrastructure as Code	0   0   0   0	View in Orca
Passed	SAST	0   0   0   0	View in Orca
Passed	Secrets	0   0   0   0	View in Orca
Passed	Vulnerabilities	0   0   0   0	View in Orca

[Copilot]

🟢 Approval recommended

The serialization fix matches the server schema and is adequately covered by focused round-trip tests.

0 open findings

What changed in this PR

Fixes exported collection configs so multi-vector indexes can be recreated correctly.

Changes:

• Serializes multi-vector settings using the server-compatible multivector structure.
• Adds round-trip tests for plain and MUVERA configurations.

File	Description
weaviate/​collections/​classes/​config.py	Corrects multi-vector serialization.
test/​collection/​test_config_methods.py	Tests multi-vector configuration preservation.

🧠 Review effort: Balanced

---

💡 Add a code-review agent skill or configure MCP servers for context-aware, tailored reviews. Learn more in the docs.