Implement federated multi search

[LukasKalbertodt]

Pull Request

Related issue

Fixes #609

What does this PR do?

This PR adds types and methods to use the federated multi search API. There are multiple ways to add this to this library, depending on how strictly one wants to type everything. I decided to:

• Add a new FederatedMultiSearchResponse, which also required a new Client::execute_federated_multi_search_query. The response of federated searches is just very different from normal multi searches, and I didn't think having an enum returned by execute_multi_search_query was a particularly nice design (and it would have been a breaking change).
• Add a federation: Option<FederationHitInfo> to each SearchResult (which is None for non-federated searches). I don't think it's worth to have a dedicated FederatedSearchResult.
• Add MultiSearchQuery::with_federation which returns a new FederatedMultiSearchQuery. One could also change MultiSearchQuery to be able to represent federated queries, but I had a slight preference for my solution.

PR checklist

Please check if your PR fulfills the following requirements:

• [x] Does this PR fix an existing issue, or have you listed the changes applied in the PR description (and why they are needed)?
• [x] Have you read the contributing guidelines?
• [x] Have you made sure that the title is accurate and descriptive of the changes?

Thank you so much for contributing to Meilisearch!

Summary by CodeRabbit

• New Features

  • Introduced federated multi-search, allowing users to combine results from multiple indexes into a single, merged list.
  • Added support for specifying federation options such as weighting, offsets, limits, and facet merging in multi-search queries.
  • Search results now include federation metadata, providing more context about each hit in federated searches.
  • Enhanced test support for federated multi-search scenarios across diverse document types.
  • Improved test macro to support automatic creation and management of multiple indexes in test functions.

• Documentation

  • Expanded documentation to explain federated search usage and options in multi-search queries.
  • Clarified test macro documentation to reflect support for multiple index parameters in tests.

[coderabbitai[bot]]

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Walkthrough

The changes introduce federated multi-search support by adding new query and response types, federation-specific options, and related metadata structures. The client gains an asynchronous method to execute federated multi-search queries. Documentation is updated to explain usage and type requirements for federated queries. No changes to error handling or unrelated functionality are made.

Changes

File(s)	Change Summary
src/client.rs	Added execute_federated_multi_search_query async method to Client for federated multi-search; expanded documentation for multi_search to explain federated search usage and type requirements.
src/search.rs	Added federated multi-search support: - Added FederatedMultiSearchQuery, FederationOptions, FederatedMultiSearchResponse, FederationHitInfo, and QueryFederationOptions structs. - Enhanced MultiSearchQuery and SearchQuery with federation methods and fields. - Updated SearchResult to include federation metadata. - Replaced manual Serialize impl for MultiSearchQuery with derived version. - Added tests for federated multi-search functionality.
meilisearch-test-macro/src/lib.rs	Modified meilisearch_test macro to support multiple Index parameters by creating and managing separate indices for each parameter instead of a single shared index.
meilisearch-test-macro/README.md	Updated documentation to clarify that multiple Index parameters are supported and multiple indices are created automatically.

Sequence Diagram(s)

sequenceDiagram
    participant User
    participant Client
    participant Server

    User->>Client: Build MultiSearchQuery
    User->>Client: Call with_federation(federation_options)
    User->>Client: Call execute() on FederatedMultiSearchQuery
    Client->>Server: POST /multi-search with federated query
    Server-->>Client: FederatedMultiSearchResponse<T>
    Client-->>User: Merged federated search results

Assessment against linked issues

Objective	Addressed	Explanation
Add a new search parameter named federation to the multiSearch method (#609)	✅
Add capability for a new search parameter federationOptions in the search query (#609)	✅
Add tests (#609)	✅

Poem

A rabbit leaps through fields of search,
Now federated queries it can perch!
With options and weights, results combine,
Across indexes, the answers align.
🥕✨
New structs and methods—what a feat!
Our search is now truly elite. """

---

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[LukasKalbertodt]

@curquiza Any chances you got time to look at this?

Also: am I supposed to pay attention to the AI bot? I assumed I can ignore it like most other AI slop (I merely skimmed it), but if you think its comments make sense, I will take another look at it.

[LukasKalbertodt]

@martin-g Thanks for the review! I updated the PR to include the change you requested and also add a unit test. To do that, I had to adjust the #[meilisearch_test] macro and make it a bit more powerful.

Oh and: CI fails, but the three test failures don't seem to be related to my PR at all.

[LukasKalbertodt]

Again, for the record: I am pretty sure the CI failures have nothing to do with this PR. So I think this should be ready.