How do I perform multi-field search in Haystack?

To perform multi-field search in Haystack, you use document stores and retrievers configured to query across multiple fields simultaneously. Haystack supports this through document stores like Elasticsearch or OpenSearch, which handle structured data with multiple fields. When indexing documents, you define fields such as title, content, or author. During retrieval, you specify which fields to search using parameters like search_fields in the retriever. For example, with the ElasticsearchRetriever, you can set search_fields=["title", "content"] to search both fields. This approach combines results from all specified fields, using the underlying search engine’s scoring mechanism to rank matches.

You can customize the search behavior using field-specific boosts and query logic. Boosts (e.g., title^2) prioritize matches in certain fields—useful when some fields are more relevant than others. For instance, a title match might be weighted higher than a body text match. Additionally, you can control how terms are combined using operators like AND or OR in the query string. If using Elasticsearch, leverage its Query String syntax to define complex logic, such as (title:"database" AND content:"search"). For non-text fields like dates or numbers, ensure they’re mapped correctly in the document store to enable range queries or filtering alongside text search.

Practical implementation involves three steps:

1. Define fields during indexing: When adding documents, include metadata like author or date alongside the main content.
2. Configure the retriever: Use ElasticsearchRetriever(search_fields=["title^3", "content", "author"]) to search with boosts.
3. Execute queries: Pass a query string to the retriever’s retrieve() method. For example, searching for "machine learning" across title and content might return documents where either field contains the term, with titles scored higher. Testing with different field combinations and boosts helps optimize relevance. If performance is critical, index settings (e.g., analyzers, n-grams) can be tuned to improve speed and accuracy. Always validate results using Haystack’s evaluation tools to ensure the multi-field setup meets your requirements.