A complete guide to MongoDB in Ruby — installing the mongo gem, connections and connection pools, CRUD with filters and update operators, the Aggregation Pipeline, compound and text search indexing, the Mongoid ODM with validations and callbacks, embedded documents, reference relations, change streams, GridFS for large files, multi-document transactions, and when to choose MongoDB vs a relational database.
A complete guide to Elasticsearch in Ruby — index and document concepts, installing elasticsearch-ruby, document CRUD, the Query DSL (match, term, bool, range, wildcard), Aggregations, highlight, suggest, pagination with from/size and search_after, the Bulk API, Rails integration via searchkick, database-to-Elasticsearch sync, and efficient indexing strategies.