A Mongoid developer specializes in using Mongoid, a Ruby Object-Document-Mapper (ODM) for MongoDB. They design, develop, and maintain applications using Ruby on Rails with Mongoid as the data store. They handle the mapping of Ruby objects to MongoDB documents, manage relationships between objects, and implement complex queries. They also deal with performance tuning of MongoDB and ensure data consistency and integrity. Their role involves understanding application requirements, designing data schemas, and integrating with other services. They work closely with other developers and stakeholders, often contributing to the architecture and design decisions of the systems they build.