Does Ruby on Rails Work With MongoDB?
Rails works with MongoDB through the Mongoid ODM, but you lose many Rails conveniences designed around relational databases.
Quick Facts
How Ruby on Rails Works With MongoDB
Rails uses Active Record by default, which is built for relational databases. To use MongoDB, you replace it with Mongoid, a mature ODM (Object-Document Mapper) that provides a Rails-like API for document-based data. The developer experience is familiar—you define models with validations, associations, and callbacks similarly to Active Record—but there are architectural differences. MongoDB's document model means you can embed related data, reducing the need for joins, which can improve performance for certain access patterns.
However, you sacrifice some Rails magic. Features like database-level transactions (until MongoDB 4.0+), complex joins, and some migration tooling work differently or not at all. Many gems assume Active Record, so you may encounter compatibility issues with popular plugins. The relational mindset that Rails enforces through conventions becomes optional, which is powerful but requires more thoughtful schema design. Mongoid handles most common operations smoothly, but complex queries or aggregations often require learning MongoDB's aggregation framework rather than relying on Rails' query interface.
Best Use Cases
Rails + Mongoid Quick Setup
bundle add mongoid && rails g mongoid:config# Gemfile
gem 'mongoid'
# app/models/post.rb
class Post
include Mongoid::Document
field :title, type: String
field :content, type: String
field :published_at, type: Time
has_many :comments
validates :title, presence: true
end
class Comment
include Mongoid::Document
field :body, type: String
belongs_to :post
end
# Usage
post = Post.create!(title: 'Hello', content: 'World')
post.comments.create!(body: 'Great post!')
Post.where(published_at: { '$gte' => 1.week.ago }).countKnown Issues & Gotchas
Many Rails gems expect Active Record and won't work with Mongoid without modification
Fix: Check gem compatibility before adding dependencies; search GitHub for 'mongoid' issues in gem repos
Transactions in MongoDB require replica sets; single-node deployments don't support multi-document ACID transactions
Fix: Use MongoDB Atlas or deploy a replica set; for simple cases, design for eventual consistency
Rails migration generators and rake tasks don't work with Mongoid; schema management is implicit in model definitions
Fix: Use Mongoid's index management via model definitions; document schema changes manually
N+1 query problems are less obvious with MongoDB since embedding is common, but still occur with associations
Fix: Use `.includes()` or aggregation pipelines; monitor with tools like New Relic
Alternatives
- •Rails + PostgreSQL (native JSON columns): Stay with Active Record while handling semi-structured data
- •Node.js + Express + MongoDB: Avoid Rails overhead; better native async/await for MongoDB operations
- •Django + MongoDB (MongoEngine): Python alternative with similar ODM approach to Mongoid
Resources
Related Compatibility Guides
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