Switching from SQl to MongoDB in Rails 3 - ruby-on-rails

Question

I am considering switching a quite big application (Rails 3.0.10) from our SQL database (SQLite and Postgres) to MongoDB. I plan to put everything in it, mainly utf-8 string, binary file and user data. (Maybe also a little full text search) I have complex relationships (web structure: categories, tags, translations..., polymorphic also) and I feel that MongoDB philosophy is to avoid that and to put everything in big document, am I right ?
Does anyone have experience with MongoDB in Rails ? Particularly switching a app from ActiveRecord to Mongoid ? Do you think it's a good idea ? Do you know a guide/article to learn the MongoDB way to organize complex data ?
ps : In MongoDB, I particularly like the freedom offers by its architecture and its performance orientation. It's my main personal motivations to consider the switch.

Answer 1

I am using mongodb with mongoid, for 5-6 months. Have also worked with postgres + AR, MySQL + AR. Have no experience with switching AR to mongoid.
Are you facing any performance issues or expect to face them soon? If not I would advice to avoid the switch, as the decision seems just to be based on coolness factor of Mongodb.
They both have their pros and cons, I like the speed of mongodb, but there are many restrictions on what you can do to achieve that(like no joins, no transaction support and slow field vs. field(updated_at > created_at) queries).
If there are performance issues, I would still recommend to stick with your current system, as the switch might be a big task and it would be better if you spend half the time in optimizing the current system. After reading the question, I get a feeling that you have never worked with mongodb before, there are a many things which can bite you and you would not be fully aware of how to solve them.
However, If you still insist on switching, you need to carefully evaluate you data structure and the way you query them. In relational database, you have the normal forms, which have the advantage that whatever structure you start with, you will roughly reach the same end result once you do the normalization. In mongodb, there are virtually unlimited ways in which you can model your documents. You need to carefully model your documents to avail the benefits of mongodb. The queries you need to run play a very important role in your structuring along with the actual data you want to store.
Keep in mind, you do not have joins in mongodb(can be mitigated, with good modeling). As of now you can not have queries like, field1 = field2, i.e. you can't compare fields, but need to provide a literal to query against.
Take a look at this question: Efficient way to store data in MongoDB: embedded documents vs individual documents. Somebody points the OP to a discussion where embedded documents are recommended, but pretty much similar scenario, OP chooses to go with standalone documents, because of the nature of the queries he will be using to fetch the data.
All I want to say is, it should be a informed decision, which should be taken after you completely model your system with mongodb, have some performance tests with some real data to see if mongodb will solve your problem and should not be based on coolness factor.
UPDATE:
You can do field1 = field2 using $where clause, but its slow and is advised to be avoided.

Answer 2

We are currently switching from PostgreSQL, tsearch, and PostGIS in a production application. It has been a challenging process to say the least. Our data model is a better fit for mongodb because we don't need to do complex joins. We can model our data very easily into the nested document structure mongodb provides.
We have started a mirror site with the mongodb changes in it so we can leave the production site alone, while we stumble through the process. I don't want to scare you, because in the end, we will be happy we made the switch - but it is a lot of work. I would agree with the answer from rubish: be informed, and make the decision you feel is best. Don't base it on the 'coolness' factor.
If you must change, here are some tips from our experience:
ElasticSearch fits well with mongo's document structure to replace PostgreSQL's tsearch full text search extensions.
It also has great support for point based geo indexing. (Points of interest closest to, or within x miles/kilometers)
We are using Mongo's built in GridFS to store files, which works great. It simplifies the sharing of user contributed images, and files across our cluster of servers.
We are using rake tasks to dump data out of postgresql into yaml format. Then, we have another rake task in the mirror site which imports and converts the data into models stored in mongodb.
The data export/import might work using a shared redis database, resque on both sides, and an observer in the production application to log changes as they happen.
We are using Mongoid as our ODM, and there are a lot of scopes within our models that needed to be rewritten to work with Mongoid vs ActiveRecord.
Over all, we are very happy with MongoDB. It offers us much more flexibility in the way we model our data. I just wish we would have discovered it before the project were started.

Answer 3

skip active record,
Alternatively, if you’ve already created your app, have a look at config/application.rb
and change the first lines from this:
require "rails/all"
to this:
require "action_controller/railtie"
require "action_mailer/railtie"
require "active_resource/railtie"
require "rails/test_unit/railtie"
It’s also important to make sure that the reference to active_record in the generator block is commented out:
Configure generators values. Many other options are available, be sure to check the documentation.
# config.generators do |g|
# g.orm :active_record
# g.template_engine :erb
# g.test_framework :test_unit, :fixture => true
# end
As of this this writing, it’s commented out by default, so you probably won’t have to change anything here.
I hope it will be helpful to you while you switching app from AR to mongo.
Thanks.