Mongoid ODM

Using a low-level driver to connect to the MongoDB database is often not the most efficient route. All the flexibility that a low-level driver provides is offset against longer development times and code to glue our models with the database.

An ODM (Object Document Mapper) can be the answer to these problems. Just like ORMs, ODMs bridge the gap between our models and the database. In Rails, the most widely used MVC framework for Ruby, Mongoid, can be used to model our data in a similar way to Active Record.

Installing the gem is similar to the Mongo Ruby driver, by adding a single file in the Gemfile:

gem 'mongoid', '~> 6.1.0'

Depending on the version of Rails, we may need to add the following to application.rb as well:

config.generators do |g|
g.orm :mongoid
end

Connecting to the database is done through a config file, mongoid.yml. Configuration options are passed as key-value pairs with semantic indentation. Its structure is similar to the database.yml used for relational databases.

Some of the options that we can pass through the mongoid.yml file are:

 

The next step is to modify our models to be stored in MongoDB. This is as simple as including one line of code in the model declaration:

class Person
  include Mongoid::Document
 End

We can also use the following:

include Mongoid::Timestamps

We use it to generate created_at and updated_at fields in a similar way to Active Record. Data fields do not need to be declared by type in our models but it's a good practice to do so. The supported data types are:

• Array
• BigDecimal
• Boolean
• Date
• DateTime
• Float
• Hash
• Integer
• BSON::ObjectId
• BSON::Binary
• Range
• Regexp
• String
• Symbol
• Time
• TimeWithZone

If the types of fields are not defined, fields will be cast to the object and stored in the database. This is slightly faster but doesn't support all types. If we try to use BigDecimal, Date, DateTime, or Range we will get back an error.