Store mongo-backed data as virtual fields of an ActiveRecord model
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.gitignore chore(gem) [AS] Version bump and README Dec 9, 2016
stores_in_mongo.gemspec chore(gem) [AS] Version bump and README Dec 9, 2016


Create virtual attributes of an ActiveRecord model that are stored in a Mongo database

Have you ever wanted to perform relational JOINs between your SQL tables and Mongo documents?

Have you ever abused ActiveRecord's serialize feature and stored way too much complex data in your SQL table that you know deep in your heart shuld be offloaded, perhaps to a Mongo document?

Now you can with ease!

This gem allows you to interact with fields that you define as if they were fields of your model, but actually store their data in a Mongo document. The class representing the Mongo document itself can be created dynamically by stores_in_mongo, or you can provide an existing class that includes Mongoid::Document.

An example use case is a Transcript model that you also wish to store in a relational database, for example to join to other resources (e.g. :project and :user):

class Transcript < ActiveRecord::Base
  belongs_to: :project
  belongs_to: :user

  stores_in_mongo :words, Hash

This setup requires the transcript table to have a String column called mongo_document_id, which can be customized (see below).

You can then use Transcript#words as if it were a field of Transcript. This is functionally similar to defining a has_one_document association using a gem such as active_mongoid, but this implementation allows developers to skip any fussing with a separate object, instead treating the data as a native field. Here, the words field is saved with the model, reloaded with the model, destroyed with the model, and deep_dup'd with the model. Data is loaded on-demand, meaning you don't have to worry about slow performance due to loading from an external Mongo database unless you're actually referencing the data.

You can specify multiple fields and additional options in a single model using block syntax:

class Transcript < ActiveRecord::Base
  belongs_to: :project
  belongs_to: :user

  stores_in_mongo do
    field :words, Hash
    field :keywords, Array

What's new


  • Block syntax, allowing multiple fields
  • Add options to specify existing class and foreign_key column name
  • Remove dup override in favor of more aggressive deep_dup implementation.
  • Change default document_id column from document_id to mongo_document_id
  • Rename private document* methods to safer, more explicit mongo_document* names

NOTE: to upgrade to v2.0 from previous versions, you will either have to migrate your database to rename the document_id column to mongo_document_id, or else provide foreign_key: "document_id" as an argument to stores_in_mongo.


  • Allow non-Hash data types

Detailed spec

The basic use case is described in the example above, using the stores_in_mongo method with the provided block. There are some document-level options and field-level options available.

StoresInMongo::Base is included by default in ActiveRecord::Base. This defined the method stores_in_mongo, which you can use to attach Mongo-backed virtual-fields to your object. By default, calling stores_in_mongo will create a new class nested underneath your model called MongoDocument and its correspoding collection in your Mongo database (e.g. Transcript::MongoDocument, whose collection name will be transcript_mongo_documents)

There are two ways you can call stores_in_mongo:

  • Inline (quick) syntax, which allows only a single field stores_in_mongo(field_name, data_type = nil, **options)

  • Block syntax, which allows mutliple fields and additional options

stores_in_mongo(**options) do
 field field_name_1, data_type_1 = nil
 field field_name_2, data_type_2 = nil
 # ...etc

Args for stores_in_mongo

  • field_name: a String or Symbol namining the field. This argument can only be used with inline syntax.
  • data_type (default: nil): a data type (class), such as Hash or Array, that you could pass as type to a Mongoid class. This argument can only be used with inline syntax.
  • :class_name (default: nil): provide an existing Mongoid class instead of having stores_in_mongo define one on the fly. This class must already exist (or be auto-loadable) by the time this code executes.
  • :foreign_key (default: class_name.foreign_key): specify the column of the model that holds the id of the mongo document. This will be mongo_document_id by default if the class_name argument is also not specified. If class_name is specified to be, for example, "Transcript", this will be transcript_id by default.
  • :as: String specifying a polymorphic association to the Mongoid class. E.g. as: "document" will tell stores_in_mongo to look for the columns document_type and document_id to fetch and persist the Mongo document associated with this record. You cannot use this option with :class_name or :foreign_key options.

Inside the block

The block passed to stores_in_mongo currently accepts two methods:

  • field(name, data_type=nil): same as inline syntax, but you can call this multiple times to define multiple fields.
  • session do ... end: define an inline method that determines which mongoid session to use for a given model. This is useful if you need to store sensitive documents in different locations depending on instance attributes (e.g. client), but don't want to have to define polymorphic active-mongoid associations to the document classes and carry around all that bookeeping. For example:
stores_in_mongo do
  field :name
  field :interview
  session do
    self.protected_client? ? "secure" : "default"

This setup uses the secure session in your mongoid.yml file if self.protected_client? returns true for a given record, and default otherwise. For more, see the Mongoid docs on custom persistence.

Other details

Setting fields assigns them in memory, but you need to save your object before these are persisted to the Mongo database.

The mongo document is automatically saved when the model is saved and destroyed when the model is destroyed. There is no auto creation (for the reason specified above), but you can add it if you like! You can try after_initialize :find_or_initialize_mongo_document or after_initialize :find_or_initialize_mongo_document :if => :new_record? to be slightly less aggressive.

Calling reload on your object will reload the Mongo fields as well (but only if the document has already been loaded, saving performance when this behavior is not needed)

Calling deep_dup on your object will load the document if not already loaded and deep_dup it.

Changing the document data will cause Rails to update the updated_at field on save, if appropriate. stores_in_mongo does this by overriding the dirty method to also check for possible changes to the mongo document. Currently, we do not have a good way for detecting in-place changes to the document, so your model will always be marked as dirty if the document has been loaded.

Methods defined by stores_in_mongo are owned by a dynamically created module model::MongoDocumentMethods, where model is the class that called stores_in_mongo. Thus you can override these in your model and still have access to super if you would like to hook into the document creation, loading, or saving.


Figure out if we can safely hack into the changes data and any other ActiveRecord features that described changed or non-persisted data.

Enjoy! Feedback and feature requests welcome!