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Ohm ॐ

Object-hash mapping library for Redis.


Ohm is a library for storing objects in Redis, a persistent key-value database. It includes an extensible list of validations and has very good performance.


Join the mailing list:

Meet us on IRC: #ohm on

Getting started

Install Redis. On most platforms it's as easy as grabbing the sources, running make and then putting the redis-server binary in the PATH.

Once you have it installed, you can execute redis-server and it will run on localhost:6379 by default. Check the redis.conf file that comes with the sources if you want to change some settings.

If you don't have Ohm, try this:

$ sudo gem install ohm

Or you can grab the code from

Now, in an irb session you can test the Redis adapter directly:

>> require "ohm"
=> true
>> Ohm.connect
=> []
>> Ohm.redis.set "Foo", "Bar"
=> "OK"
>> Ohm.redis.get "Foo"
=> "Bar"


Ohm's purpose in life is to map objects to a key value datastore. It doesn't need migrations or external schema definitions. Take a look at the example below:


class Event < Ohm::Model
  attribute :name
  reference :venue, Venue
  set :participants, Person
  counter :votes

  index :name

  def validate
    assert_present :name

class Venue < Ohm::Model
  attribute :name
  collection :events, Event

class Person < Ohm::Model
  attribute :name

All models have the id attribute built in, you don't need to declare it.

This is how you interact with IDs:

event = Event.create :name => "Ohm Worldwide Conference 2031"
# => 1

# Find an event by id
event == Event[1]
# => true

# Trying to find a non existent event
# => nil

This example shows some basic features, like attribute declarations and validations. Keep reading to find out what you can do with models.

Attribute types

Ohm::Model provides four attribute types: {Ohm::Model::attribute attribute}, {Ohm::Model::set set}, {Ohm::Model::list list} and {Ohm::Model::counter counter}; and two meta types: {Ohm::Model::reference reference} and {Ohm::Model::collection collection}.


An attribute is just any value that can be stored as a string. In the example above, we used this field to store the event's name. You can use it to store numbers, but be aware that Redis will return a string when you retrieve the value.


A set in Redis is an unordered list, with an external behavior similar to that of Ruby arrays, but optimized for faster membership lookups. It's used internally by Ohm to keep track of the instances of each model and for generating and maintaining indexes.


A list is like an array in Ruby. It's perfectly suited for queues and for keeping elements in order.


A counter is like a regular attribute, but the direct manipulation of the value is not allowed. You can retrieve, increase or decrease the value, but you can not assign it. In the example above, we used a counter attribute for tracking votes. As the incr and decr operations are atomic, you can rest assured a vote won't be counted twice.


It's a special kind of attribute that references another model. Internally, Ohm will keep a pointer to the model (its ID), but you get accessors that give you real instances. You can think of it as the model containing the foreign key to another model.


Provides an accessor to search for all models that reference the current model.

Persistence strategy

The attributes declared with attribute are only persisted after calling save. If the object is in an invalid state, no value is sent to Redis (see the section on Validations below).

Operations on attributes of type list, set and counter are possible only after the object is created (when it has an assigned id). Any operation on these kinds of attributes is performed immediately, without running the object validations. This design yields better performance than running the validations on each operation or buffering the operations and waiting for a call to save.

For most use cases, this pattern doesn't represent a problem. If you need to check for validity before operating on lists, sets or counters, you can use this pattern:

if event.valid?
  event.comments << "Great event!"

If you are saving the object, this will suffice:

  event.comments << "Wonderful event!"

Working with Sets

Given the following model declaration:

class Event < Ohm::Model
  attribute :name
  set :attendees, Person

You can add instances of Person to the set of attendees with the << method:

@event.attendees << Person.create(name: "Albert")

# And now...
@event.attendees.each do |person|
  # what you want with this person.


Since attendees is a {Ohm::Model::Set Set}, it exposes two sorting methods: {Ohm::Model::Collection#sort sort} returns the elements ordered by id, and {Ohm::Model::Collection#sort_by sort_by} receives a parameter with an attribute name, which will determine the sorting order. Both methods receive an options hash which is explained in the documentation for {Ohm::Model::Collection#sort sort}.


Ohm lets you declare references and collections to represent associations.

class Post < Ohm::Model
  attribute :title
  attribute :body
  collection :comments, Comment

class Comment < Ohm::Model
  attribute :body
  reference :post, Post

After this, every time you refer to post.comments you will be talking about instances of the model Comment. If you want to get a list of IDs you can use post.comments.raw.


An index is a set that's handled automatically by Ohm. For any index declared, Ohm maintains different sets of objects IDs for quick lookups.

In the Event example, the index on the name attribute will allow for searches like Event.find(:name => "some value").

Note that the assert_unique validation and the methods find and except need a corresponding index in order to work.

Finding records

You can find a collection of records with the find method:

# This returns a collection of users with the username "Albert"
User.find(:username => "Albert")

Filtering results

# Find all users from Argentina
User.find(:country => "Argentina")

# Find all activated users from Argentina
User.find(:country => "Argentina", :status => "activated")

# Find all users from Argentina, except those with a suspended account.
User.find(:country => "Argentina").except(:status => "suspended")

Note that calling these methods results in new sets being created on the fly. This is important so that you can perform further operations before reading the items to the client.

For more information, see SINTERSTORE and SDIFFSTORE.


Before every save, the validate method is called by Ohm. In the method definition you can use assertions that will determine if the attributes are valid. Nesting assertions is a good practice, and you are also encouraged to create your own assertions. You can trigger validations at any point by calling valid? on a model instance.


Ohm ships with some basic assertions. Check Ohm::Validations to see the method definitions.


The assert method is used by all the other assertions. It pushes the second parameter to the list of errors if the first parameter evaluates to false.

def assert(value, error)
  value or errors.push(error) && false


Checks that the given field is not nil or empty. The error code for this assertion is :not_present.

assert_present :name


Checks that the given field matches the provided format. The error code for this assertion is :format.

assert_format :username, /^\w+$/


Checks that the given field holds a number as a Fixnum or as a string representation. The error code for this assertion is :not_numeric.

assert_numeric :votes


Validates that the attribute or array of attributes are unique. For this, an index of the same kind must exist. The error code is :not_unique.

assert_unique :email


When an assertion fails, the error report is added to the errors array. Each error report contains two elements: the field where the assertion was issued and the error code.

Validation example

Given the following example:

def validate
  assert_present :foo
  assert_numeric :bar
  assert_format :baz, /^\d{2}$/
  assert_unique :qux

If all the assertions fail, the following errors will be present:

# => [[:foo, :not_present], [:bar, :not_numeric], [:baz, :format], [:qux, :not_unique]]

Presenting errors

Unlike other ORMs, that define the full error messages in the model itself, Ohm encourages you to define the error messages outside. If you are using Ohm in the context of a web framework, the views are the proper place to write the error messages.

Ohm provides a presenter that helps you in this quest. The basic usage is as follows:

error_messages = @model.errors.present do |e|
  e.on [:name, :not_present], "Name must be present"
  e.on [:account, :not_present], "You must supply an account"

# => ["Name must be present", "You must supply an account"]

Having the error message definitions in the views means you can use any sort of helpers. You can also use blocks instead of strings for the values. The result of the block is used as the error message:

error_messages = @model.errors.present do |e|
  e.on [:email, :not_unique] do
    "The email #{} is already registered."

# => ["The email is already registered."]

Ohm Extensions

Ohm is rather small and can be extended in many ways.

A lot of amazing contributions are available at Ohm Contrib, make sure to check them if you need to extend Ohm's functionality.


Ohm uses features from Redis > 1.3.10. If you are stuck in previous versions, please use Ohm 0.0.35 instead.

Upgrading from 0.0.x to 0.1

Since Ohm 0.1 changes the persistence strategy (from 1-key-per-attribute to Hashes), you'll need to run a script to upgrade your old data set. Fortunately, it is built in:

require "ohm/utils/upgrade"

Ohm.connect :port => 6380[:User, :Post, :Comment]).run

Yes, you need to provide the model names. The good part is that you don't have to load your application environment. Since we assume it's very likely that you have a bunch of data, the script uses Batch to show you some progress while the process runs.