Ohm ॐ
Object-hash mapping library for Redis.
Description
Ohm is a library for storing objects in Redis, a persistent key-value database. It has very good performance.
Community
Join the mailing list: http://groups.google.com/group/ohm-ruby
Meet us on IRC: #ohm on freenode.net
Related projects
These are libraries in other languages that were inspired by Ohm.
- JOhm for Java, created by xetorthio
- Lohm for Lua, created by slact
- ohm.lua for Lua, created by amakawa
- Nohm for Node.js, created by maritz
- Redisco for Python, created by iamteem
- redis3m for C++, created by luca3m
- Ohmoc for Objective-C, created by seppo0010
- Sohm for Lua, compatible with Twemproxy
Articles and Presentations
- Simplicity
- How to Redis
- Redis and Ohm
- Ohm (Redis ORM) (Japanese)
- Redis and Ohm
- Ruby off Rails
- Data modeling with Redis and Ohm
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 http://github.com/soveran/ohm.
Now, in an irb session you can test the Redis adapter directly:
>> require "ohm"
=> true
>> Ohm.redis.call "SET", "Foo", "Bar"
=> "OK"
>> Ohm.redis.call "GET", "Foo"
=> "Bar"
Connecting to a Redis database
Ohm uses a lightweight Redis client called Redic. To connect
to a Redis database, you will need to set an instance of Redic
, with
an URL of the form redis://:<passwd>@<host>:<port>/<db>
, through the
Ohm.redis=
method, e.g.
require "ohm"
Ohm.redis = Redic.new("redis://127.0.0.1:6379")
Ohm.redis.call "SET", "Foo", "Bar"
Ohm.redis.call "GET", "Foo"
# => "Bar"
Ohm defaults to a Redic connection to "redis://127.0.0.1:6379". The example above could be rewritten as:
require "ohm"
Ohm.redis.call "SET", "Foo", "Bar"
Ohm.redis.call "GET", "Foo"
# => "Bar"
All Ohm models inherit the same connection settings from Ohm.redis
.
For cases where certain models need to connect to different databases,
they simple have to override that, i.e.
require "ohm"
Ohm.redis = Redic.new(ENV["REDIS_URL1"])
class User < Ohm::Model
end
User.redis = Redic.new(ENV["REDIS_URL2"])
Models
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:
Example
class Event < Ohm::Model
attribute :name
reference :venue, :Venue
set :participants, :Person
counter :votes
index :name
end
class Venue < Ohm::Model
attribute :name
collection :events, :Event
end
class Person < Ohm::Model
attribute :name
end
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"
event.id
# => 1
# Find an event by id
event == Event[1]
# => true
# Update an event
event.update :name => "Ohm Worldwide Conference 2032"
# => #<Event:0x007fb4c35e2458 @attributes={:name=>"Ohm Worldwide Conference"}, @_memo={}, @id="1">
# Trying to find a non existent event
Event[2]
# => nil
# Finding all the events
Event.all.to_a
# => [<Event:1 name='Ohm Worldwide Conference 2031'>]
This example shows some basic features, like attribute declarations and querying. Keep reading to find out what you can do with models.
Attribute types
Ohm::Model provides 4 attribute types:
- {Ohm::Model.attribute attribute},
- {Ohm::Model.set set}
- {Ohm::Model.list list}
- {Ohm::Model.counter counter}
and 2 meta types:
- {Ohm::Model.reference reference}
- {Ohm::Model.collection collection}.
attribute
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.
set
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.
list
A list
is like an array in Ruby. It's perfectly suited for queues
and for keeping elements in order.
counter
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 increment
and decrement
operations are atomic, you can rest assured a vote won't be counted twice.
reference
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.
collection
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
.
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. This design yields better performance than buffering
the operations and waiting for a call to save
.
For most use cases, this pattern doesn't represent a problem. If you are saving the object, this will suffice:
if event.save
event.comments.add(Comment.create(body: "Wonderful event!"))
end
Working with Sets
Given the following model declaration:
class Event < Ohm::Model
attribute :name
set :attendees, :Person
end
You can add instances of Person
to the set of attendees with the
add
method:
event.attendees.add(Person.create(name: "Albert"))
# And now...
event.attendees.each do |person|
# ...do what you want with this person.
end
Sorting
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 below:
:order
Order direction and strategy. You can pass in any of the following:
- ASC
- ASC ALPHA (or ALPHA ASC)
- DESC
- DESC ALPHA (or ALPHA DESC)
It defaults to ASC
.
Important Note: Starting with Redis 2.6, ASC
and DESC
only
work with integers or floating point data types. If you need to sort
by an alphanumeric field, add the ALPHA
keyword.
:limit
The offset and limit from which we should start with. Note that
this is 0-indexed. It defaults to 0
.
Example:
limit: [0, 10]
will get the first 10 entries starting from offset 0.
:by
Key or Hash key with which to sort by. An important distinction with
using {Ohm::Model::Collection#sort sort} and
{Ohm::Model::Collection#sort_by sort_by} is that sort_by
automatically
converts the passed argument with the assumption that it is a hash key
and it's within the current model you are sorting.
Post.all.sort_by(:title) # SORT Post:all BY Post:*->title
Post.all.sort(by: :title) # SORT Post:all BY title
Tip: Unless you absolutely know what you're doing, use sort
when you want to sort your models by their id
, and use sort_by
otherwise.
:get
A key pattern to return, e.g. Post:*->title
. As is the case with
the :by
option, using {Ohm::Model::Collection#sort sort} and
{Ohm::Model::Collection#sort_by sort_by} has distinct differences in
that sort_by
does much of the hand-coding for you.
Post.all.sort_by(:title, get: :title)
# SORT Post:all BY Post:*->title GET Post:*->title
Post.all.sort(by: :title, get: :title)
# SORT Post:all BY title GET title
Associations
Ohm lets you declare references
and collections
to represent associations.
class Post < Ohm::Model
attribute :title
attribute :body
collection :comments, :Comment
end
class Comment < Ohm::Model
attribute :body
reference :post, :Post
end
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.ids
.
References explained
Doing a {Ohm::Model.reference reference} is actually just a shortcut for the following:
# Redefining our model above
class Comment < Ohm::Model
attribute :body
attribute :post_id
index :post_id
def post=(post)
self.post_id = post.id
end
def post
Post[post_id]
end
end
The only difference with the actual implementation is that the model is memoized.
The net effect here is we can conveniently set and retrieve Post
objects,
and also search comments using the post_id
index.
Comment.find(post_id: 1)
Collections explained
The reason a {Ohm::Model.reference reference} and a {Ohm::Model.collection collection} go hand in hand, is that a collection is just a macro that defines a finder for you, and we know that to find a model by a field requires an {Ohm::Model.index index} to be defined for the field you want to search.
# Redefining our post above
class Post < Ohm::Model
attribute :title
attribute :body
def comments
Comment.find(post_id: self.id)
end
end
The only "magic" happening is with the inference of the index
that was used
in the other model. The following all produce the same effect:
# easiest, with the basic assumption that the index is `:post_id`
collection :comments, :Comment
# we can explicitly declare this as follows too:
collection :comments, :Comment, :post
# finally, we can use the default argument for the third parameter which
# is `to_reference`.
collection :comments, :Comment, to_reference
# exploring `to_reference` reveals a very interesting and simple concept:
Post.to_reference == :post
# => true
Indices
An {Ohm::Model.index 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 methods {Ohm::Model::Set#find find} and {Ohm::Model::Set#except 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 active users from Argentina
User.find(country: "Argentina", status: "active")
# Find all active users from Argentina and Uruguay
User.find(status: "active").combine(country: ["Argentina", "Uruguay"])
# Find all users from Argentina, except those with a suspended account.
User.find(country: "Argentina").except(status: "suspended")
# Find all users both from Argentina and Uruguay
User.find(country: "Argentina").union(country: "Uruguay")
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, SDIFFSTORE and SUNIONSTORE
Uniques
Uniques are similar to indices except that there can only be one record per entry. The canonical example of course would be the email of your user, e.g.
class User < Ohm::Model
attribute :email
unique :email
end
u = User.create(email: "[email protected]")
u == User.with(:email, "[email protected]")
# => true
User.create(email: "[email protected]")
# => raises Ohm::UniqueIndexViolation
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.
Upgrading
Ohm 2 breaks the compatibility with previous versions. If you're upgrading an existing application, it's nice to have a good test coverage before going in. To know about fixes and changes, please refer to the CHANGELOG file.