Object-hash mapping library for Redis.
Ohm is a library for storing objects in Redis, a persistent key-value database. It has very good performance.
Join the mailing list: http://groups.google.com/group/ohm-ruby
Meet us on IRC: #ohm on freenode.net
These are libraries in other languages that were inspired by Ohm.
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"
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"])
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
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 2032'>]
This example shows some basic features, like attribute declarations and querying. Keep reading to find out what you can do with models.
Ohm::Model provides 4 attribute types:
Ohm::Model.attribute,Ohm::Model.setOhm::Model.listOhm::Model.counterand 2 meta types:
Ohm::Model.referenceOhm::Model.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 increment and decrement
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.
Besides the provided attribute types, it is possible to instruct Ohm to track arbitrary keys and tie them to the object's lifecycle.
For example:
class Log < Ohm::Model
track :text
def append(msg)
redis.call("APPEND", key[:text], msg)
end
def tail(n = 100)
redis.call("GETRANGE", key[:text], -(n), -1)
end
end
log = Log.create
log.append("hello\n")
assert_equal "hello\n", log.tail
log.append("world\n")
assert_equal "world\n", log.tail(6)
When the log object is deleted, the :text key will be deleted
too. Note that the key is scoped to that particular instance of
Log, so if log.id is 42 then the key will be Log:42:text.
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
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
Since attendees is a Ohm::Model::Set, it exposes two sorting
methods: Ohm::Model::Collection#sort returns the elements
ordered by id, and Ohm::Model::Collection#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 direction and strategy. You can pass in any of the following:
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.
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.
Key or Hash key with which to sort by. An important distinction with
using Ohm::Model::Collection#sort and
Ohm::Model::Collection#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.
A key pattern to return, e.g. Post:*->title. As is the case with
the :by option, using Ohm::Model::Collection#sort and
Ohm::Model::Collection#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
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.
Doing a Ohm::Model.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)
The reason a Ohm::Model.reference and a
Ohm::Model.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 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
If your models are defined inside a module, you will have to define the references and collections as in the following example:
module SomeNamespace
class Foo < Ohm::Model
attribute :name
end
class Bar < Ohm::Model
reference :foo, 'SomeNamespace::Foo'
end
end
An Ohm::Model.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 and
Ohm::Model::Set#except need a corresponding index in order to work.
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")
# 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 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: "foo@bar.com")
u == User.with(:email, "foo@bar.com")
# => true
User.create(email: "foo@bar.com")
# => raises Ohm::UniqueIndexViolation
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 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.
FAQs
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