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: 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.
- JOhm for Java, created by xetorthio
- Lohm for Lua, created by slact
- Nohm for Node.js, created by maritz
- Redisco for Python, created by iamteem
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.connect
=> []
>> Ohm.redis.set "Foo", "Bar"
=> "OK"
>> Ohm.redis.get "Foo"
=> "Bar"
There are a couple of different strategies for connecting to your Redis
database. The first is to explicitly set the :host, :port, :db and
:timeout options. You can also set only a few of them, and let the other
options fall back to the default.
The other noteworthy style of connecting is by just doing Ohm.connect and
set the environment variable REDIS_URL.
Here are the options for {Ohm.connect} in detail:
:url
: A Redis URL of the form redis://:<passwd>@<host>:<port>/<db>.
Note that if you specify a URL and one of the other options at
the same time, the other options will take precedence. Also, if
you try and do Ohm.connect without any arguments, it will check
if ENV["REDIS_URL"] is set, and will use it as the argument for
:url.
:host
: Host where the Redis server is running, defaults to "127.0.0.1".
:port
: Port number, defaults to 6379.
:db
: Database number, defaults to 0.
:password
: It is the secret that will be sent to the Redis server. Use it if the server
configuration requires it. Defaults to nil.
:timeout
: Database timeout in seconds, defaults to 0.
:thread_safe
: Initializes the client with a monitor. It has a small performance penalty, and
it's off by default. For thread safety, it is recommended to use a different
instance per thread. I you have no choice, then pass :thread_safe => true
when connecting.
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
end
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
# Trying to find a non existent event
Event[2]
# => nil
This example shows some basic features, like attribute declarations and validations. Keep reading to find out what you can do with models.
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.
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 << Comment.create(:body => "Great event!")
end
If you are saving the object, this will suffice:
if event.save
event.comments << 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
<< method:
event.attendees << 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 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:
1. ASC
2. ASC ALPHA (or ALPHA ASC)
3. DESC
4. DESC ALPHA (or ALPHA DESC)
It defaults to `ASC`.
:start
: The offset from which we should start with. Note that
this is 0-indexed. It defaults to 0.
:limit : The number of entries to get. If you don't pass in anything, it will get all the results from the LIST or SET that you are sorting.
: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
: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
:store : An optional key which you may use to cache the sorted result. The key may or may not exist.
This option can only be used together with `:get`.
The type that is used for the STORE key is a LIST.
Post.all.sort_by(:title, :store => "FOO")
# Get all the results stored in FOO.
Post.db.lrange("FOO", 0, -1)
When using temporary values, it might be a good idea to use a `volatile`
key. In Ohm, a volatile key means it just starts with a `~` character.
Post.all.sort_by(:title, :get => :title,
:store => Post.key.volatile["FOO"])
Post.key.volatile["FOO"].lrange 0, -1
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.key.smembers.
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)
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
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 {Ohm::Model::Validations#assert_unique assert_unique} validation and the methods {Ohm::Model::Set#find find} and {Ohm::Model::Set#except 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 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
end
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+$/
Checks that the given field holds a number as a Fixnum or as a string representation. The error code for this assertion is :not_numeric.
assert_numeric :votes
Validates that the attribute or array of attributes are unique. For this, an index of the same kind must exist. The error code is :not_unique.
assert_unique :email
When an assertion fails, the error report is added to the errors array. Each error report contains two elements: the field where the assertion was issued and the error code.
Given the following example:
def validate
assert_present :foo
assert_numeric :bar
assert_format :baz, /^\d{2}$/
assert_unique :qux
end
If all the assertions fail, the following errors will be present:
obj.errors
# => [[:foo, :not_present], [:bar, :not_numeric], [:baz, :format], [:qux, :not_unique]]
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"
end
error_messages
# => ["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 #{@model.email} is already registered."
end
end
error_messages
# => ["The email foo@example.com is already registered."]
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.
Check the examples to get a feeling of the design patterns for Redis.
- Activity Feed
- Chaining finds
- Serialization to JSON
- One to many associations
- Philosophy behind Ohm
- Learning Ohm internals
- Slugs and permalinks
- Tagging
- Polymorphism
- Serialized attributes
Ohm uses features from Redis > 1.3.10. If you are stuck in previous versions, please use Ohm 0.0.35 instead.
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
Ohm::Utils::Upgrade.new([: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.