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https://github.com/soveran/ohm

Object-Hash Mapping for Redis
https://github.com/soveran/ohm

lesscode ohm redis ruby

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Object-Hash Mapping for Redis

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README 

        
Ohm ॐ

=====



Object-hash mapping library for Redis.



Description

-----------



Ohm is a library for storing objects in [Redis][redis], a persistent key-value

database. It has very good performance.



Community

---------



Join the mailing list: [http://groups.google.com/group/ohm-ruby](http://groups.google.com/group/ohm-ruby)



Meet us on IRC: [#ohm](irc://chat.freenode.net/#ohm) on [freenode.net](http://freenode.net/)



Related projects

----------------



These are libraries in other languages that were inspired by Ohm.



* [Ohm](https://github.com/soveran/ohm-crystal) for Crystal, created by soveran

* [JOhm](https://github.com/xetorthio/johm) for Java, created by xetorthio

* [Lohm](https://github.com/slact/lua-ohm) for Lua, created by slact

* [ohm.lua](https://github.com/amakawa/ohm.lua) for Lua, created by amakawa

* [Nohm](https://github.com/maritz/nohm) for Node.js, created by maritz

* [Redisco](https://github.com/iamteem/redisco) for Python, created by iamteem

* [redis3m](https://github.com/luca3m/redis3m) for C++, created by luca3m

* [Ohmoc](https://github.com/seppo0010/ohmoc) for Objective-C, created by seppo0010

* [Sohm](https://github.com/xxuejie/sohm.lua) for Lua, compatible with Twemproxy



Articles and Presentations

--------------------------



* [Simplicity](http://files.soveran.com/simplicity)

* [How to Redis](http://www.paperplanes.de/2009/10/30/how_to_redis.html)

* [Redis and Ohm](http://carlopecchia.eu/blog/2010/04/30/redis-and-ohm-part1/)

* [Ohm (Redis ORM)](http://blog.s21g.com/articles/1717) (Japanese)

* [Redis and Ohm](http://www.slideshare.net/awksedgreep/redis-and-ohm)

* [Ruby off Rails](http://www.slideshare.net/cyx.ucron/ruby-off-rails)

* [Data modeling with Redis and Ohm](http://www.sitepoint.com/semi-relational-data-modeling-redis-ohm/)



Getting started

---------------



Install [Redis][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][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][redic]. To connect

to a Redis database, you will need to set an instance of `Redic`, with

an URL of the form `redis://:@:/`, through the

`Ohm.redis=` method, e.g.



```ruby

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:



```ruby

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.



```ruby

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



```ruby

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:



```ruby

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"

# => #"Ohm Worldwide Conference"}, @_memo={}, @id="1">



# Trying to find a non existent event

Event[2]

# => nil



# Finding all the events

Event.all.to_a

# => []

```



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`,

* `Ohm::Model.set`

* `Ohm::Model.list`

* `Ohm::Model.counter`



and 2 meta types:



* `Ohm::Model.reference`

* `Ohm::Model.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.



Tracked keys

------------



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:



```ruby

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`.



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:



```ruby

if event.save

  event.comments.add(Comment.create(body: "Wonderful event!"))

end

```



Working with Sets

-----------------



Given the following model declaration:



```ruby

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:



```ruby

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`, 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



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`.



__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` 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.



```ruby

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` and

`Ohm::Model::Collection#sort_by` has distinct differences in

that `sort_by` does much of the hand-coding for you.



```ruby

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.



```ruby

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` is actually just a shortcut for

the following:



```ruby

# 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.



```ruby

Comment.find(post_id: 1)

```



### Collections explained



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.



```ruby

# 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:



```ruby

# 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

```



Modules

-------



If your models are defined inside a module, you will have to define

the references and collections as in the following example:



```ruby

module SomeNamespace

  class Foo < Ohm::Model

    attribute :name

  end

  

  class Bar < Ohm::Model

    reference :foo, 'SomeNamespace::Foo'

  end

end

```



Indices

-------



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.



### Finding records



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



```ruby

# This returns a collection of users with the username "Albert"

User.find(username: "Albert")

```



### Filtering results



```ruby

# 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](http://redis.io/commands/sinterstore),

[SDIFFSTORE](http://redis.io/commands/sdiffstore) and

[SUNIONSTORE](http://redis.io/commands/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.



```ruby

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][contrib]

make sure to check them if you need to extend Ohm's functionality.



[contrib]: http://cyx.github.io/ohm-contrib/



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.



[redis]: http://redis.io

[ohm]: http://github.com/soveran/ohm

[redic]: https://github.com/amakawa/redic