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https://github.com/soveran/ohm-crystal

Ohm for Crystal
https://github.com/soveran/ohm-crystal

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Ohm for Crystal

Lists

README 

        
Ohm ॐ

=====



Object-hash mapping library for Redis.



![CI](https://github.com/soveran/ohm-crystal/workflows/Crystal%20CI/badge.svg)



Description

-----------



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

database. It has very good performance.



Community

---------



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) for Ruby, 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.



Add this to your application's `shard.yml`:



```yaml

dependencies:

  ohm:

    github: soveran/ohm-crystal

```



Or you can grab the code from [http://github.com/soveran/ohm-crystal][ohm].



## Connecting to a Redis database



Ohm uses a lightweight Redis client called [Resp][resp]. To connect

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

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

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



```crystal

require "ohm"



Ohm.redis = Resp.new("redis://127.0.0.1:6379")



Ohm.redis.call "SET", "Foo", "Bar"



Ohm.redis.call "GET", "Foo"

# => "Bar"

```



Ohm defaults to a Resp connection to "redis://127.0.0.1:6379". The

example above could be rewritten as:



```crystal

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`.



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



```crystal

class Party < Ohm::Model

  attribute :name

  reference :venue, Venue

  set :participants, Person

  counter :votes



  index :name

end



class Venue < Ohm::Model

  attribute :name

  collection :parties, Party, :venue_id

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:



```crystal

party = Party.create({"name" => "Ohm Worldwide Party 2031"})

party.id

# => "1"



# Find an party by id

party == Party[1]

# => true



# Update an party

party.update({"name" => "Ohm Worldwide Party 2032"})

party.name

# => "Ohm Worldwide Party 2032"



# Trying to find a non existent party

Party[2]

# => nil



# Finding all the parties

Party.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 party's `name`.

If you want to store any other data type, you have to convert it

to a string first. 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:



```crystal

class Log < Ohm::Model

  track :text



  def append(msg)

    key["text"].call("APPEND", msg)

  end



  def tail(n = 100)

    key["text"].call("GETRANGE", -n.to_s, "-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:



```crystal

if party.save

  party.comments.add(Comment.create({"body" => "Wonderful party!"}))

end

```



Working with Sets

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



Given the following model declaration:



```crystal

class Party < Ohm::Model

  attribute :name

  set :attendees, Person

end

```



You can add instances of `Person` to the set of attendees with the

`add` method:



```crystal

party.attendees.add(Person.create({"name" => "Albert"}))



# And now...

party.attendees.each do |person|

  # ...do what you want with this person.

end

```



Working with Lists

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



Given the following model declaration:



```crystal

class Queue < Ohm::Model

  attribute :name

  list :people, Person

end

```



You can add instances of `Person` to the list of people with the

`push` method:



```crystal

queue.people.push(Person.create({"name" => "Albert"}))



# And now...

queue.people.each do |person|

  # ...do what you want with this person.

end

```



Working with Counters

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



Given the following model declaration:



```crystal

class Site < Ohm::Model

  attribute :url

  counter :visits

end

```



You can increment or decrement the visits:



```crystal

site.visits     #=> 0

site.visits(+1) #=> 1

site.visits(+1) #=> 2

site.visits(+5) #=> 7

site.visits(-4) #=> 3

site.visits     #=> 3

```



Associations

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



Ohm lets you declare `references` and `collections` to represent

associations.



```crystal

class Post < Ohm::Model

  attribute :title

  attribute :body

  collection :comments, Comment, :post_id

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:



```crystal

# 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 net effect here is we can conveniently set and retrieve `Post` objects,

and also search comments using the `post_id` index.



```crystal

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.



Here's again the `collection` macro in use:



```crystal

collection :comments, Comment, :post_id

```



When it expands, what you get is this method definition:



```crystal

def comments

  Comment.find({"post_id" => self.id })

end

```



Both examples are equivalent.



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 `Party` example, the index on the name attribute will

allow for searches like `Party.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:



```crystal

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

User.find({"username" => "Albert"})

```



### Filtering results



```crystal

# 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.



```crystal

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

```



[redis]: http://redis.io

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

[resp]: https://github.com/soveran/resp-crystal