Getting Started with Ohm: A Cheat Sheet Guide

Ohm Cheatsheet

Ohm: a library for storing data in Redis, a key-value store

Ohm Version: 2.3.0

I. Prerequisites

  • Redis
  • Ruby

II. Installation

Just install the ohm gem, fire up an irb session or write a Ruby script and require it.

gem install ohm

III. Connecting to Redis

A. Single Connection

By default, Ohm connects to Redis at localhost (, port 6379. If you wish to override this, you may set a different Redis URL with Redic, a lightweight Redis client.

require "ohm"
Ohm.redis ="redis://<IP>:<PORT>")

# Sample: Ohm.redis ="redis://")

B. Multiple Connections

Certain Models could connect to different Redis servers.

Ohm.redis =

class Student < Ohm::Model

Student =

IV. Simple Key-Value Fetch and Get

require "ohm" "SET", "Key", "Value" "GET", "Key"
# => "Value"

V. Mapping Objects to Key Value Store

A. Class Declaration

class Student < Ohm::Model

B. Classes Attributes, References, and More

 class Student < Ohm::Model
   attribute :student_number
   attribute :first_name
   attribute :last_name
   attribute :birthdate
   index :student_number

class Course < Ohm::Model
  attribute :name
  attribute :domain
  reference :classroom, :Classroom
  set :students, :Student
  counter :waitlist
  counter :drops
  index :name

class Classroom < Ohm::Model
  attribute :name
  collection :courses, :Course

i. attribute

Any value that can be stored in a string. Numbers stored would be returned as a string.

ii. set

Similar to an unordered list.

a. Adding To a Set
course = Course.create(name: 'English 1')

student1 = Student.create(first_name: 'Adelen', last_name: 'Festin')

course.students.add(Student.create(first_name: 'Victoria', last_name: 'Po'))

# => 2
b. Iterating From a Set

Elements of a set are returned like enumerables to which you can apply iterative methods.

course.students.each do |student|
  puts student.last_name
# => Festin
#    Po
# => ["Adelen", "Victoria"]
c. Removing From a Set
  • index starts at 1

Proper way:

Usage of the delete method on the set to actually remove the element from the set.


Improper way:

Deleting the object itself.

student = course.students[1]

Doing so would result to having the deleted element still part of the list

# count would still include deleted element

But accessing the set element would result to nil:

# [.., nil, ..]

iii. list

class PreenlistmentList
  reference :course, :Course
  list :students, :Student

taekwondo = Course.create(name: 'PE 2 TKD')

student1 = Student.create(student_number: '2010-00033')
student2 = Student.create(student_number: '2010-18415')
student3 = Student.create(student_number: '2010-30011')

pl = PreenlistmentList.create(course: taekwondo)

# Pushes element to end of list
pl.students.push(student1) # necessary for list to persist

# Places element to front of list

# Push student3 twice
# => 3

# => 4

# Deletes all occurences of student3

# 2

iv. counter

Just like a regular attribute but direct manipulation and assignment is not allowed. Can only increment or decrement.

Course[1].incr(:waitlist) # 0 + 1
# => 1

course = Course[1]
course.incr(:waitlist) # 1 + 1
# => 2

course.decr(:waitlist) # 2 - 1
# => 1

For multiple attributes, you may all increase and/or decrease in one line by separating the attributes with a comma.

course.decr(:waitlist, :drops)

v. reference

Reference to another model; similar to a foreign key.

ph108 = Classroom.create(name: 'Palma Hall, Room 108')

course = Course.create(name: 'Geog 1', classroom: ph108)

# <Classroom:0x007f8dc9f2bb70 @attributes= ...
# "Palma Hall, Room 108"

vi. collection

A shortcut accessor to search for all models that reference the current model. Returns an enumerable



VI. CRUD Operations

A. Creating Records

i. Immediate Create
course = Course.create name: 'Math 17'
# => "1"
# => "Math 17"

ii. Initialize and Save
another_course = name: 'CS 11'
# => "CS 11"

B. Reading / Looking up a record

i. By Index
course = Course[1]
# => "Math 17"
ii. By Query (id)
course = Course.find(id: 1).first
# => "Math 17"
iii. By Query (attributes)
course = Course.find(name: 'Math 17').first
# => "Math 17"

C. Updating Records

course = Course.find(name: 'Math 17').first
# => 1

course.update(name: "Math 53")
# => "Math 53"
# => 1

D.Deleting Records

i. Direct Access
ii. Prior Assignment
course = Course[1]

VII. Filtering

A. Single Attribute

Course.find(domain: 'Math')

B. Multiple Attribute

Course.find(domain: 'English', waitlist: 0)

C. Multiple Values for an Attribute

# Find all courses with waitlist count = 0 and
# has domains of either Math or English

Course.find(waitlist: 0).combine(domain: ["Math", "English"])

D. With Exceptions

# Find all courses under the Math domain except
# for courses with Math 2 as the name

Course.find(domain: "Math").except(name: "Math 2")

VIII. Indices

Adding indices to models would allow you to execute find operations on the indexed attributes.

class People < Ohm::Model
  attribute :name
  attribute :gender

  index :name


Valid find lookups:

People.find(name: 'John Doe')

Invalid find lookups:

People.find(gender: 'Female')
# => Ohm::IndexNotFound: Ohm::IndexNotFound

# To fix: add gender to the index list
# index :name, :gender

IX. Sorting

All Sets can be sorted with sort which by default sorts by ID but can be overriden when passed with the by parameter.

On Ohm Version 2.3.0, you can only sort by numeric fields else a runtime error or an unsorted output may result

A. By

Indicate the attribute to which sorting will be based on.

For accuracy, use sort_by

 courses = Course.all.sort_by(:waitlist)
 => [0, 0, 1]
B. Order
Course.all.sort_by(:waitlist, order: 'ASC').map(&:waitlist)
# => [0, 0, 1]

Course.all.sort_by(:waitlist, order: 'DESC').map(&:waitlist)
# => [1, 0, 0]

B. Limit

Course.all.sort(limit: [1, 2])
# Gets 2 entries starting from offset 1

Course.all.sort(limit: [0, 1])
# Gets the first entry

X. Uniqueness

class Room < Ohm::Model
  attribute :name
  attribute :building
  unique :name

Room.create(name: 'Rm 180', building: 'Palma Hall')
# success

Room.create(name: 'Rm 180', building: 'DCS')
# Ohm::UniqueIndexViolation: UniqueIndexViolation: name

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