This document discusses one-to-one, one-to-many, and many-to-many relationships in Active Record. One-to-one relationships involve a belongs_to and has_one association, like users and profiles. One-to-many relationships are common, like users having many orders. Many-to-many relationships use a join table to connect two models, such as writing prompts belonging to multiple categories. The document provides guidance on when to use each type of relationship.

2. Basic Active Record
Relationships
One-to-one, one-to-many, many-to-many

3. One to One
▪ The table of the belongs_to
model contains a column with
the foreign key (often id) of the
has_one model
▪ Not always clear when to use
one
▪ Users and social media profiles
– A user only has_one Facebook
profile, and that profile only
belongs_to that user

5. So when do I use it?
One indicator: the potential for
a lot of null (empty) fields

6. One to Many
▪ Very common relationship
▪ Like one to one, the table of the belongs_to model contains a column
with the foreign key (often id) of the has_many model
▪ Users and orders
– A user has_many orders, and each order only belongs_to that user

9. Many to Many
▪ Also very common
▪ Writing prompts and categories
– Categories can contain many prompts, and those prompts can belong to many
categories
▪ Two ways of describing the relationship
– has_many JOINTABLE & has_manyOTHERTABLE through JOINTABLE
– has_and_belongs_to_many (Rails/ActiveRecord magic creates the join table)
▪ Join table contains the foreign keys (ids) of what's connected to it

11. When to use has_and_belongs_to_many
When your join table only contains the foreign keys(ids)
and no other useful information

12. When to use has_and_belongs_to_many
When your join table only contains the foreign keys(ids)
and no other useful information
not conventional

13. When to use has_and_belongs_to_many
When your join table only contains the foreign keys(ids)
and no other useful information

14. t.what_now?
▪ t.belongs_to, t.references, and t.integer
– Everyone has their own preference, but any of these will work
– t.belongs_to :user
– t.references :user
– t.integer :user_id
▪ Note: when you use t.integer you need to specify the id because integer just means a
number
▪ With t.belongs_to or t.references the Rails/ActiveRecord magic can safely assume
what you mean

15. More resources
▪ http://www.mutuallyhuman.com/blog/2014/06/12/activerecord-
associations-and-you/
▪ http://guides.rubyonrails.org/association_basics.html
▪ https://github.com/kdmcclin/schema-practice
▪ https://github.com/kdmcclin/chained-relationships
@scarletalphabet