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Associations

has_many, belongs_to — defining relationships between models

ActiveRecord Associations declare relationships between models in Ruby code.

Basic relationships:

class User < ApplicationRecord
  has_many :posts              # User 1:N Post
  has_one :profile             # User 1:1 Profile
end

class Post < ApplicationRecord
  belongs_to :user             # posts table has user_id foreign key
  has_many :comments
  has_many :tags, through: :post_tags  # N:M relationship
end

Usage:

user.posts                    # all posts for this user
user.posts.create(title: '...')  # create through association
post.user                     # the post's author

Options:

  • dependent: :destroy — delete children when parent is deleted

  • class_name: — specify relationship class name (non-convention)

  • foreign_key: — specify foreign key column name

  • inverse_of: — explicitly declare inverse relationship

Architecture Diagram

1:N (has_many / belongs_to)
User
has_many :posts
1 ←→ N
Post
belongs_to :user
user_id foreign key
1:1 (has_one / belongs_to)
User
has_one :profile
1 ←→ 1
Profile
belongs_to :user
user_id foreign key
N:M (has_many :through)
Post
has_many :tags,
through: :post_tags
N
PostTag
post_id + tag_id
N
Tag
has_many :posts,
through: :post_tags
Key point: <strong>Declarative relationship setup</strong> → methods like user.posts, post.user are auto-generated

Key Points

1

belongs_to :user — declares user_id foreign key exists in this table

2

has_many :posts — declares other model has this foreign key

3

has_one :profile — declares 1:1 relationship

4

has_many :tags, through: :post_tags — N:M via join table

5

Set cascade delete with dependent: :destroy

6

Methods like user.posts, post.user auto-generated

Pros

  • ✓ Declarative — relationships visible at a glance
  • ✓ Auto-generated CRUD methods through associations
  • ✓ Solve N+1 with eager loading
  • ✓ Minimal config with foreign key conventions

Cons

  • ✗ Complex relationships are hard to trace
  • ✗ Data loss if dependent option misconfigured
  • ✗ Polymorphic associations are complex
  • ✗ Need to learn optimization options like counter_cache

Use Cases

User-Post 1:N relationship Post-Tag N:M relationship User-Profile 1:1 relationship Express complex relationships with has_many :through

Related

💎
ActiveRecord
ORM — manipulate databases with objects
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