> ## Documentation Index
> Fetch the complete documentation index at: https://nikita-shkoda.mintlify.site/llms.txt
> Use this file to discover all available pages before exploring further.
# JOINs and relations
> Declare associations, query across collections, and model common relationship patterns.
Related: JOINs, References & JOINs — Deep dive, JOINs, Selection, and Grouping, Cascading
Use this page when you need to join across collections or model relationships.
## Declare associations
```ruby app/search_engine/publisher.rb theme={null}
class SearchEngine::Publisher < SearchEngine::Base
collection :publishers
attribute :publisher_id, :string
attribute :name, :string
identify_by :publisher_id
end
```
```ruby app/search_engine/book.rb theme={null}
class SearchEngine::Book < SearchEngine::Base
collection :books
attribute :publisher_id, :string
attribute :name, :string
belongs_to :publisher
end
```
## Query across collections
```ruby theme={null}
SearchEngine::Book
.joins(:publisher)
.where(publisher: { name: "Acme" })
.to_a
```
## One-to-one relations
```ruby app/search_engine/user.rb theme={null}
class SearchEngine::User < SearchEngine::Base
collection :users
attribute :user_id, :string
attribute :email, :string
identify_by :user_id
has_one :profile, foreign_key: :user_id
end
```
```ruby app/search_engine/profile.rb theme={null}
class SearchEngine::Profile < SearchEngine::Base
collection :profiles
attribute :user_id, :string
attribute :display_name, :string
belongs_to :user, local_key: :user_id, foreign_key: :user_id
end
```
```ruby theme={null}
SearchEngine::User.find("user_1").profile
```
## One-to-many relations
```ruby app/search_engine/author.rb theme={null}
class SearchEngine::Author < SearchEngine::Base
collection :authors
attribute :author_id, :string
attribute :name, :string
has_many :books, foreign_key: :author_id
end
```
```ruby app/search_engine/book.rb theme={null}
class SearchEngine::Book < SearchEngine::Base
collection :books
attribute :author_id, :string
attribute :title, :string
belongs_to :author, local_key: :author_id, foreign_key: :author_id
end
```
```ruby theme={null}
SearchEngine::Author.find("author_1").books.where(published: true)
```
## Many-to-many relations
```ruby app/search_engine/enrollment.rb theme={null}
class SearchEngine::Enrollment < SearchEngine::Base
collection :enrollments
attribute :student_id, :string
attribute :course_id, :string
end
```
```ruby app/search_engine/student.rb theme={null}
class SearchEngine::Student < SearchEngine::Base
collection :students
attribute :student_id, :string
attribute :name, :string
has_many :enrollments, foreign_key: :student_id
end
```
```ruby app/search_engine/course.rb theme={null}
class SearchEngine::Course < SearchEngine::Base
collection :courses
attribute :course_id, :string
attribute :title, :string
has_many :enrollments, foreign_key: :course_id
end
```
```ruby theme={null}
SearchEngine::Course
.joins(:enrollments)
.where(enrollments: { student_id: "student_1" })
.to_a
```
Use belongs\_to\_many when a single local key maps to many target records via a shared foreign key.
## Next steps
* Learn join rules and nested selection in JOINs.
* Explore cascading reindex behavior in Cascading.