Describe types of associations in Active Record.

In Active Record, which is the ORM framework used in Ruby on Rails, associations define the relationships between different models. These associations enable developers to express relationships between tables in a database using object-oriented concepts. Active Record supports several types of associations, each representing a different type of relationship between models. Here are the main types of associations:

1. **belongs_to**: This association indicates a one-to-one or many-to-one relationship between two models, where each instance of the associated model belongs to exactly one instance of the parent model. The foreign key is typically stored on the table of the associated model. For example:
```ruby
class Book < ApplicationRecord
belongs_to :author
end

class Author < ApplicationRecord
has_many :books
end
```
In this example, each book belongs to an author, and an author can have many books.

2. **has_one**: This association indicates a one-to-one relationship between two models, where each instance of the parent model has one associated instance of the associated model. The foreign key is typically stored on the table of the parent model. For example:
```ruby
class User < ApplicationRecord
has_one :profile
end

class Profile < ApplicationRecord
belongs_to :user
end
```
In this example, each user has one profile, and a profile belongs to a user.

3. **has_many**: This association indicates a one-to-many relationship between two models, where each instance of the parent model can have multiple associated instances of the associated model. The foreign key is typically stored on the table of the associated model. For example:
```ruby
class Author < ApplicationRecord
has_many :books
end

class Book < ApplicationRecord
belongs_to :author
end
```
In this example, an author can have many books, and each book belongs to one author.

4. **has_many :through**: This association allows for a many-to-many relationship between two models through a join model. It's useful when you need to associate two models with each other through a third model. For example:
```ruby
class Doctor < ApplicationRecord
has_many :appointments
has_many :patients, through: :appointments
end

class Patient < ApplicationRecord
has_many :appointments
has_many :doctors, through: :appointments
end

class Appointment < ApplicationRecord
belongs_to :doctor
belongs_to :patient
end
```
In this example, a doctor can have many appointments, a patient can have many appointments, and each appointment belongs to a doctor and a patient.

5. **has_one :through**: Similar to `has_many :through`, this association allows for a one-to-one relationship between two models through a join model. For example:
```ruby
class Supplier < ApplicationRecord
has_one :account
has_one :account_history, through: :account
end

class Account < ApplicationRecord
belongs_to :supplier
has_one :account_history
end

class AccountHistory < ApplicationRecord
belongs_to :account
end
```
In this example, a supplier has one account, and each account has one account history.

6. **polymorphic**: This association allows a model to belong to more than one other model, on a single association. It's useful when a model can belong to multiple other models with a single association. For example:
```ruby
class Image < ApplicationRecord
belongs_to :imageable, polymorphic: true
end

class Product < ApplicationRecord
has_many :images, as: :imageable
end

class User < ApplicationRecord
has_many :images, as: :imageable
end
```
In this example, an image can belong to either a product or a user.

These associations simplify database interactions by allowing developers to express relationships between models using object-oriented concepts. They provide methods for accessing and manipulating associated records, and they handle the complexities of database queries and joins behind the scenes.

No target file, test, or entry point is named. Start by locating the repository's documentation structure and determine where this Active Record association overview belongs; done means the documented association types and examples are accurate, complete, and integrated in the appropriate documentation location.

Written by the indexing model from the issue text.