# *Flexirest:* Associations

There are two types of association. One assumes when you call a method you actually want it to call the method on a separate class (as that class has other methods that are useful). The other is lazy loading related classes from a separate URL.

## Type 1 - Loading Other Classes

If the call would return a single instance or a list of instances that should be considered another object, you can also specify this when mapping the method using the `:has_one` or `:has_many` options respectively. It doesn't call anything on that object except for instantiate it, but it does let you have objects of a different class to the one you initially called.

```ruby
class Expense < Flexirest::Base
  def inc_vat
    ex_vat * 1.20
  end
end

class Address < Flexirest::Base
  def full_string
    "#{self.street}, #{self.city}, #{self.region}, #{self.country}"
  end
end

class Person < Flexirest::Base
  get :find, "/people/:id", :has_many => {:expenses => Expense}, 
    :has_one => {:address => Address}
end

@person = Person.find(1)
puts @person.expenses.reduce {|e| e.inc_vat}
puts @person.address.full_string
```

You can also use `has_one`/`has_many` on the class level to allow chaining of classes. You can specify the class name or allow the system to automatically convert it to the singular class. For example:

```ruby
class Expense < Flexirest::Base
  def inc_vat
    ex_vat * 1.20
  end
end

class Address < Flexirest::Base
  def full_string
    "#{self.street}, #{self.city}, #{self.region}, #{self.country}"
  end
end

class Person < Flexirest::Base
  has_one :addresses
  has_many :expenses, Expense
  get :find, "/people/:id"
end

class Company < Flexirest::Base
  has_many :people
  get :find, "/companies/:id"
end
```

Sometimes we want attributes to just return a simple Ruby Array. To achieve this we can add an `:array` option to the method. This is especially useful when the attribute contains an array of scalar values. If you don't specify the `:array` option Flexirest will return a `Flexirest::ResultIterator`. To illustrate the difference consider the following example:

```ruby
class Book < Flexirest::Base
  # :authors attribute ["Robert T. Kiyosaki", "Sharon L. Lechter C.P.A"]
  # :genres attribute ["self-help", "finance", "education"]
  get :find, "/books/:name", array: [:authors]
end
```

In the example above, the following results can be observed:

```ruby
@book = Book.find("rich-dad-poor-dad")
puts @book.authors
#=> ["Robert T. Kiyosaki", "Sharon L. Lechter C.P.A"]
puts @book.authors.class
#=> Array
puts @book.genres
#=> #<Flexirest::ResultIterator:0x007ff420fe7a88 @_status=nil, @_headers=nil, @items=["self-help", "finance", "education"]>
puts @books.genres.class
#=> Flexirest::ResultIterator
puts @books.genres.items
#=> ["self-help", "finance", "education"]
```

When the `:array` option includes an attribute, it is assumed the values were returned with the request, and they will not be lazily loaded. It is also assumed the attribute values do not map to a Flexirest resource.

## Type 2 - Lazy Loading From Other URLs

When mapping the method, passing a list of attributes will cause any requests for those attributes to mapped to the URLs given in their responses. The response for the attribute may be one of the following:

```ruby
"attribute" : "URL"
"attribute" : ["URL", "URL"]
"attribute" : { "url" : "URL"}
"attribute" : { "href" : "URL"}
"attribute" : { "something" : "URL"}
```

The difference between the last 3 examples is that a key of `url` or `href` signifies it's a single object that is lazy loaded from the value specified. Any other keys assume that it's a nested set of URLs (like in the array situation, but accessible via the keys - e.g. object.attribute.something in the above example).

It is required that the URL is a complete URL including a protocol starting with "http". To configure this use code like:

```ruby
class Person < Flexirest::Base
  get :find, "/people/:id", :lazy => [:orders, :refunds]
end
```

And use it like this:

```ruby
# Makes a call to /people/1
@person = Person.find(1)

# Makes a call to the first URL found in the "books":[...] array in the article response
# only makes the HTTP request when first used though
@person.books.first.name
```

## Type 3 - HAL Auto-loaded Resources

You don't need to define lazy attributes if they are defined using [HAL](http://stateless.co/hal_specification.html) (with an optional embedded representation). If your resource has an `_links` item (and optionally an `_embedded` item) then it will automatically treat the linked resources (with the `_embedded` cache) as if they were defined using `:lazy` as per type 2 above.

If you need to, you can access properties of the HAL association. By default just using the HAL association gets the embedded resource (or requests the remote resource if not available in the `_embedded` list).

```ruby
@person = Person.find(1)
@person.students[0]._hal_attributes("title")
```

## Type 4 - Nested Resources

It's common to have resources that are logically children of other resources. For example, suppose that your API includes these endpoints:

| HTTP Verb | Path                        |                                          |
|-----------|-----------------------------|------------------------------------------|
| POST      | /magazines/:magazine_id/ads | create a new ad belonging to a magazine  |
| GET       | /magazines/:magazine_id/ads | display a list of all ads for a magazine |

In these cases, your child class will contain the following:

```ruby
class Ad < Flexirest::Base
  post :create, "/magazines/:magazine_id/ads"
  get :all, "/magazines/:magazine_id/ads"
end
```

You can then access Ads by specifying their magazine IDs:

```ruby
Ad.all(magazine_id: 1)
Ad.create(magazine_id: 1, title: "My Add Title")
```

## Type 5 - JSON API Auto-loaded Resources

If attributes are defined using [JSON API](http://jsonapi.org), you don't need to define lazy attributes. If your resource has a `links` object with a `related` item, it will automatically treat the linked resources as if they were defined using `:lazy`.

You need to activate JSON API by specifying the `json_api` proxy:

```ruby
class Article < Flexirest::Base
  proxy :json_api
end
```

If you want to embed linked resources directly in the response (i.e. request a JSON API compound document), use the `includes` class method. The linked resource is accessed in the same was as if it was lazily loaded, but without the extra request:

```ruby
# Makes a call to /articles with parameters: include=images
Article.includes(:images).all

# For nested resources, the include parameter becomes: include=images.tags,images.photographer
Article.includes(:images => [:tags, :photographer]).all
```


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