Ruby on Rails

John H. wrote in post #1063205:
> I'm a bit unclear what's going on with relations. I'm hoping someone can
> help explain it to me.
>
> Lets say we have a model with orders and line_items such that an order
> has
> many line_items. Thus:
>
> class Order
> has_may :line_items
> end
> class LineItem
> belongs_to :order
> end
>
> When I have an order and call line_items what's going on? e.g.
>
> Order order = Order.first
> order.line_items.each {|li| puts li }
>
> I thought that was basically an alias for:
>
> Order order = Order.first
> LineItem.where(:order_id => order.id).each {|li| puts li }

This is an over-simplification. Let me illustrate by example:

1.9.3p194 :010 > order = Order.first
Order Load (0.3ms) SELECT "orders".* FROM "orders" LIMIT 1
=> #<Order id: 1, name: "First item", description: "First one", price:
nil, created_at: "2012-06-06 01:20:03", updated_at: "2012-06-06
01:20:03">

Notice first that selecting the order does not touch the line_items
table at all. At this point the order.line_items method is represented
by an ActiveRecord::Relation object. Accessing this relation object will
load (fire) the relation and populate the array.

Now let's take look at the line_items:

1.9.3p194 :002 > order.line_items
LineItem Load (0.2ms) SELECT "line_items".* FROM "line_items" WHERE
"line_items"."order_id" = 1
=> [#<LineItem id: 1, order_id: 1, name: "Line 1", quantity: 5,
created_at: "2012-06-06 01:21:24", updated_at: "2012-06-06 01:21:24">]
1.9.3p194 :003 > order.line_items
=> [#<LineItem id: 1, order_id: 1, name: "Line 1", quantity: 5,
created_at: "2012-06-06 01:21:24", updated_at: "2012-06-06 01:21:24">]

Notice the first time we access line_items ActiveRecord will load the
association by running a generated SQL statement. Subsequent calls do
not reissue the SQL statement.

Let's take a look at what is reported as the class for order.line_items:

1.9.3p194 :003 > puts order.line_items.class
Array

This appears to be a typical Array object, but it's really more
complicated than that. What you're seeing is not a simple Array object,
but rather an Array that has been extended with some Rails magic. Let's
take a look that the ancestry of that resulting "Array" object:

1.9.3p194 :004 > puts order.line_items.class.ancestors
Array
JSON::Ext::Generator::GeneratorMethods::Array
Enumerable
Object
PP::ObjectMixin
JSON::Ext::Generator::GeneratorMethods::Object
ActiveSupport::Dependencies::Loadable
Kernel
BasicObject

Here's the ancestry of a plain old Ruby Array:

# Ruby (No Rails)
$ irb
1.9.3p194 :001 > Array.ancestors
=> [Array, Enumerable, Object, Kernel, BasicObject]

In fact all Array objects in rails have the additional magic added to
them:

# Ruby on Rails
$ rails console
Loading development environment (Rails 3.2.3)
1.9.3p194 :001 > puts Array.ancestors
Array
JSON::Ext::Generator::GeneratorMethods::Array
Enumerable
Object
PP::ObjectMixin
JSON::Ext::Generator::GeneratorMethods::Object
ActiveSupport::Dependencies::Loadable
Kernel
BasicObject

> but, this isn't consistent with batching.
>
> So if we do
>
> Order order = Order.first
>
> # this doesn't appear to work, it's not batching, just finding all of
> the
> line items and iterating
> order.line_items.find_each {|li| puts li }

Hopefully what I explained above will clear up what's happening here.
That very first access of order.line_items is causing that relation
object to fire and load the objects.

What you probably want is the find_in_batches method provided by
ActiveRecord:

http://api.rubyonrails.org/classes/ActiveRecord/Batches.html#method-i-find_in_batches

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