Thanks.
(the find_each I referenced in my question is (basically) the same as
find_in_batches)
The question is really why:
order.line_items != LineItem.where(:order_id => order.id)
So, I think what you're saying is that
order.line_items returns an array, not a relation.
On Tuesday, June 5, 2012 7:02:25 PM UTC-7, Ruby-Forum.com User wrote:
>
> 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
>
>
> --
> Posted via http://www.ruby-forum.com/.
>
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