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Eager Loading (revised)

#22 Eager Loading (revised)

Mar 20, 2013 | 7 minutes | Performance, Active Record
One way to improve performance is to reduce the number of database queries through eager loading. Here I demonstrate this and compare the difference between the "includes" and "joins" methods.
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Florian Görlitz over 11 years ago

good episode. But what about delegate? Where is the differnence to your:

ruby 
def category_name
  read_attribute("category_name") || category.name
end

Thank you

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Lee Jarvis over 11 years ago

Using delegate :name, :to => :category, :prefix => 'category' would simply do category.name rather than read the loaded/cached category_name attribute. AFAIK

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9zb over 7 years ago

Pretty cool !

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baby90 over 7 years ago

That's okay for me

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Ashley Connor over 11 years ago

Off topic but how do you get SQL syntax highlighting when using tail?

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punaise-de-lit over 7 years ago

It's really easy, just type it on Google ;)

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Greg Silcox over 11 years ago

I went through many of the original RailsCasts about a year ago. They where are a great way to refresh on the basics, but rails has changed so much that it would be difficult for a new rails hopeful, such as my son, to make some of them work. Thanks for giving us a fresh, back-to-basics episode.

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Dom over 11 years ago

I agree. They are they a good reminder for optimization techniques and what has been deprecated as we move move into new Rails versions.

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Yossi S over 11 years ago

Great episode! But I was hoping you'd touch eager loading with polymorphic associations. I couldn't find good information about it, and I feel it's a common problem.

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Weddingful over 11 years ago

The way I approached eager loading with polymorphic associations is to define the relationship as non-polymorphic for the types of polymorphic you need to eager load. Only works for cases where your eager loading is of the same type.

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Yossi S over 11 years ago

Yeah, but thats exactly what I wanted to avoid doing. My model is this message board:

A User has FeedItems. Each feed item is polymorphically related to one other model object e.g. TextPost, Picture, File, SpreadSheet and many more, via FeedItem's postable polymorphic property

So the problem here is with FeedItem: When showing User's FeedItems, I want to do something like:

ruby 
@feed_items = current_user.feed_items.includes(:postable)

But since postable is polymorphic, i cant and i get the n+1 problem...

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Andrei Stoicescu over 11 years ago

By using includes there, the polymorphic association is loaded in the best possible way (meaning that first it runs a query to load the feed items, and then, judging by the id/types it does a query for each type, and not for each instance, which from my point of view it's the best possible way to do it)

I had a more difficult problem posted here where I wanted to do an includes on the polymorphic model to eager-load other associated models, but ended up doing a manual sweep

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Yossi S over 11 years ago

You are totally correct, it really does what you said, which is the best solution. But I too share your problem: TextPost and Picture have comments (which I'd like to eagerly load), and SpreadSheet has collaborators, which I'd also like to eagerly load.

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Barry D'arcy over 10 years ago

I think this can be resolved with default scopes? If you want your Picture to include :comments and your Spreadsheet to include :collaborators:

ruby 
class Picture #model
  default_scope -> { includes(:comments).order('id DESC') }
end

class Spreadsheet #model
  default_scope -> { includes(:coolaborators).order('id DESC') }
end

... and if you didn't want to include somewhere, use something like Picture.all.unscoped

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Bharat Ruparel over 11 years ago

+1 for eager loading with polymorphic associations. A very common and real problem that should be addressed in a Pro episode. Thanks.

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Carlos Júnior over 10 years ago

Unfortunately eager loading polymorphic associations isn't actually possible. The polymorphic association holds a a class name and an id present on that class. The polymorphic associations, though, don't have a way to join them in a SQL query since SQL doesn't support any sort of meta-programing.

For instance:

Address is kept in addresses table, and has an addressable reference. In the database there would be "addressable_id" and "addressable_type". There's no way to use "addressable_type" to make an INNER JOIN in the SQL query.

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Carlos Júnior over 10 years ago

Just to add up more info for future visitors:

You can, instead of "include" use "preload". But this will cause a second query to be triggered. This method is recommended since it'll help you to avoid N+1 queries.

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Marc Guillaume about 7 years ago

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Marc Guillaume about 7 years ago

THX SBU, It's for me

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Happy Diwali 2016 about 8 years ago

I went through many of the original RailsCasts about a year ago. They where are a great way to refresh on the basics, but rails has changed so much that it would be difficult for a new rails hopeful, such as my son, to make some of them work. Thanks for giving us a fresh, back-to-basics episode.

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