Tricky find on associated conditions

My head is spinning a bit this morning - can somebody help me with this?

Given these two models with the following associations:

class Article < ActiveRecord::Base
belongs_to :category

  • has an attribute called “published_on”
    end

class Category < ActiveRecord::Base
has_many :articles
end

I need to list all categories, the order should be determined by the
“published_on” date of their latest associated article.

So for example:

cat1 = Category.create
cat1.articles.create(:published_on => 5.days.ago)
cat1.articles.create(:published_on => 4.days.ago)

cat2 = Category.create
cat2.articles.create(:published_on => 2.days.ago)

Then my list should be ordered:

[cat2, cat1]

I believe this would be trivial to make in raw SQL, but I have trouble
figuring out, how to do it with AR’s find() method.

  • Carsten

Carsten G. wrote:

My head is spinning a bit this morning - can somebody help me with this?

Given these two models with the following associations:

class Article < ActiveRecord::Base
belongs_to :category

  • has an attribute called “published_on”
    end

class Category < ActiveRecord::Base
has_many :articles
end

I need to list all categories, the order should be determined by the
“published_on” date of their latest associated article.

So for example:

cat1 = Category.create
cat1.articles.create(:published_on => 5.days.ago)
cat1.articles.create(:published_on => 4.days.ago)

cat2 = Category.create
cat2.articles.create(:published_on => 2.days.ago)

Then my list should be ordered:

[cat2, cat1]

I believe this would be trivial to make in raw SQL, but I have trouble
figuring out, how to do it with AR’s find() method.

  • Carsten

Hey Carsten

Are you aware that you can include associations in find queries with
“:include”

http://api.rubyonrails.org/classes/ActiveRecord/Base.html

Off the top of my head, try:

Category.all :include => :articles, :order => “articles.published_on
DESC”

Let me know if that works

Cheers,

Gavin

http://handyrailstips.com

Sweet - wasn’t aware of that one

Thanks

:slight_smile:

Gavin

http://handyrailstips.com

Gavin M. wrote:

Off the top of my head, try:

Category.all :include => :articles, :order => “articles.published_on
DESC”

Works like a charm - thanks!

Now that you got my head pointed in the right direction :wink: I noticed,
that :joins does the same job for me (ie. same result) with some slight
internal difference:

:include - uses LEFT OUTER JOIN and explicitely selects all attributes
from both tables (specifying each attribute by name)

:joins - uses INNER JOIN and only selects categories.*

While it probably won’t matter with my 300-400 articles, in a larger
dataset I should think, that :joins is the faster of the two.

Thank you for your help. :slight_smile:

  • Carsten

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