41 lines
1.0 KiB
Markdown
41 lines
1.0 KiB
Markdown
---
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title: "has_one - find all that have no associated object"
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kind: article
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slug: has_one-find-all-that-have-no-associated-object
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created_at: 2009-06-07
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tags:
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- General
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- Ruby
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- Rails
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- has_one
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- has_many
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- belongs_to
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---
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Let me pose a typical Rails situation:
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<pre lang="ruby">class Person < ActiveRecord::Base
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has_one :fancy_hat
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end
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class FancyHat < ActiveRecord::Base
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belongs_to :person
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end</pre>
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Now, how can you get all the people that don't have a fancy hat?
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<pre lang="ruby">class Person < ActiveRecord::Base
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has_one :fancy_hat
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named_scope :hatless, :joins => 'LEFT JOIN fancy_hats ON fancy_hats.person_id = people.id', :conditions => 'fancy_hats.person_id IS NULL'
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end</pre>
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Now you can find all the hatless people you want.
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FYI: Finding fancy hats that have no one to wear them is a lot easier, because the foreign key is stored in the <code>fancy_hats</code> table.
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<pre lang="ruby">class FancyHat < ActiveRecord::Base
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belongs_to :person
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named_scope :wearerless, :conditions => { :person_id => nil }
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end</pre> |