36 lines
1.0 KiB
Markdown
36 lines
1.0 KiB
Markdown
+++
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date = "2009-06-07"
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title = "has_one - find all that have no associated object"
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tags = ["General", "Ruby", "Rails", "has_one", "has_many", "belongs_to"]
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slug = "has_one-find-all-that-have-no-associated-object"
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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>
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