Rails: Find by related model attribute?

So, I tried to开发者_运维知识库 search for an example of how to do this, and I'm not sure I even know how to describe what I'm trying to do. I'm an utter noob when it comes to SQL, and I'm sure this is really basic but I'm totally lost:

I have a model, Photo, which has_many :tags, :through => :taggings. Tags have a name and an id.

I want to do something like: Photo.where( #tag_name in [array] )

... but like I said I have no idea how to write something like that, or what to search to see an example on Google.

Can anyone give me an example of that kind of query, and what it might be called?

Thanks!

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Just tried this on a similar model of my own and seemed to work fine:

Photo.joins(:tags).where('tags.name' => ['herp','derp']).group(:id)

Also, here's a great resource on the AREL querying interface rails 3 uses, with information on these and other ActiveRecord calls related to querying.

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So, as a twist on Brett's method, it turns out the following works without breaking PostgreSQL:

def self.tagged_with( string )
  array = string.split(',').map{ |s| s.lstrip }
  select('distinct photos.*').joins(:tags).where('tags.name' => array )
end

Problem solved! See this article for a great explanation as to why this is a better idea than 'group'.