ActiveRecord query assistance

Question

Hey all,

i need a little help with a AR query. This is how my models look like:

class User < AR:B
  has_many :publications
end

class Publication < AR:B
  belongs_to :user
  belongs_to :category
end

class Category < AR:B
  has_many :publications
end

Now let's say I want to iterate over all existing categories and either display the user's publications, or display something like "#{current_user.name} has no publications in this category".

class PublicationsController < AC:B

  def index
    @categories = Category.find(:all, :include => :publications, :conditions => { 'publications.user_id' => current_user })
  end

end

This gives me all Categories the user actually has publications, but lacks the "empty" ones.

Any suggestions? :-)

Answer 1

This gives you all the Category objects:

@categories = Category.all

Then, if you declare has_many :through associations you can do something like the following:

@categories.each do |category|
  if category.users.include?(current_user)
    # User has publications
    publications = category.publications.select { |pub| pub.user == current_user }
  else
    # User has no publications
  end
end

(has-many-through declarations:

class User < AR:B
  has_many :publications
  has_many :categories, :through => :publication
end

class Publication < AR:B
  belongs_to :user
  belongs_to :category
end

class Category < AR:B
  has_many :publications
  has_many :users, :through => :publication
end

... warning: drycode)

There's probably an neater way to do this using named scopes though.

Answer 2

You might be able to just modify the find call:

@categories = Category.find(:all, :include => :publications, :conditions => [ 'publications.user_id=? OR publications.user_id IS NULL', current_user ])

Notice that we use the Array variant here rather than the Hash variant, since the example in the documentation implies this is the correct usage.